Search Results for "cyclone cyclones hurricane hurricanes":
Walsh, Kevin J., Suzana J Camargo, Gabriel A Vecchi, A S Daloz, J B Elsner, Kerry A Emanuel, M Horn, Y-K Lim, Malcolm J Roberts, Christina M Patricola, E Scoccimarro, Adam H Sobel, S E Strazzo, Gabriele Villarini, Michael F Wehner, Ming Zhao, James Kossin, T LaRow, K Oouchi, S D Schubert, H Wang, Julio T Bacmeister, P Chang, F Chauvin, Christiane Jablonowski, Arun Kumar, and Hiroyuki Murakami, et al., July 2015: Hurricanes and climate: the U.S. CLIVAR working group on hurricanes. Bulletin of the American Meteorological Society, 96(6), DOI:10.1175/BAMS-D-13-00242.1. Abstract
While a quantitative climate theory of tropical cyclone formation remains elusive, considerable progress has been made recently in our ability to simulate tropical cyclone climatologies and understand the relationship between climate and tropical cyclone formation. Climate models are now able to simulate a realistic rate of global tropical cyclone formation, although simulation of the Atlantic tropical cyclone climatology remains challenging unless horizontal resolutions finer than 50 km are employed. This article summarizes published research from the idealized experiments of the Hurricane Working Group of U.S. CLIVAR (CLImate VARiability and predictability of the ocean-atmosphere system). This work, combined with results from other model simulations, has strengthened relationships between tropical cyclone formation rates and climate variables such as mid-tropospheric vertical velocity, with decreased climatological vertical velocities leading to decreased tropical cyclone formation. Systematic differences are shown between experiments in which only sea surface temperature is increased versus experiments where only atmospheric carbon dioxide is increased, with the carbon dioxide experiments more likely to demonstrate the decrease in tropical cyclone numbers previously shown to be a common response of climate models in a warmer climate. Experiments where the two effects are combined also show decreases in numbers, but these tend to be less for models that demonstrate a strong tropical cyclone response to increased sea surface temperatures. Further experiments are proposed that may improve our understanding of the relationship between climate and tropical cyclone formation, including experiments with two-way interaction between the ocean and the atmosphere and variations in atmospheric aerosols.
Villarini, Gabriele, James A Smith, Mary Lynn Baeck, Timothy Marchok, and Gabriel A Vecchi, December 2011: Characterization of rainfall distribution and flooding associated with U.S. landfalling tropical cyclones: Analyses of hurricanes Frances, Ivan, and Jeanne (2004). Journal of Geophysical Research: Atmospheres, 116, D23116, DOI:10.1029/2011JD016175. Abstract
Rainfall and flooding associated with landfalling tropical cyclones are examined through empirical analyses of three hurricanes (Frances, Ivan, and Jeanne) that affected large portions of the eastern U.S. during September 2004. Three rainfall products are considered for the analyses: NLDAS, Stage IV, and TMPA. Each of these products has strengths and weaknesses related to their spatio-temporal resolution and accuracy in estimating rainfall. Based on our analyses, we recommend using the Stage IV product when studying rainfall distribution in landfalling tropical cyclones due to its fine spatial and temporal resolutions (about 4-km and hourly) and accuracy, and the capability of estimating rainfall up to 150 km from the coast. Lagrangian analyses of rainfall distribution relative to the track of the storm are developed to represent evolution of the temporal and spatial structure of rainfall. Analyses highlight the profound changes in rainfall distribution near landfall, the changing contributions to the rainfall field from eyewall convection, inner rain bands and outer rain bands, and the key role of orographic amplification of rainfall. We also present new methods for examining spatial extreme of flooding from tropical cyclones and illustrate the links between evolving rainfall structure and spatial extent of flooding.
Skillful seasonal forecasting of tropical cyclone (TC; wind speed ≥17.5 m s−1) activity is challenging, even more so when the focus is on major hurricanes (wind speed ≥49.4 m s−1), the most intense hurricanes (Category 4–5; wind speed ≥58.1 m s−1), and landfalling TCs. Here we show that a 25-km resolution global coupled model (HiFLOR) developed at the Geophysical Fluid Dynamics Laboratory (GFDL) has improved skill in predicting the frequencies of major hurricanes and Category 4–5 hurricanes in the North Atlantic, and landfalling TCs over the United States and Caribbean Islands a few months in advance, relative to its 50-km resolution predecessor climate model (FLOR). HiFLOR also shows significant skill in predicting Category 4–5 hurricanes in the western North Pacific and eastern North Pacific, while both models show comparable skills in predicting basin-total and landfalling TC frequency in the basins. The improved skillful forecasts of basin-total TCs, major hurricanes, and Category 4–5 hurricane activity in the North Atlantic by HiFLOR are obtained mainly by improved representation of the TCs and their response to climate from the increased horizontal resolution, rather than improvements in large-scale parameters.
Ross, R, and Yoshio Kurihara, 1993: Hurricane-environment interaction in hurricanes Gloria and Gilbert In 20th Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 27-30.
Following Hurricane Katrina and the parade of storms that affected the conterminous United States in 2004–2005, the apparent recent increase in intense hurricane activity in the Atlantic basin, and the reported increases in recent decades in some hurricane intensity and duration measures in several basins have received considerable attention. An important ongoing avenue of investigation in the climate and meteorology research communities is to determine the relative roles of anthropogenic forcing (i.e., global warming) and natural variability in producing the observed recent increases in hurricane frequency in the Atlantic, as well as the reported increases of tropical cyclone activity measures in several other ocean basins. A survey of the existing literature shows that many types of data have been used to describe hurricane intensity, and not all records are of sufficient length to reliably identify historical trends. Additionally, there are concerns among researchers about possible effects of data inhomogeneities on the reported trends. Much of the current debate has focused on the relative roles of sea-surface temperatures or large-scale potential intensity versus the role of other environmental factors such as vertical wind shear in causing observed changes in hurricane statistics. Significantly more research – from observations, theory, and modeling – is needed to resolve the current debate around global warming and hurricanes.
Bender, Morris A., 1997: The effect of relative flow on the asymmetric structure in the interior of hurricanes. Journal of the Atmospheric Sciences, 54(6), 703-724. Abstract PDF
Asymmetric structure of tropical cyclones simulated by the Geophysical Fluid Dynamics Laboratory high-resolution triply nested movable-mesh hurricane model was analyzed. Emphasis was placed on the quasi-steady component of the asymmetric structure in the region of the eyewall. It was found that the asymmetry was primarily caused by the relative wind, that is, the flow entering and leaving the storm region relative to the moving storm. A set of idealized numerical experiments was first performed both with a constant and a variable Coriolis parameter (f) and the addition of basic flows that were either constant or sheared with height. Analysis was then made for one case of Hurricane Gilbert (1988) to demonstrate that the quasi-steady asymmetric structure analyzed in the idealized studies could be identified in this real data case.
Vorticity analysis in the variable f experiment indicated that quasi-steady asymmetries resulted in the eyewall region through the effect of vorticity advection due to differences between the beta gyre flow in the lower free atmosphere and the storm motion. This was roughly matched with a persistent area of divergence and vorticity compression in the lower free atmosphere ahead of the storm and enhanced convergence and vorticity stretching to the rear. An asymmetric structure in the upward motion and accumulated precipitation, when averaged over a sufficiently long period of time, exhibited a corresponding maximum in the eyewall's rear quadrant.
With the addition of an easterly basic flow, a pronounced change in the asymmetry of the time-averaged boundary layer convergence resulted, with maximum convergence located ahead of the storm. However, the asymmetries in the average vertical motion in the middle troposphere and accumulated precipitation were more affected by the convergence field in the lower free atmosphere produced by the relative flow there. The relative flow depended on both the basic and beta gyre flow. With the addition of an easterly vertical shear to the easterly basic flow, the storm moved faster than the lower-level winds, and strong relative wind was from the front to the rear in the lower free atmosphere and from the opposite direction in the outflow layer aloft. As a result, the upward motion was significantly increased in the front of the storm and reduced in the rear, and the precipitation maximum shifted to the left front quadrant
Overall, analysis results suggest that the flow relative to the storm motion is an important factor contributing to the formation of quasi-steady asymmetries in the convergence and vertical motion fields, as well as in the mean precipitation pattern of tropical cyclones.
Kurihara, Yoshio, 1993: Hurricanes and atmospheric processes In Relating Geophysical Structures and Processes: The Jeffreys Volume, Geophysical Monograph 76, IUGG Volume 16, Washington, DC, American Geophysical Union, 19-26. Abstract
Hurricanes are among cyclonic vortices in which the gradient wind relationship holds during their evolution. The development of hurricanes proceeds with a persistent thermal forcing and the continual adjustment of fields to a new state of gradient wind balance. The forcing is largely due to the release of the latent heat received at the ocean surface. The adjustment is achieved by the development of a transverse circulation and the generation of inertia gravity waves. The behavior of the vortex also strongly depends on how it responds and adjusts itself to the environmental forcing. Thus, the spatial and temporal variability of the tropical cyclone climatology is related to regional and seasonal changes in the conditions of the larger scale environment. As exemplified by the evolution of hurricanes, the dynamics of an atmospheric system which undergoes slow structural change is controlled by the processes of forcing and adjustment. An atmospheric process may play a dual role: it contributes to the adjustment of one system while it provides forcing to another system, thus linking atmospheric systems of distinctly different scales.
Prominent multidecadal fluctuations of India summer rainfall, Sahel summer rainfall, and Atlantic Hurricane activity have been observed during the 20th century. Understanding their mechanism(s) will have enormous social and economic implications. We first use statistical analyses to show that these climate phenomena are coherently linked. Next, we use the GFDL CM2.1 climate model to show that the multidecadal variability in the Atlantic ocean can cause the observed multidecadal variations of India summer rainfall, Sahel summer rainfall and Atlantic Hurricane activity (as inferred from vertical wind shear changes). These results suggest that to interpret recent climate change we cannot ignore the important role of Atlantic multidecadal variability.
Shen, W, Isaac Ginis, and Robert E Tuleya, 2002: A numerical investigation of land surface water on landfalling hurricanes. Journal of the Atmospheric Sciences, 59(4), 789-802. Abstract PDF
Little is known about the effects of surface water over land on the decay of landfalling hurricanes. This study, using the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory hurricane model, examines the surface temperature changes due to hurricane-land surface water interactions, and their effects on the surface heat fluxes, hurricane structure, and intensity. Different water depths and surface conditions are incorporated for a variety of experiments starting with a hurricane bogus embedded in a uniform easterly mean flow of 5 m s-1.
A salient feature of hurricane-land surface water interaction is the local surface cooling near the hurricane core with the largest cooling behind and on the right side of the hurricane center. Unlike the surface cooling due to hurricane-ocean interaction, the largest cooling in hurricane-land surface water interaction can be much closer to the hurricane core. Without solar radiation during night, the surface evaporation dominates the local surface cooling. This causes a surface temperature contrast between the core area and its environment. During the day, the surface temperature contrast is enhanced due to additional influence from the reduced solar radiation under the core. Related to the local surface cooling, there is a significant reduction of surface evaporation with a near cutoff behind the hurricane center. A layer of half-meter water can noticeably reduce landfall decay although the local surface temperature around the hurricane core region is more than 4°C lower than in its environment. Further experiments indicate that an increase of roughness reduces the surface winds but barely changes the surface temperature and evaporation patterns and their magnitudes since the increase of roughness also increases the efficiency of surface evaporation.
Several recent models suggest that the frequency of Atlantic tropical cyclones could decrease as the climate warms. However, these models are unable to reproduce storms of category 3 or higher intensity. We explored the influence of future global warming on Atlantic hurricanes with a downscaling strategy by using an operational hurricane-prediction model that produces a realistic distribution of intense hurricane activity for present-day conditions. The model projects nearly a doubling of the frequency of category 4 and 5 storms by the end of the 21st century, despite a decrease in the overall frequency of tropical cyclones, when the downscaling is based on the ensemble mean of 18 global climate-change projections. The largest increase is projected to occur in the Western Atlantic, north of 20°N.
Zhao, Ming, and Isaac M Held, December 2010: An analysis of the effect of global warming on the intensity of Atlantic hurricanes using a GCM with statistical refinement. Journal of Climate, 23(23), DOI:10.1175/2010JCLI3837.1. Abstract
A statistical intensity adjustment is utilized to extract information from tropical cyclone
simulations in a 50km-resolution global model. A simple adjustment based on the modeled
and observed probability distribution of storm life-time maximum wind speed allows the
GCM to capture the differences between observed intensity distributions in active/inactive
year composites from the 1981-2008 period in the N. Atlantic. This intensity adjustment is
then used to examine the atmospheric model’s responses to different sea surface temperature
anomalies generated by coupled models for the late 21st century. In the North Atlantic all
simulations produce a reduction in the total number of cyclones, but with large inter-model
spread in the magnitude of the reduction. The intensity response is positively correlated with
changes in frequency across the ensemble. Yet there is, on average, an increase in intensity
in these simulations despite the mean reduction in frequency. We argue that it is useful to
decompose these intensity changes into two parts: an increase in intensity that is intrinsic to
the climate change experiments; and a change in intensity positively correlated with frequency,
just as in the active/inactive historical composites. Isolating the intrinsic component, which
is relatively independent of the details of the SST warming pattern, we find an increase in
storm-lifetime maximum winds of 5-10 ms−1 for storms with intensities of 30-60 ms−1, by
the end of the 21st century. The effects of change in frequency, which are dependent on the
details of the spatial structure of the warming, must then be superimposed on this intrinsic
change.
In this study we assess the impact of imperfect sampling in the pre-satellite era (between
1878 and 1965) on North Atlantic hurricane activity measures, and on the long-term
trends in those measures. Our results suggest that a substantial upward adjustment of
hurricane counts is needed prior to 1965 to account for likely ‘missed’ hurricanes due to
sparse density of reporting ship traffic. After adjusting for our estimate of ‘missed’
hurricanes in the basin, the long-term (1878-2008) trend in hurricane counts changes
from significantly positive to no significant change (with a nominally negative trend).
The adjusted hurricane count record is more strongly connected to the difference between
main development region (MDR) sea surface temperature (SST) and tropical-mean SST,
than with MDR SST. Our results do not support the notion that the warming of the
tropical North Atlantic due to anthropogenic greenhouse gas emissions has caused
Atlantic hurricane frequency to increase.
A newly developed global model, the Geophysical Fluid Dynamics Laboratory (GFDL)
High-Resolution Atmospheric Model (HiRAM) which is designed for both weather predictions
and climate-change simulations, is used to predict the tropical cyclone activity at 25-km
resolution. Assuming the persistence of the sea surface temperature anomaly during the forecast
period, we show that the inter-annual variability of seasonal prediction for hurricane counts in
the North Atlantic basin is highly predictable during the past decade (2000-2010). A remarkable
correlation of 0.96 between the observed and model predicted hurricane counts is achieved. The
root mean square error of the predicted hurricane number is less than 1 per year after correcting
the model’s negative bias. The predictive skill of the model in the tropics is further supported by
the successful prediction of a Madden-Julian Oscillation event initialized 7-day in advance of its
onset.
Villarini, Gabriele, Gabriel A Vecchi, and James A Smith, January 2012: U.S. landfalling and North Atlantic hurricanes: Statistical modeling of their frequencies and ratios. Monthly Weather Review, 140(1), DOI:10.1175/MWR-D-11-00063.1. Abstract
Time series of US landfalling and North Atlantic hurricane counts and their ratios over the period 1878–2008 are modeled using tropical Atlantic sea surface temperature (SST), tropical mean SST, North Atlantic Oscillation (NAO), and Southern Oscillation Index (SOI). Two SST input data are employed to examine the uncertainties in the reconstructed SST data on the modeling results. Due to the likely undercount of recorded hurricanes in the earliest part of the record, we consider both the uncorrected hurricane record (HURDAT), and a time series with a recently proposed undercount correction.
Modeling of the count data is performed using a conditional Poisson regression model, in which the rate of occurrence can depend linearly or nonlinearly on the climate indices. Model selection is performed following a stepwise approach and using two penalty criteria. These results do not allow identifying a single “best” model due to the different model configurations (different SST data, corrected versus uncorrected datasets, penalty criteria). Despite the lack of an objectively identified unique final model, we recommend a set of models in which the parameter of the Poisson distribution depends linearly on tropical Atlantic and tropical mean SSTs.
Modeling of the fractions of North Atlantic hurricanes making US landfall is performed using a binomial regression model. Similar to the count data, it is not possible to identify a single “best” model, but different model configurations are obtained depending on the SST data, undercount correction, and penalty criterion. These results suggest that these fractions are controlled by local (related to the NAO) and remote (SOI and tropical mean SST) effects.
Tropical cyclones (TCs) are a hazard to life and property (1, 2), as was tragically apparent following Super Typhoon Haiyan's landfall in the Philippines in 2013 and Hurricane/extratropical system Sandy's landfall in the New York tri-state area in 2012. Yet TCs also provide vital water, sometimes relieving drought (3). Predictions of the path and intensity of individual TCs are usually sufficiently good several days in advance that action can be taken. In contrast, predictions of seasonal TC activity months in advance must still be made more regionally relevant to produce information that can be acted on, for example, to improve storm preparedness.
A new high-resolution Geophysical Fluid Dynamics Laboratory (GFDL) coupled model (HiFLOR) has been developed and used to investigate potential skill in simulation and prediction of tropical cyclone (TC) activity. HiFLOR comprises of high-resolution (~25-km mesh) atmosphere and land components and a more moderate-resolution (~100-km mesh) sea ice and ocean components. HiFLOR was developed from the Forecast Oriented Low Resolution Ocean model (FLOR) by decreasing the horizontal grid spacing of the atmospheric component from 50-km to 25-km, while leaving most of the sub-gridscale physical parameterizations unchanged. Compared with FLOR, HiFLOR yields a more realistic simulation of the structure, global distribution, and seasonal and interannual variations of TCs, and a comparable simulation of storm-induced cold wakes and TC-genesis modulation induced by the Madden Julian Oscillation (MJO). Moreover, HiFLOR is able to simulate and predict extremely intense TCs (categories 4 and 5) and their interannual variations, which represents the first time a global coupled model has been able to simulate such extremely intense TCs in a multi-century simulation, sea surface temperature restoring simulations, and retrospective seasonal predictions.
Hazelton, Andrew T., Robert Rogers, and R E Hart, August 2017: Analyzing Simulated Convective Bursts in Two Atlantic Hurricanes. Part I: Burst Formation and Development. Monthly Weather Review, 145(8), DOI:10.1175/MWR-D-16-0267.1. Abstract
Understanding the structure and evolution of the tropical cyclone (TC) inner core remains an elusive challenge in tropical meteorology, especially the role of transient asymmetric features such as localized strong updrafts known as convective bursts (CBs). This study investigates the formation of CBs and their role in TC structure and evolution using high-resolution simulations of two Atlantic hurricanes (Dean in 2007 and Bill in 2009) with the Weather Research and Forecasting (WRF) Model.
Several different aspects of the dynamics and thermodynamics of the TC inner-core region are investigated with respect to their influence on TC convective burst development. Composites with CBs show stronger radial inflow in the lowest 2 km, and stronger radial outflow from the eye to the eyewall around z = 2–4 km, than composites without CBs. Asymmetric vorticity associated with eyewall mesovortices appears to be a major factor in some of the radial flow anomalies that lead to CB development. The anomalous outflow from these mesovortices, along with outflow from supergradient parcels above the boundary layer, favors low-level convergence and also appears to mix high-θe air from the eye into the eyewall. Analyses of individual CBs and parcel trajectories show that parcels are pulled into the eye and briefly mix with the eye air. The parcels then rapidly move outward into the eyewall, and quickly ascend in CBs, in some cases with vertical velocities of over 20 m s−1. These results support the importance of horizontal asymmetries in forcing extreme asymmetric vertical velocity in tropical cyclones.
Hazelton, Andrew T., R E Hart, and Robert Rogers, August 2017: Analyzing Simulated Convective Bursts in Two Atlantic Hurricanes. Part II: Intensity Change due to Bursts. Monthly Weather Review, 145(8), DOI:10.1175/MWR-D-16-0268.1. Abstract
This paper investigates convective burst (CB) evolution in Weather Research and Forecasting (WRF) Model simulations of two tropical cyclones (TCs), focusing on the relationship between CBs and TC intensity change. Analysis of intensity change in the simulations shows that there are more CBs inside the radius of maximum winds (RMW) during times when the TCs are about to intensify, while weakening/steady times are associated with more CBs outside the RMW, consistent with past observational and theoretical studies. The vertical mass flux distributions show greater vertical mass flux at upper levels both from weaker updrafts and CBs for intensifying cases. The TC simulations are further dissected by past intensity change, and times of sustained intensification have more CBs than times when the TC has been weakening but then intensifies. This result suggests that CB development may not always be predictive of intensification, but rather may occur as a result of ongoing intensification and contribute to ongoing intensification. Abrupt short-term intensification is found to be associated with an even higher density of CBs inside the RMW than is slower intensification. Lag correlations between CBs and intensity reveal a broad peak, with the CBs leading pressure falls by 0–3 h. These relationships are further confirmed by analysis of individual simulation periods, although the relationship can vary depending on environmental conditions and the previous evolution of the TC. These results show that increased convection due to both weak updrafts and CBs inside the RMW is favorable for sustained TC intensification and show many details of the typical short-term response of the TC core to CBs.
Moon, Il-Ju, Thomas R Knutson, Hye-Ji Kim, Alexander V Babanin, and Jin-Yong Jeong, November 2022: Why do eastern North Pacific hurricanes intensify more and faster than their western-counterpart typhoons with less ocean energy?Bulletin of the American Meteorological Society, 103(11), DOI:10.1175/BAMS-D-21-0131.1E2604-E2627. Abstract
Tropical cyclones operate as heat engines, deriving energy from the thermodynamic disequilibrium between ocean surfaces and atmosphere. Available energy for the cyclones comes primarily from upper-ocean heat content. Here, we show that eastern North Pacific hurricanes reach a given intensity 15% faster on average than western North Pacific typhoons despite having half the available ocean heat content. Eastern North Pacific hurricanes also intensify on average 16% more with a given ocean energy (i.e., air–sea enthalpy flux) than western North Pacific typhoons. As efficient intensifiers, eastern Pacific hurricanes remain small during their intensification period, tend to stay at lower latitudes, and are affected by relatively lower vertical wind shear, a colder troposphere, and a drier boundary layer. Despite a shallower warm upper-ocean layer in the eastern North Pacific, average hurricane-induced sea surface cooling there is only slightly larger than in the western North Pacific due to the opposing influences of stronger density stratification, smaller size, and related wave-interaction effects. In contrast, western North Pacific typhoons encounter a more favorable oceanic environment for development, but several factors cause typhoons to greatly increase their size during intensification, resulting in a slow and inefficient intensification process. These findings on tropical cyclones’ basin-dependent characteristics contribute toward a better understanding of TC intensification.
Kurihara, Yoshio, R Ross, and Morris A Bender, 1991: Toward improvement of the dynamical prediction of tropical cyclones: A hurricane model initialization scheme In 19th Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Geophysical Union, 326-327.
Frappier, A, Thomas R Knutson, K-B Liu, and Kerry A Emanuel, 2007: Perspective: coordinating paleoclimate research on tropical cyclones with hurricane-climate theory and modelling. Tellus A, 59(4), 529-537. Abstract PDF
Extending the meteorological record back in time can offer critical data for assessing tropical cyclone-climate links. While paleotempestology, the study of ancient storms, can provide a more realistic view of past ‘worst case scenarios’, future environmental conditions may have no analogues in the paleoclimate record. The primary value in paleotempestology proxy records arises from their ability to quantify climate–tropical cyclone interactions by sampling tropical cyclone activity during pre-historic periods with a wider range of different climates. New paleotempestology proxies are just beginning to be applied, encouraging new collaboration between the paleo and tropical cyclone dynamics communities. The aim of this paper is to point out some paths toward closer coordination by outlining target needs of the tropical cyclone theory and modelling community and potential contributions of the paleotempestology community. We review recent advances in paleotempestology, summarize the range of types and quality of paleodata generation, and identify future research opportunities for paleotempestology, tropical cyclone dynamics and climate change impacts and attribution communities.
Knutson, Thomas R., Robert E Tuleya, W Shen, and Isaac Ginis, 2001: Impact of CO2-induced warming on hurricane intensities simulated in a hurricane model with ocean coupling. Journal of Climate, 14(11), 2458-2468. Abstract PDF
This study explores how a carbon dioxide (CO2) warming-induced enhancement of hurricane intensity could be altered by the inclusion of hurricane-ocean coupling. Simulations are performed using a coupled version of the Geophysical Fluid Dynamics Laboratory hurricane prediction system in an idealized setting with highly simplified background flow fields. The large-scale atmospheric boundary conditions for these high-resolution experiments (atmospheric temperature and moisture profiles and moisture profiles and SSTs) are derived from control and high-CO2 climatologies obtained from a low-resolution (R30) global coupled ocean-atmosphere climate model. The high-CO2 conditions are obtained from years 71-120 of a transient +1% yr -1 CO2-increase experiment with the global model. The CO2-induced SST changes from the global climate model range from +2.2° to +2.7°C in the six tropical storm basins studied. In the storm simulations, ocean coupling significantly reduces the intensity of simulated tropical cyclones, in accord with previous studies. However, the net impact of ocean coupling on the simulated CO2 warming-induced intensification of tropical cyclones is relatively minor. For both coupled and uncoupled simulations, the percentage increase in maximum surface wind speeds averages about 5%-6% over the six basins and varies from about 3% to 10% across the different basins. Both coupled and uncoupled simulations also show strong increases of near-storm precipitation under high-CO2 climate conditions, relative to control (present day) conditions.
Knutson, Thomas R., and Robert E Tuleya, 1999: Increased hurricane intensities with CO2 -induced global warming as simulated using the GFDL hurricane prediction system. Climate Dynamics, 15, 503-519. Abstract PDF
The impact of CO2 -induced global warming on the intensities of strong hurricanes is investigated using the GFDL regional high-resolution hurricane prediction system. The large-scale initial conditions and boundary conditions for the regional model experiments, including SSTs, are derived from control and transient CO2 increase experiments with the GFDL R30-resolution global coupled climate model. In a case study approach, 51 northwest Pacific storm cases derived from the global model under present-day climate conditions are simulated with the regional model, along with 51 storm cases for high CO2 conditions. For each case, the regional model is integrated forward for five days without ocean coupling. The high CO2 storms, with SSTs warmer by about 2.2° C on average and higher environmental convective available potential energy (CAPE), are more intense than the control storms by about 3-7 m/s (5%-11%) for surface wind speed and 7 to 24 hPa for central surface pressure. The simulated intensity increases are statistically significant according to most of the statistical tests conducted and are robust to changes in storm initialization methods. Near-storm precipitation is 28% greater in the high CO2 sample. In terms of storm tracks, the high CO2 sample is quite similar to the control. The mean radius of hurricane force winds is 2 to 3% greater for the composite high CO2 storm than for the control,and the high CO2 storms penetrate slightly higher into the upper troposphere. More idealized experiments were also performed in which an initial storm disturbance was embedded in highly simplified flow fields using time mean temperature and moisture conditions from the global climate model. These idealized experiments support the case study results and suggest that, in terms of thermodynamic influences, the results for the NW Pacific basin are qualitatively applicable to other tropical storm basins.
Wu, C-C, and Kerry A Emanuel, 1995: Potential vorticity diagnostics of hurricane movement. Part I: A case study of Hurricane Bob (1991). Monthly Weather Review, 123(1), 69-92. Abstract
Potential vorticity (PV) diagnostics are applied to evaluate the control by the large-scale environment of hurricane movement and, more importantly, to assess the storm's influence on its own track. As a first application of these diagnostics, an observational case study of Hurricane Bob (1991) is presented using the twice-daily National Meteorological Center Northern Hemisphere final analyses gridded datasets. Defining the seasonal climatology as the mean reference state, piecewise potential vorticity inversions are performed under the non-linear balance condition. This allows one to determine the balanced flows associated with any individual perturbation of PV. By examining the balanced flows at the central position of the hurricane, one can identify the influence of each PV perturbation on hurricane movement. The hurricane advection flow is also defined as the balanced flow at the storm center associated with the whole PV distribution, excluding the positive PV anomaly of the hurricane itself.
The results from the observational study of Bob show that the hurricane advection flow is a good approximation t the real storm motion. The results also show that the balanced flows associated with the climatological mean PV and perturbation PV distribution in both the lower and upper troposphere are both important in contributing to Bob's movement. However, it is difficult to separate PV anomalies directly or indirectly attributable to the storm from ambient PV anomalies. Results from other cases will be presented in a companion paper.
Wu, C-C, and Kerry A Emanuel, 1995: Potential vorticity diagnostics of hurricane movement. Part II: Tropical Storm Ana (1991) and Hurricane Andrew (1992). Monthly Weather Review, 123(1), 93-109. Abstract
The validity of balance dynamics in the Tropics allows an exploration of the dynamics of hurricanes using the potential vorticity (PV) framework. Part I demonstrated the use of PV diagnostics in understanding the hurricane steering flow and also the interaction between the cyclone and its environment. To obtain a broader understanding of this PV methodology, two other observational case studies are performed (Tropical Storm Ana of 1991 and Hurricane Andrew of 1992) emphasizing the same methods of analysis.
The results are consistent with a previous finding that the hurricane advection flow, defined by inverting the entire PV distribution excluding the storm's own positive anomaly, is a good approximation to real cyclone movement, even though the original data cannot capture the actual hurricane strength. This study confirms that upper-level PV anomalies can play an important role in the motion of the storm. But their quantitative effect on the cyclone's motion depends strongly on the relative location of the vortex and the upper-air PV features. Due to the limitations of the data, the Beta effect or the mechanism proposed by Wu and Emanuel was not able to be supported or disproved.
Shen, B-W, R Atlas, O Reale, Shian-Jiann Lin, J-D Chern, J Chang, C Henze, and J-L Li, 2006: Hurricane forecasts with a global mesoscale-resolving model: Preliminary results with Hurricane Katrina (2005). Geophysical Research Letters, 33, L13813, DOI:10.1029/2006GL026143. Abstract
It is known that General Circulation Models (GCMs) have insufficient resolution to accurately simulate hurricane near-eye structure and intensity. The increasing capabilities of high-end computers have changed this. The mesoscale-resolving finite-volume GCM (fvGCM) has been experimentally deployed on the NASA Columbia supercomputer, and its performance is evaluated in this study by choosing hurricane Katrina as an example. In late August 2005, Katrina underwent two stages of rapid intensification, and became the sixth most intense hurricane in the Atlantic. Six 5-day simulations of Katrina at both 0.25° and 0.125° show comparable track forecasts but the 0.125° runs provide much better intensity forecasts, producing the center pressure with errors of only ±12 hPa. In the runs examined in this study, the 0.125° simulates better near-eye wind distributions and a more realistic average intensification rate. To contribute to the ongoing research on the effects of disabling convection parameterization (CP), we present promising results by comparing 0.125° runs with disabled CPs against runs with enabled CPs.
Knutson, Thomas R., Robert E Tuleya, and Yoshio Kurihara, 1997: Exploring the sensitivity of hurricane intensity to CO2-induced global warming using the GFDL Hurricane Prediction System In 22nd Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 587-588.
Kurihara, Yoshio, Morris A Bender, and Robert E Tuleya, 1997: For hurricane intensity forecast: Formulation of a new initialization method for the GFDL Hurricane Prediction Model In 22nd Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 543-544.
Kurihara, Yoshio, Morris A Bender, and Robert E Tuleya, 1995: Performance evaluation of the GFDL Hurricane Prediction System in the 1994 hurricane season In 21st Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 41-43.
Persing, J, M T Montgomery, and Robert E Tuleya, 2002: Environmental interactions in the GFDL Hurricane Model for Hurricane Opal. Monthly Weather Review, 130(2), 298-317. Abstract PDF
Hurricane Opal (1995) crossed the Gulf of Mexico rapidly intensifying to a 130-kt storm, then fortunately weakening before landfall on the Florida panhandle. This intensification was underforecast by the National Hurricane Center. Forecast fields from the 1997 version of the Geophysical Fluid Dynamics Laboratory Hurricane Prediction System (GFDL model) for Hurricane Opal are used to diagnose the rapid intensification of the tropical cyclone. While falling short of the realized peak intensity, the simulation did capture the phase of intensification. This study presents the first step toward diagnosing the mechanisms for intensification within a moderate resolution (~15 km) hydrostatic model and testing the extant hypotheses in the literature.
Using a mean tangential wind budget, and the Eliassen balanced vortex model, positive eddy vorticity fluxes aloft are identified in the vicinity (~600 km) of Opal, but are not found to aid intensification. A detailed examination of each of the terms of the budget (mean and eddy vorticity flux, mean and eddy vertical advection, and "friction") shows for the most rapidly intensifying episodes a greater forcing for mean tangential winds near the center of the storm, particularly from the mean vertical advection and mean vorticity flux terms. Variations in these mean terms can be primarily attributed to variations in the heating rate. Upper-level divergence exhibits significant vertical structure, such that single-level or layer-average analysis techniques do not capture the divergence signature aloft. Far from the storm (400 km), divergence features near 200 mb are significantly influenced by convective events over land that are, perhaps, only indirectly influenced by the hurricane.
While there is a trough interaction simulated within the model, we suggest that the hurricane develops strongly without an important interaction with the trough. A synthetic removal of specific potential vorticity features attributed to the trough is proposed to test this hypothesis. Imposed shear is proposed to weaken the storm at later times, which is at odds with other recent "nontrough" theories for the behavior of Opal.
Bender, Morris A., and Isaac Ginis, 2000: Real-case simulations of hurricane-ocean interaction using a high-resolution coupled model: effects on hurricane intensity. Monthly Weather Review, 128(4), 917-946. Abstract PDF
In order to investigate the effect of tropical cyclone-ocean interaction on the intensity of observed hurricanes, the GFDL movable triply nested mesh hurricane model was coupled with a high-resolution version of the Princeton Ocean Model. The ocean model had 1/6º uniform resolution, which matched the horizontal resolution of the hurricane model in its innermost grid. Experiments were run with and without inclusion of the coupling for two cases of Hurricane Opal (1995) and one case of Hurricane Gilbert (1988) in the Gulf of Mexico and two cases each of Hurricanes Felix (1995) and Fran (1996) in the western Atlantic. The results confirmed the conclusions sugggested by the earlier idealized studies that the cooling of the sea surface induced by the tropical cyclone will have a significant impact on the intensity of observed storms, particularly for slow moving storms where the SST decrease is greater. In each of the seven forecasts, the ocean coupling led to substantial improvements in the prediction of storm intensity measured by the storm's minimum sea level pressure.
Without the effect of coupling the GFDL model incorrectly forecasted 25-hPa deepening of Gilbert as it moved across the Gulf of Mexico. With the coupling included, the model storm deepened only 10 hPa, which was much closer to the observed amount of 4 hPa. Similarly, during the period that Opal moved very slowly in the southern Gulf of Mexico, the coupled model produced a large SST decrease northwest of the Yucatan and slow deepening consistent with the observations. The uncoupled model using the initial NCEP SST's predicted rapid deepening of 58 hPa during the same period.
Improved intensity prediction was achieved both for Hurricanes Felix and Fran in the western Atlantic. For the case of Hurricane Fran, the coarse resolution of the NCEP-SST analysis could not resolve Hurricane Edouard's wake, which was produced when Edouard moved in nearly an identical path to Fran four days earlier. As a result, the operational GFDL forecast using the operational SST's and without coupling incorrectly forecasted 40-hPa deepening while Fran remained at nearly constant intensity as it crossed the wake. When the coupled model was run with Edouard's cold wake generated by imposing hurricane wind forcing during the ocean initialization, the intensity prediction was significantly improved. The model also correctly predicted the rapid deepening that occurred as Fran began to move away from the cold wake. These results suggest the importance of an accurate initial SST analysis as well as the inclusion of the ocean coupling, for accurate hurricane intensity prediction with a dynamical model.
Recently, the GFDL hurricane-ocean coupled model used in these case studies was run on 163 forecasts during the 1995-98 seasons. Improved intensity forecasts were again achieved with the mean absolute error in the forecast of central pressure reduced by about 26% compared to the operational GFDL model. During the 1998 season, when the system was run in near-real time, the coupled model improved the intensity forecasts for all storms with central pressure higher than 940 hPa although the most significant improvement (~60%) occurred in the intensity range of 960-970 hPa. These much larger sample sets confirmed the conclusion from the case studies, that the hurricane-ocean interaction is an important physical mechanism in the intensity of observed tropical cyclones.
Wu, C-C, and Yoshio Kurihara, 1996: A numerical study of the feedback mechanisms of hurricane-environment interaction on hurricane movement from the potential vorticity perspective. Journal of the Atmospheric Sciences, 53(15), 2264-2282. Abstract PDF
The interaction between a hurricane and its environment is studied by analyzing the generation and influence of potential vorticity (PV) from the Geophysical Fluid Dynamics Laboratory hurricane model analysis system. Two sets of numerical experiments are performed: one with and the other without a bogused hurricane vortex in the initial time, for cases of Hurricanes Bob (1991), Gilbert (1988), and Andrew (1992).
The PV budget analysis of Bob shows that the condensational heating within the vortex redistributes the PV, causing a PV sink in the upper part of the vortex and a PV source in the lower part. This tendency is compensated for largely, but not entirely, by the upward transport of high-PV air from the lower levels to the upper levels. The net effect contributes to the increase of the negative upper-level PV anomaly during the vortex intensification period. This result indicates that the diabatic heating effect plays a crucial role in the evolution of the PV field in hurricanes. It also suggests the importance of accurate representation of the heating profile in hurricane models.
It is shown that the negative upper-level PV anomaly is spread out by the upper-level outflow and the large-scale background flow. The impact of the spread of the negative upper PV anomaly to the storm is quantitatively evaluated by computing the nonlinear balanced flow associated with the PV perturbation. Notable contribution to the steering of the storm from the upper-level PV anomaly is found. The result supports the theory advanced by Wu and Emanuel concerning the effect of the upper negative PV anomaly on hurricane motion. This study also indicates the need of enhanced observation and accurate analysis and prediction in the upper troposphere in order to improve hurricane track forecasting.
The Geophysical Fluid Dynamics Laboratory (GFDL) Hurricane Prediction System was adopted by the U.S. National Weather Service as an operational hurricane prediction model in the 1995 hurricane season. The framework of the prediction model is described with emphasis on its unique features. The model uses a multiply nested movable mesh system to depict the interior structure of tropical cyclones. For cumulus parameterization, a soft moist convective adjustment scheme is used. The model initial condition is defined through a method of vortex replacement. It involves generation of a realistic hurricane vortex by a scheme of controlled spinup. Time integration of the model is carried out by a two-step iterative method that has a characteristic of frequency-selective damping.
The outline of the prediction system is presented and the system performance in the 1995 hurricane season is briefly summarized. Both in the Atlantic and the eastern Pacific, the average track forecast errors are substantially reduced by the GFDL model, compared with forecasts by other models, particularly for the forecast periods beyond 36 h. Forecasts of Hurricane Luis and Hurricane Marilyn were especially skillful. A forecast bias is noticed in cases of Hurricane Opal and other storms in the Gulf of Mexico. The importance of accurate initial conditions, in both the environmental flow and the storm structure, is argued.
Zhang, Zhan, and Mingjing Tong, et al., July 2020: The Impact of Stochastic Physics-Based Hybrid GSI/EnKF Data Assimilation on Hurricane Forecasts Using EMC Operational Hurricane Modeling System. Atmosphere, 11(8), 801, DOI:10.3390/atmos11080801. Abstract
The National Oceanic and Atmospheric Administration’s (NOAA) cloud-permitting high-resolution operational Hurricane Weather and Research Forecasting (HWRF) model includes the sophisticated hybrid grid-point statistical interpolation (GSI) and Ensemble Kalman Filter (EnKF) data assimilation (DA) system, which allows assimilating high-resolution aircraft observations in tropical cyclone (TC) inner core regions. In the operational HWRF DA system, the flow-dependent background error covariance matrix is calculated from the HWRF self-cycled 40-member ensemble. This DA system has proved to provide improved initial TC structure and therefore improved TC track and intensity forecasts. However, the uncertainties from the model physics are not taken into account in the FY2017 version of the HWRF DA system. In order to further improve the HWRF DA system, the stochastic physics perturbations are introduced in the HWRF DA, including the cumulus convection scheme, the planetary boundary layer (PBL) scheme, and model surface physics (drag coefficient), for HWRF-based ensembles. This study shows that both TC initial conditions and TC track and intensity forecast skills are improved by adding stochastic model physics in the HWRF self-cycled DA system. It was found that the improvements in the TC initial conditions and forecasts are the results of ensemble spread increases which realistically represent the model background error covariance matrix in HWRF DA. For all 2016 Atlantic storms, the TC track and intensity forecast skills are improved by about ~3% and 6%, respectively, compared to the control experiment. The case study shows that the stochastic physics in HWRF DA is especially helpful for those TCs that have inner-core high-resolution aircraft observations, such as tail Doppler radar (TDR) data.
Hazelton, Andrew T., Zhan Zhang, Bin Liu, Jili Dong, Ghassan Alaka, Weiguo Wang, Timothy Marchok, Avichal Mehra, Sundararaman Gopalakrishnan, Xuejin Zhang, Morris A Bender, Vijay Tallapragada, and Frank D Marks, April 2021: 2019 Atlantic hurricane forecasts from the global-nested Hurricane Analysis and Forecast System: Composite statistics and key events. Weather and Forecasting, 36(2), DOI:10.1175/WAF-D-20-0044.1519-538. Abstract
NOAA’s Hurricane Analysis and Forecast System (HAFS) is an evolving FV3-based hurricane modeling system that is expected to replace the operational hurricane models at the National Weather Service. Supported by the Hurricane Forecast Improvement Program (HFIP), global-nested and regional versions of HAFS were run in real time in 2019 to create the first baseline for the HAFS advancement. In this study, forecasts from the global-nested configuration of HAFS (HAFS-globalnest) are evaluated and compared with other operational and experimental models. The forecasts by HAFS-globalnest covered the period from July through October during the 2019 hurricane season. Tropical cyclone (TC) track, intensity, and structure forecast verifications are examined. HAFS-globalnest showed track skill superior to several operational hurricane models and comparable intensity and structure skill, although the skill in predicting rapid intensification was slightly inferior to the operational model skill. HAFS-globalnest correctly predicted that Hurricane Dorian would slow and turn north in the Bahamas and also correctly predicted structural features in other TCs such as a sting jet in Hurricane Humberto during extratropical transition. Humberto was also a case where HAFS-globalnest had better track forecasts than a regional version of HAFS (HAFS-SAR) due to a better representation of the large-scale flow. These examples and others are examined through comparisons with airborne tail Doppler radar from the NOAA WP-3D to provide a more detailed evaluation of TC structure prediction. The results from this real-time experiment motivate several future model improvements, and highlight the promise of HAFS-globalnest for improved TC prediction.
Hazelton, Andrew T., Kun Gao, Morris A Bender, Levi Cowan, Ghassan J Alaka Jr, Alex Kaltenbaugh, Lew Gramer, Xuejin Zhang, Lucas Harris, Timothy Marchok, Matthew J Morin, Avichal Mehra, Zhan Zhang, Bin Liu, and Frank D Marks, January 2022: Performance of 2020 real-time Atlantic hurricane forecasts from high-resolution global-nested hurricane models: HAFS-globalnest and GFDL T-SHiELD. Weather and Forecasting, 37(1), DOI:10.1175/WAF-D-21-0102.1143-161. Abstract
The global-nested Hurricane Analysis and Forecast System (HAFS-globalnest) is one piece of NOAA’s Unified Forecast System (UFS) application for hurricanes. In this study, results are analyzed from 2020 real-time forecasts by HAFS-globalnest and a similar global-nested model, the Tropical Atlantic version of GFDL’s System for High‐resolution prediction on Earth‐to‐Local Domains (T-SHiELD). HAFS-globalnest produced the highest track forecast skill compared to several operational and experimental models, while T-SHiELD showed promising track skills as well. The intensity forecasts from HAFS-globalnest generally had a positive bias at longer lead times primarily due to the lack of ocean coupling, while T-SHiELD had a much smaller intensity bias particularly at longer forecast lead times. With the introduction of a modified planetary boundary layer scheme and an increased number of vertical levels, particularly in the boundary layer, HAFS forecasts of storm size had a smaller positive bias than occurred in the 2019 version of HAFS-globalnest. Despite track forecasts that were comparable to the operational GFS and HWRF, both HAFS-globalnest and T-SHiELD suffered from a persistent right-of-track bias in several cases at the 4–5-day forecast lead times. The reasons for this bias were related to the strength of the subtropical ridge over the western North Atlantic and are continuing to be investigated and diagnosed. A few key case studies from this very active hurricane season, including Hurricanes Laura and Delta, were examined.
The GFDL hurricane modelling system, initiated in the 1970s, has progressed from a research tool to an operational system over four decades. This system is still in use today in research and operations, and its evolution will be briefly described. This study used an idealized version of the 2014 GFDL model to test its sensitivity across a wide range of three environmental factors that are often identified as key factors in tropical cyclone (TC) evolution: SST, atmospheric stability (upper air thermal anomalies), and vertical wind shear (westerly through easterly). A wide range of minimum central pressure intensities resulted (905 to 980hPa). The results confirm that a scenario (e.g., global warming) in which the upper troposphere warms relative to the surface will have less TC intensification than one with a uniform warming with height. TC rainfall is also investigated for the SST-stability parameter space. Rainfall increases for combinations of SST increase and increasing stability similar to global warming scenarios, consistent with climate change TC downscaling studies with the GFDL model. The forecast system’s sensitivity to vertical shear was also investigated. The idealized model simulations showed weak disturbances dissipating under strong easterly and westerly shear of 10 m s-1. A small bias for greater intensity under easterly sheared versus westerly sheared environments was found at lower values of SST. The impact of vertical shear on intensity was different when a strong vortex was used in the simulations. In this case none of the initial disturbances weakened, and most intensified to some extent.
Wang, H, M Richardson, R John Wilson, A Ingersoll, A D Toigo, and R W Zurek, 2003: Cyclones, tides, and the origin of a cross-equatorial dust storm on Mars. Geophysical Research Letters, 30(9), 1488, DOI:10.1029/2002GL016828. Abstract
We investigate the triggering mechanism of a cross-equatorial dust storm observed by Mars Global Surveyor in 1999. This storm, which had a significant impact on global mean temperatures, was seen in visible and infrared data to commence with the transport of linear dust fronts from the northern high latitudes into the southern tropics. However, other similar transport events observed in northern fall and winter did not lead to large dust storms. Based on off-line Lagrangian particle transport analysis using a high resolution Mars general circulation model, we propose a simple explanation for the diurnal, seasonal and interannual variability of this type of frontal activity, and of the resulting dust storms, that highlights the cooperative interaction between northern hemisphere fronts associated with low pressure cyclones and tidally-modified return branch of the Hadley circulation.
Kurihara, Yoshio, Robert E Tuleya, Morris A Bender, and R Ross, 1993: Advanced modeling of tropical cyclones In Tropical Cyclone Disasters, Proceedings of ICSU/WMO International Symposium, October 12-16, 1992, Beijing, China, Peking University Press, 190-201. Abstract
Advanced tropical cyclone models of sufficiently fine resolution are capable of representing important internal structure of the vortex. In the model, a vortex should interact with ocean and land in a realistic manner. Interaction with the ocean can significantly moderate the storm intensity. Inclusion of the heat budget of the soil layer retards the storm intensity over land. How to improve the treatment of deep convection is an issue which is wide open for future study. Specification of a realistic, yet model-adapted vortex in the initial condition of the model is essential for improvement of tropical cyclone track and intensity prediction.
Broccoli, Anthony J., and Syukuro Manabe, 1991: Will global warming increase the frequency of tropical cyclones? In Fifth Conference on Climate Variations, Boston, MA, American Meteorological Society, 46.
Bender, Morris A., 1995: Numerical study of the asymmetric structure in the interior of tropical cyclones In 21st Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 600-602.
Broccoli, Anthony J., Syukuro Manabe, J F B Mitchell, and L Bengtsson, 1995: Comments on "Global climate change and tropical cyclones": Part II. Bulletin of the American Meteorological Society, 76(11), 2243-2245. PDF
Philander, S G., 1995: Comments on "Global Climate Change and Tropical Cyclones" by J. Lighthill, et al. Bulletin of the American Meteorological Society, 76(3), 380.
Kurihara, Yoshio, and Morris A Bender, 1989: On the structure of moving tropical cyclones In 18th Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 188-189.
Orlanski, Isidoro, M Marino, C Menendez, and J J Katzfey, 1989: The role of cyclones in the daily variability of Antarctic ozone In Third International Conference on Southern Hemisphere Meteorology and Oceanography, Boston, MA, American Meteorological Society, 416-420.
Bender, Morris A., and Yoshio Kurihara, 1987: A numerical study of the effect of the mountainous terrain of Japan on tropical cyclones In Short- and Medium-Range Numerical Weather Prediction, Collection of Papers Presented At WMO/IUGG NWP Symposium Tokyo, Japan, Aug. 4-8, 1986, Geneva, Switz, World Meteorological Organization, 651-663. Abstract PDF
A triply-nested, movable mesh model was used to study the effects of mountainous terrain on the landfall of tropical cyclones onto the islands of Japan. The integration domain spanned 43° latitude and 47° longitude with finest resolution of 1/6°. Numerical experiments were separately performed for three cases. In each experiment a storm was embedded onto a stationary Haurwitz type wave at the initial time, and moved in a north-northeast direction at about 10 to 12 m s-1. In the first experiment, the tropical cyclone struck the southwest Izu Peninsula in eastern Japan. The second and third respectively made landfall on the Kii Peninsula in the central part of Japan and on the island of Kyushu in western Japan. In order to isolate some of the effects on the storm system resulting from its interaction with the mountainous terrain, these simulations were compared with supplemental experiments performed with a flat land condition. In all three cases it was found that the presence of the mountainous terrain greatly enhanced the storm decay after landfall. As the storm approached Tokyo Bay rapid weakening occurred as dry air from the mountain region to the west of Tokyo was advected into the eye and eyewall region of the storm. Upon leaving eastern Japan and again moving over open water, the storm never underwent reintensification. In the case of the storm leaving western Japan, reintensification over the Sea of Japan occurred very slowly as compared with the experiment run with a flat land distribution. Apparently, the above behavior was related to the structural change which occurred to the storm system during the passage over the mountainous islands. The precipitation pattern was also greatly affected by the presence of the mountainous terrain. As the storm made landfall over central Japan, the area of heaviest rainfall shifted to the right of the storm track, where strong upslope winds developed. This storm eventually travelled over the high mountains of east-central Japan and rapidly decayed by the end of the experiment.
Although performed for an idealized experimental design, these experiments reveal some of the important effects the mountainous terrain may have on the behavior of tropical cyclones making landfall on Japan. Understanding these effects should prove useful in forecasting more accurately the behavior of the storms.
A triply-nested. movable mesh model was used to study the behavior of tropical cyclones encountering island mountain ranges. The integration domain consisted of a 37° wide and 45° long channel, with an innermost mesh resolution of 1/6°. The storms used for this study were embedded in easterly flows of ~ 5 and ~ 10 m s-1 initially. Realistic distributions of island topography at 1/6° resolution were inserted into the model domain for the region of the Caribbean, including the islands of Cuba, Hispaniola, and Puerto Rico; the island of Taiwan; and the region of Luzon in the northern Philippines.
It was found that the islands affected the basic flow as well as the wind field directly associated with the storm system. The combination of these effects caused changes in the track and translational speed of the storm. In particular, in the case of the 5 m s-1 easterly flow, the storm accelerated and veered to the north well before reaching Taiwan. For the other island distributions, the northward deflection of the track and the increase of translational speed occurred near and over the islands. After landfall, the surface pressure underwent rapid filling. As the tropical cyclone passed over Hispaniola, the surface low continued to move along with the upper level vortex as it transversed the mountain range, while over Luzon it became obscure before reforming on the lee side slope of the mountain. In case of Taiwan and the 10 m s-1 easterly zonal flow, secondary surface lows developed behind the mountain range. The upper level vortex in this case became detached from the original surface low and eventually coupled with a secondary one.
The intensity changes of the storm near and over the islands were strongly related to the latent energy supply and the vertical coherence of the storm system. Advection of dry air from near or above the mountain tops into the storm area caused significant weakening of all the storm moving with the weaker easterly flow. Storms leaving Hispaniola and moving over open sea quickly reintensified as their vertical structure remained coherent. On the other hand, storms leaving Luzon were disorganized and did not reintensify until several hours later when the vertical coherence of the systems was reestablished
Although these experiments were performed for an idealized experimental design and basic flow, many observed storms have exhibited similar behavior in track deviation and decay. This implies that the effect of detailed topography should be considered if an accurate forecast of the storm direction and behavior is to be made.
Kurihara, Yoshio, 1985: Numerical modeling of tropical cyclones. Advances in Geophysics, 28B, 255-281.
Oey, Leo, and H C Zhang, 2004: The generation of subsurface cyclones and jets through eddy-slope interaction. Continental Shelf Research, 24(18), DOI:10.1016/j.csr.2004.07.007. Abstract
A mechanism for the generation of subsurface cyclones and jets when a warm ring smashes onto a continental slope and shelf is proposed based on the results of a primitive-equation three-dimensional numerical model. The warm ring initially ‘sits’ over a slope with an adjoining shelf in a periodic channel, and its subsequent evolution is examined. The ‘inviscid’ response is cyclonic ‘peeling-off’ of the on-slope portion of the warm ring. The cyclone propagates away (to the left looking on-slope) from the warm ring, and is bottom-intensified as well as slope-trapped (cross-slope scale ≈ Rossby radius). The near-surface flow ‘leaks’ further onto the shelf while subsurface currents are blocked by the slope. The ‘viscous’ response consists of the formation of a bottom boundary layer (BBL) with a temporally and spatially dependent displacement thickness. The BBL ‘lifts’ the strong along-slope (leftward) current or jet (speed >0.5 m s-1) away from the bottom. The jet, coupled with weak stratification within the BBL and convergence due to downwelling across the slope, becomes supercritical. Super-inertial disturbance in the form of a hydraulic jump or front, with strong upwelling and downwelling cell, and the jet, propagate along the slope as well as off-slope and upward into the water column. The upward propagation is halted at z≈ztrap when mixing smoothes out the ‘jump’ to an along-slope scale λtrap that allows the ambient jet to bend the propagation path horizontal. At this ‘matured’ stage, ztrap≈−250 m, λtrap≈50 km, and the jet's cross-slope and vertical scales are ≈30 km and 50 m, respectively. An example that illustrates the process under a more realistic setting in the Gulf of Mexico when the Loop Current impinges upon the west Florida slope is given. The phenomenon may be relevant to the recent oil industry's measurements in the Gulf, which at times indicate jets at z≈−150 m through −400 m over the slope.
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A GFDL tropical cyclone model was applied to simulate storm landfall. The numerical model is a three-dimensional, primitive equation model and has 11 vertical levels with four in the planetary boundary layer. The horizontal grid spacing is variable with finest resolution being 20 km near the center. This model was used successfully in the past to investigate the development of tropical cyclones over the ocean.
In the present experiments, a simple situation is assumed where a mature tropical cyclone drifts onto flat land. In such a case, the landfall can be simulated by changing the position of the coastline in the computational domain rather than by moving the storm. As the coastline moves with a specified speed, the surface boundary conditions are altered at the shore from those for the ocean to those for the land by increasing the surface roughness length and also by suppressing the evaporation.
Despite the simplicity and idealization of the experiments, the cyclone's filling rates are quite reasonable and a decay sequence is obtained. Notable asymmetries in the wind, moisture and precipitation fields exist relative to the coastline at the time of landfall. Roughness-induced, quasi-steady convergence and divergence zones are observed where onshore and offshore winds encounter the coastline. Spiral bands propagate and exist over the land area. A comparison of the energy and angular momentum budgets between ocean and land surface boundary conditions indicates a simultaneous broadening and weakening of the storm system in the decay process. The latent energy release through condensational processes is initially augmented over land by greater moisture convergence in the planetary boundary layer which counteracts the lack of evaporation from the land surface.
Supplementary experiments indicate that the suppression of evaporation is the most important factor in the decay of a storm upon landfall. When the evaporation is suppressed, the storm eventually weakens whether the surface roughness is increased or not. An increased surface roughness, which causes increased inflow in the boundary layer, has little immediate negative impact on the storm intensity. Indeed, if the supply of latent energy is sufficient, a storm can deepen when encountering an increase in surface roughness. The decay rate in a later period well after landfall is influenced by the rate with which the water vapor of the storm system is depleted in the earlier period immediately after landfall.
Mechoso, C R., 1980: The atmospheric circulation around Antarctica: Linear stability and finite-amplitude interactions with migrating cyclones. Journal of the Atmospheric Sciences, 37(10), 2209-2233. Abstract
Available observations of the atmospheric circulation over the coast of Antarctica indicate the presence of a core of westerly winds in the upper troposphere. The linear stability of these westerlies is studied by using a semi-spectral numerical model with which the linearized, shallow, anelastic hydrostatic equations are integrated. The influence on the stability of the westerlies of both the slope and amplitude of the topography representative of East Antarctica is analyzed. The results obtained for several basic flows taken as idealizations of possible mean states indicate that although the topography exerts a somewhat stabilizing influence, the doubling times for the unstable perturbations are less than two days in all cases.
It is shown by using a three-level primitive equation model that the combined action of finite-amplitude baroclinic waves migrating from middle latitudes, the topography of Antarctica, and the meridional temperature gradients around the continent can generate westerlies with a jetlike structure over the topographic slopes. Furthermore, none of those mechanisms acting separately can generate such a jet.
The results suggest that the region around Antarctica, far from being a place where all baroclinic processes are damped out by topographic slopes, is baroclinically very active with a complicated energy cascade, and that the distinctive topographic characteristics of Antarctica are fundamental to the permanence of low temperatures in its overlying atmosphere.
By use of a triply nested, movable mesh model, several ideal simulations of tropical cyclone landfall were performed for a strong zonal flow of ~10 m s-1. The integration domain was a 37 X 45 degree channel with the innermost mesh having a 22 X 22 point resolution of 1/6 degree. General characteristics similar to observed landfalling tropical cyclones are obtained in the primary simulation experiment including an abrupt change in the low level (~68 m) winds at the coastline and a decay of the tropical cyclone as it moves inland. Additional interesting features subject to model and experimental limitations include: little noticeable track change of the model storm when compared to a control experiment with an ocean surface only; a possible temporary displacement of the center of the surface wind circulation from the surface pressure center at landfall; and a distinct decrease in kinetic energy generation and precipitation a few hours after landfall. The sensitivity to the specified land surface conditions was analyzed by performing additional experiments in which the land surface conditions including surface temperature, moisture, and distribution of surface roughness were changed. It was found that a reasonable change in some of these land conditions can make a considerable difference in behavior for a landfalling tropical cyclone. It was also shown that a small, less intense model storm fills less rapidly. This corresponds well with observations that many landfalling hurricanes decay to approximately the same asymptotic value one day after landfall.
Orlanski, Isidoro, 1986: Localized baroclinicity: a source for meso-a cyclones. Journal of the Atmospheric Sciences, 43(23), 2857-2885. Abstract
An investigation has been made using a two-dimensional model to solve the initial value problem describing the evolution of disturbances on a mean baroclinic state. Three main problems are considered:
the effect of static stability on meso-baroclinic waves in a periodic domain;
downstream instability in an open domain and the effect of surface sensible heat; and
the effect of moisture on these unstable waves.
It was found that a flow can be unstable to mesoscale baroclinic waves. The requirement for instability of wavelengths less than 1000 km is similar to that for the planetary quasi-geostrophic baroclinic waves.
The Rossby penetration height can be derived from the solution of the unstable waves as these unstable waves will only be sensitive to the baroclinicity of the atmosphere in a layer with a depth delta.
The characteristics of the finite-amplitude unstable waves suggest that the limiting amplitude for the baroclinic waves is achieved by an energy cascade to frontal scales.
Perhaps the most signicant finding of this study has been to demonstrate the importance of localized surface heating in producing the more intense development of short baroclinic waves. It was found that waves in the presence of surface heating grew twice as fast as those without. These waves, having a depth on the order of the boundary layer and horizontal scales of a few hundred kilometers, can organize convergence of surface moisture on these scales. With the addition of moisture, the shallow meso-baroclinic wave will explosively develop into a deep intense system.
Lighthill, J, G Holland, W Gray, Christopher Landsea, G Craig, J Evans, Yoshio Kurihara, and C Guard, 1994: Global climate change and tropical cyclones. Bulletin of the American Meteorological Society, 75(11), 2147-2157. Abstract
This paper offers an overview of the authors' studies during a specialized international symposium (Mexico, 22 November-1 December 1993) where they aimed at making an objective assessment of whether climate changes, consequent on an expected doubling of atmospheric CO2 in the next six or seven decades, are likely to increase significantly the frequency or intensity of tropical cyclones (TC). Out of three methodologies available for addressing the question they emply two, discarding the third for reasons set out in the appendix.
In the first methodology, the authors enumerate reasons why, in tropical oceans, the increase in sea surface temperature (SST) suggested by climate change models might by expected to affect either (i) TC frequency, because a well-established set of six conditions for TC formation include a condition that SST should exceed 26°C, or (ii) TC intensity, because this is indicated by thermodynamic analysis to depend critically on the temperature at which energy transfer to air near the sea surface takes place.
But careful study of both suggestions indicates that the expected effects of increased SST would be largely self-limiting (i) because the other five conditions strictly control how far the band of latitudes for TC formation can be further widened, and (ii) because intense winds at the sea surface may receive their energy input at a temperature significantly depressed by evaporation of spray, and possibly through sea surface cooling.
In the second methodology, the authors study available historical records that have very large year-to-year variability in TC statistics. They find practically no consistent statistical relationships with temperature anomalies; also, a thorough analysis of how the El Niño-Southern Oscillation cycle influences the frequency distribution of TCs shows any direct effects of local SST changes to be negligible.
The authors conclude that, even though the possibility of some minor indirect effects of global warming on TC frequency and intensity cannot be excluded, they must effectively be "swamped" by large natural variability.
Tuleya, Robert E., and Thomas R Knutson, 2002: Impact of climate change on tropical cyclones In Atmosphere-Ocean Interactions, Vol. 1, Southampton, UK, WIT Press, 293-312. Abstract
One of the possible impacts of global warming is on tropical cyclones, on their formation, track, intensity and decay rates. One of the consequences of global warming appears to be not only an increase in sea surface temperature, but more importantly a potential increase in the overall energy flux at the tropical ocean surface. Theoretical considerations imply that this increased surface disequilibrium may lead to more intense tropical storms. Three-dimensional numerical modeling is another approach to evaluating these potential consequences. Since global models are still rather limited in simulating mesoscale storm structure, this paper describes a regional modeling approach utilizing a multiple nested technique which has already been shown to be practical in operational forecasts. These 3-D model results confirm theoretical methods that indicate an increase of 3 to 10% in maximum wind speeds for a CO2 tropical SST warming of ~2.5°C. Perhaps more importantly, model results indicate a 20 to 30% increase in hurricane-related precipitation. Furthermore, the resulting increases in intensity and precipitation appear to be qualitatively insensitive to changes in convective parameterization. This paper emphasizes the impact of global warming on storm intensity and precipitation. The question of the possible impact on tropical storm frequency and track is still problematic.
Kurihara, Yoshio, Robert E Tuleya, and Morris A Bender, 1996: Simulation studies of tropical cyclones In Research Activities in Atmospheric and Oceanic Modelling, CAS/JSC Working Group on Numerical Experimentation, Report No. 23 WMO/TD No. 734, World Meteorological Organization, 5.17-5.18.
Knutson, Thomas R., and Robert E Tuleya, May 2008: Tropical cyclones and climate change: Revisiting recent studies at GFDL In Climate Extremes and Society, Diaz, H.F. and R.J. Murnane, Eds., New York, NY, Cambridge University Press, 120-144. Abstract
In this chapter, we revisit two recent studies performed at the Geophysical Fluid Dynamics Laboratory (GFDL), with a focus on issues relevant to tropical cyclones and climate change. The first study was a model-based assessment of twentieth-century regional surface temperature trends. The tropical Atlantic Main Development Region (MDR) for hurricane activity was found to have warmed by several tenths of a degree Celsius over the twentieth century. Coupled model historical simulations using current best estimates of radiative forcing suggest that the century-scale warming trend in the MDR may contain a significant contribution from anthropogenic forcing, including increases in atmospheric greenhouse gas concentrations. The results further suggest that the low-frequency variability in the MDR, apart from the trend, may contain substantial contributions from both radiative forcing (natural and anthropogenic) and internally generated climate variability. The second study used the GFDL huyrricane model, in an idealized setting, to simulate the impact of a pronounced CO2-induced warming on hurricane intensities and precipitation. A 1.75°C warming increases the intensities of hurricanes in the model by 5.8% in terms of surface wind speeds, 14% in terms of central pressure fall, or about one half category on the Saffir-Simpson Hurricane Scale. A revised storm-core accumulated (six-hour) rainfall measure shows a 21.6% increase under high CO2 conditions. Our simulated storm intensities are substantially less sensitive to sea surface temperature (SST) changes than recently reported historical observational trends are - a difference we are not able to completely reconcile at this time.
Knutson, Thomas R., Christopher Landsea, and Kerry A Emanuel, May 2010: Tropical cyclones and climate change: A review In Global Perspectives on Tropical Cyclones: From Science to Mitigation, Singapore, World Scientific Publishing Company, 243-284. Abstract
A review of the science on the relationship between climate change and tropical cyclones (TCs) is presented. Topics include changes in aspects of tropical climate that are relevant to TC activity; observed trends and low-frequency variability of TC activity; paleoclimate proxy studies; theoretical and modeling studies; future projections; roadblocks to resolution of key issues; and recommendations for making future progress.
Knutson, Thomas R., J McBride, Johnny C L Chan, Kerry A Emanuel, G Holland, Christopher Landsea, Isaac M Held, James Kossin, A K Srivastava, and M Sugi, March 2010: Tropical cyclones and climate change. Nature Geoscience, 3, DOI:doi:10.1038/ngeo779. Abstract
Whether the characteristics of tropical cyclones have changed or will change in a warming climate — and if so, how — has been the subject of considerable investigation, often with conflicting results. Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones. Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. However, future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%. Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre. For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies.
Because ocean color alters the absorption of sunlight, it can produce changes in sea surface temperatures with further impacts on atmospheric circulation. These changes can project onto fields previously recognized to alter the distribution of tropical cyclones. If the North Pacific subtropical gyre contained no absorbing and scattering materials, the result would be to reduce subtropical cyclone activity in the subtropical Northwest Pacific by 2/3, while concentrating cyclone tracks along the equator. Predicting tropical cyclone activity using coupled models may thus require consideration of the details of how heat moves into the upper thermocline as well as biogeochemical cycling.
Jiang, Xianan, Ming Zhao, and D E Waliser, October 2012: Modulation of tropical cyclones over the Eastern Pacific by the intra-seasonal variability simulated in an AGCM. Journal of Climate, 25(19), DOI:10.1175/JCLI-D-11-00531.1. Abstract
This study illustrates that observed modulations of tropical cyclone (TC) genesis over the eastern Pacific (EPAC) by large-scale intraseasonal variability (ISV) are represented well in a recently developed high-resolution atmospheric model (HiRAM) at NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL) with a horizontal resolution of about 50km. Considering the intrinsic predictability of the ISV of 2-4 weeks, this analysis thus has significant implications for dynamically based TC predictions on intraseasonal time scales. Analysis indicates that the genesis potential index (GPI) anomalies associated with the ISV can generally well depict ISV modulations of EPAC TC genesis in both observations and HiRAM simulations. Further investigation is conducted to explore the key factors associated with ISV modulation of TC activity based on an analysis of budget terms of the observed GPI during the ISV life cycle. It is found that, while relative roles of GPI factors are dependent on ISV phase and location, lower-level cyclonic vorticity, enhanced mid-level relative humidity, and reduced vertical wind shear can all contribute to the observed active TC genesis over the EPAC during particular ISV phases. In general, the observed anomalous ISV patterns of these large-scale GPI factors are well represented in HiRAM. Model deficiencies are also noted particularly in the anomalous mid-level relative humidity patterns and amplitude of vertical wind shear associated with the EPAC ISV.
Lee, Tsz-Cheung, and Thomas R Knutson, et al., May 2012: Impacts of climate change on tropical cyclones in the western North Pacific basin, Part I: Past observations. Tropical Cyclone Research and Review, 1(2), 213-230. Abstract
This paper reviews the current state of the science on the relationship
between climate change and historical tropical cyclone (TC) activity in
the western North Pacific (WNP) basin, which is the region of the
ESCAP/WMO Typhoon Committee members. Existing studies of observed
changes of TC activity in this basin, such as frequency, intensity,
precipitation, genesis location and track pattern are summarized.
Results from a survey on impacts of past TC activity on various members
of Typhoon Committee are reported, along with a review of studies of
past WNP landfalling TCs.
With considerable interannual and interdecadal variations in the TC
activity in this basin, it remains uncertain whether there has been any
detectable human influence on tropical cyclone frequency, intensity,
precipitation, track, or related aggregated storm activity metrics.
Also, the issues on of homogeneity and consistency of best track data
sets in the WNP further add uncertainty to relevant research studies.
Observations indicate some regional shifts in TC activity in the basin,
such as a decreasing trend in TC occurrence in part of the South China
Sea and an increasing trend along the east coast of China during the
past 40 years. This change is apparently related to local circulation
changes in the eastern Asia and WNP, though the cause of the
circulation changes remains unknown.
http://tcrr.typhoon.gov.cn/EN/10.6057/2012TCRR02.08
Retrospective seasonal predictions of tropical cyclones (TCs) in the three major ocean basins of the Northern Hemisphere are performed from 1990 to 2010 using the Geophysical Fluid Dynamics Laboratory High-Resolution Atmospheric Model at 25-km resolution. Atmospheric states are initialized for each forecast, with the sea surface temperature anomaly (SSTA) persisted from that at the starting time during the 5-month forecast period (July-November). Using a 5-member ensemble, it is shown that the storm counts of both tropical storm (TS) and hurricane categories are highly predictable in the North Atlantic basin during the 21-year period. The correlations between the 21-year observed and model predicted storm counts are 0.88 and 0.89 for hurricanes and TSs, respectively. The prediction in the eastern North Pacific is skillful, but it is not as outstanding as that in the North Atlantic. The persisted SSTA assumption appears to be less robust for the western North Pacific, contributing to less skillful predictions in that region. The relative skill in the prediction of storm counts is shown to be consistent with the quality of the predicted large-scale environment in the three major basins.
Finally, we show that intensity distribution of TCs can be captured well by our model if the central sea-level pressure were used as the threshold variable instead of the commonly used 10-meter wind speed. This demonstrated the feasibility of using the 25-km resolution HiRAM, a general circulation model designed initially for long-term climate simulations, to study the impacts of climate change on the intensity distribution of TCs.
Turner, A J., Arlene M Fiore, Larry W Horowitz, and M Bauer, January 2013: Summertime cyclones over the Great Lakes Storm Track from 1860–2100: variability, trends, and association with ozone pollution. Atmospheric Chemistry and Physics, 13(2), DOI:10.5194/acp-13-565-2013. Abstract
Prior work indicates that the frequency of summertime mid-latitude cyclones tracking across the Great Lakes Storm Track (GLST, bounded by: 70° W, 90° W, 40° N, and 50° N) are strongly anticorrelated with ozone (O3) pollution episodes over the Northeastern United States (US). We apply the MAP Climatology of Mid-latitude Storminess (MCMS) algorithm to 6-hourly sea level pressure fields from over 2500 yr of simulations with the GFDL CM3 global coupled chemistry-climate model. These simulations include (1) 875 yr with constant 1860 emissions and forcings (Pre-industrial Control), (2) five ensemble members for 1860–2005 emissions and forcings (Historical), and (3) future (2006–2100) scenarios following the Representative Concentration Pathways (RCP 8.5 (one member; extreme warming); RCP 4.5 (three members; moderate warming); RCP 4.5* (one member; a variation on RCP 4.5 in which only well-mixed greenhouse gases evolve along the RCP 4.5 trajectory)). The GFDL CM3 Historical simulations capture the mean and variability of summertime cyclones traversing the GLST within the range determined from four reanalysis datasets. Over the 21st century (2006–2100), the frequency of summertime mid-latitude cyclones in the GLST decreases under the RCP 8.5 scenario (m = −0.06 a−1, p < 0.01) and in the RCP 4.5 ensemble mean (m = −0.03 a−1, p < 0.01). These trends are significant when assessed relative to the variability in the Pre-industrial Control simulation (p > 0.06 for 100-yr sampling intervals; −0.01 a−1 < m < 0.02 a−1). In addition, the RCP 4.5* scenario enables us to determine the relationship between summertime GLST cyclones and high-O3 events (>95th percentile) in the absence of emission changes. The summertime GLST cyclone frequency explains less than 10% of the variability in high-O3 events over the Northeastern US in the model. Our findings imply that careful study is required prior to applying the strong relationship noted in earlier work to changes in storm counts.
Zhou, W, Isaac M Held, and Stephen T Garner, March 2014: Parameter study of tropical cyclones in rotating radiative-convective equilibrium with column physics and resolution of a 25 km GCM. Journal of the Atmospheric Sciences, 71(3), DOI:10.1175/JAS-D-13-0190.1. Abstract
Rotating radiative-convective equilibrium is studied by extracting the column physics of a meso-scale resolution global atmospheric model that simulates realistic hurricane frequency statistics and coupling it to rotating hydrostatic dynamics in doubly-periodic domains. The parameter study helps in understanding the tropical cyclones simulated in the global model and also provides a reference point for analogous studies with cloud resolving models.
The authors first examine the sensitivity of the equilibrium achieved in a large square domain (2×104 km on a side) to sea surface temperature, ambient rotation rate and surface drag coefficient. In such a large domain, multiple tropical cyclones exist simultaneously. The size and intensity of these tropical cyclones are investigated.
The variation of rotating radiative-convective equilibrium with domain size is also studied. As domain size increases, the equilibrium evolves through four regimes: a single tropical depression, an intermittent tropical cyclone with intensity widely varying, a single sustained storm, and finally multiple storms. As SST increases or ambient rotation rate f decreases, the sustained storm regime shifts towards larger domain size. The storm’s natural extent in large domains can be understood from this regime behavior.
The radius of maximum surface wind, although only marginally resolved, increases with SST and increases with f for small f when the domain is large enough. But these parameter dependencies can be modified or even reversed if the domain is smaller than the storm’s natural extent.
Villarini, Gabriele, R Goska, James A Smith, and Gabriel A Vecchi, September 2014: North Atlantic Tropical Cyclones and U.S. Flooding. Bulletin of the American Meteorological Society, 95(9), DOI:10.1175/BAMS-D-13-00060.1. Abstract
Riverine flooding associated with North Atlantic tropical cyclones (TCs) is responsible for large societal and economic impacts. The effects of TC flooding are not limited to the coastal regions, but affect large areas away from the coast, and often away from the center of the storm. Despite these important repercussions, inland TC flooding has received relatively little attention in the scientific literature, although there has been growing media attention following Hurricanes Irene (2011) and Sandy (2012). Based on discharge data from 1981 to 2011, we provide a climatological view of inland flooding associated with TCs, leveraging on the wealth of discharge measurements collected, archived, and disseminated by the U.S. Geological Survey (USGS). Florida and the eastern seaboard of the United States (from South Carolina to Maine and Vermont) are the areas that are the most susceptible to TC flooding, with typical TC flood peaks that are two to six times larger than the local 10-year flood peak, causing major flooding. We also identify a secondary swath of extensive TC-induced flooding in the central United States. These results indicate that flooding from TCs is not solely a coastal phenomenon, but affects much larger areas of the United States, as far inland as Illinois, Wisconsin and Michigan. Moreover, we highlight the dependence of the frequency and magnitude of TC flood peaks on large scale climate indices, and highlight the role played by the North Atlantic Oscillation and the El Niño-Southern Oscillation phenomenon (ENSO), suggesting potential sources of extended-range predictability.
Scoccimarro, E, Silvio Gualdi, Gabriele Villarini, Gabriel A Vecchi, and Ming Zhao, et al., June 2014: Intense Precipitation Events Associated with Landfalling Tropical Cyclones in response to a Warmer Climate and increased CO2. Journal of Climate, 27(12), DOI:10.1175/JCLI-D-14-00065.1. Abstract
In this work the authors investigate possible changes in the intensity of rainfall events associated with tropical cyclones (TCs) under idealized forcing scenarios, including a uniformly warmer climate, with a special focus on landfalling storms. A new set of experiments designed within the U.S. Climate Variability and Predictability (CLIVAR) Hurricane Working Group allows disentangling the relative role of changes in atmospheric carbon dioxide from that played by sea surface temperature (SST) in changing the amount of precipitation associated with TCs in a warmer world. Compared to the present-day simulation, an increase in TC precipitation was found under the scenarios involving SST increases. On the other hand, in a CO2-doubling-only scenario, the changes in TC rainfall are small and it was found that, on average, TC rainfall tends to decrease compared to the present-day climate. The results of this study highlight the contribution of landfalling TCs to the projected increase in the precipitation changes affecting the tropical coastal regions.
Wang, H, L Long, Arun Kumar, W Wang, J-K E Shemm, Ming Zhao, and Gabriel A Vecchi, et al., August 2014: How well do global climate models simulate the variability of Atlantic tropical cyclones associated with ENSO?Journal of Climate, 27(15), DOI:10.1175/JCLI-D-13-00625.1. Abstract
The variability of Atlantic tropical cyclones (TCs) associated with El Niño–Southern Oscillation (ENSO) in model simulations is assessed and compared with observations. The model experiments are 28-yr simulations forced with the observed sea surface temperature from 1982 to 2009. The simulations were coordinated by the U.S. CLIVAR Hurricane Working Group and conducted with five global climate models (GCMs) with a total of 16 ensemble members. The model performance is evaluated based on both individual model ensemble means and multi-model ensemble mean. The latter has the highest anomaly correlation (0.86) for the interannual variability of TCs. Previous observational studies show a strong association between ENSO and Atlantic TC activity, as well as distinctions during eastern Pacific (EP) and central Pacific (CP) El Niño events. The analysis of track density and TC origin indicates that each model has different mean biases. Overall, the GCMs simulate the variability of Atlantic TCs well with weaker activity during EP El Niño and stronger activity during La Niña. For CP El Niño, there is a slight increase in the number of TCs as compared with EP El Niño. However, the spatial distribution of track density and TC origin is less consistent among the models. Particularly, there is no indication of increasing TC activity over the U.S. southeast coastal region during CP El Niño as in observations. The difference between the models and observations is likely due to the bias of the models in response to the shift of tropical heating associated with CP El Niño, as well as the model bias in the mean circulation.
Shaevitz, D, Suzana J Camargo, Adam H Sobel, J A Jonas, D Kim, Arun Kumar, T LaRow, Y-K Lim, Hiroyuki Murakami, Kevin A Reed, Malcolm J Roberts, E Scoccimarro, Pier Luigi Vidale, H Wang, Michael F Wehner, Ming Zhao, and N Henderson, December 2014: Characteristics of tropical cyclones in high-resolution models in the present climate. Journal of Advances in Modeling Earth Systems, 6(4), DOI:10.1002/2014MS000372. Abstract
The global characteristics of tropical cyclones (TCs) simulated by several climate models are analyzed and compared with observations. The global climate models were forced by the same sea surface temperature (SST) fields in two types of experiments, using climatological SST and interannually varying SST. TC tracks and intensities are derived from each model's output fields by the group who ran that model, using their own preferred tracking scheme; the study considers the combination of model and tracking scheme as a single modeling system, and compares the properties derived from the different systems. Overall, the observed geographic distribution of global TC frequency was reasonably well reproduced. As expected, with the exception of one model, intensities of the simulated TC were lower than in observations, to a degree that varies considerably across models.
Wright, D, Thomas R Knutson, and James A Smith, December 2015: Regional climate model projections of rainfall from U.S. landfalling tropical cyclones. Climate Dynamics, 45(11-12), DOI:10.1007/s00382-015-2544-y. Abstract
The eastern United States is vulnerable to flooding from tropical cyclone rainfall. Understanding how both the frequency and intensity of this rainfall will change in the future climate is a major challenge. One promising approach is the dynamical downscaling of relatively coarse general circulation model results using higher-resolution regional climate models (RCMs). In this paper, we examine the frequency of landfalling tropical cyclones and associated rainfall properties over the eastern United States using Zetac, an 18-km resolution RCM designed for modeling Atlantic tropical cyclone activity. Simulations of 1980–2006 tropical cyclone frequency and rainfall intensity for the months of August–October are compared against results from previous studies and observation-based datasets. The 1980–2006 control simulations are then compared against results from three future climate scenarios: CMIP3/A1B (late twenty-first century) and CMIP5/RCP4.5 (early and late twenty-first century). In CMIP5 early and late twenty-first century projections, the frequency of occurrence of post-landfall tropical cyclones shows little net change over much of the eastern U.S. despite a decrease in frequency over the ocean. This reflects a greater landfalling fraction in CMIP5 projections, which is not seen in CMIP3-based projections. Average tropical cyclone rain rates over land within 500 km of the storm center increase by 8–17 % in the future climate projections relative to control. This is at least as much as expected from the Clausius–Clapeyron relation, which links a warmer atmosphere to greater atmospheric water vapor content. Over land, the percent enhancement of area-averaged rain rates from a given tropical cyclone in the warmer climate is greater for larger averaging radius (300–500 km) than near the storm, particularly for the CMIP3 projections. Although this study does not focus on attribution, the findings are broadly consistent with historical tropical cyclone rainfall changes documented in a recent observational study. The results may have important implications for future flood risks from tropical cyclones.
Knutson, Thomas R., March 2015: Tropical Cyclones and Climate Change In Encyclopedia of Atmospheric Sciences 2nd edition, Vol 6, Gerald R. North (editor-in-chief), John Pyle and Fuqing Zhang (editors), Oxford, Academic Press, 111-122.
The hindcasts of the Geophysical Fluid Dynamics Laboratory (GFDL) High-Resolution Atmospheric Model (HiRAM), which skillfully predicted the interannual variability of Atlantic tropical cyclone (TC) frequency, were analyzed to investigate what key circulation systems a model must capture in order to skillfully predict TCs. The HiRAM reproduced the leading EOF mode (M1) of the interannual variability of the Atlantic Hadley circulation and its impacts on environmental conditions. M1 represents the variability of the ITCZ intensity and width, and the predictability of Atlantic TCs can be explained by the lag correlation between M1 and SST in preceding months. Although the ITCZ displacement was not well predicted by the HiRAM, it does not affect the prediction of the basin-wide hurricane count. The analyses suggest that the leading mode of the variability of the regional Hadley circulation can serve as a useful metric to evaluate the performance of global models in TC seasonal prediction.
Recent review papers reported that many high-resolution global climate models consistently projected a reduction of global tropical cyclone (TC) frequency in a future warmer climate, although the mechanism of the reduction is not yet fully understood. Here we present a result of 4K-cooler climate experiment. The global TC frequency significantly increases in the 4K-cooler climate compared to the present climate. This is consistent with a significant decrease in TC frequency in the 4K-warmer climate. For the mechanism of TC frequency reduction in a warmer climate, upward mass flux hypothesis and saturation deficit hypothesis have been proposed. The result of the 4K-cooler climate experiment is consistent with these two hypotheses. One very interesting point is that the experiment has clearly shown that TC genesis is possible at sea surface temperature (SST) well below 26°C which has been considered as the lowest SST limit for TC genesis.
This study investigates the association between the Pacific Meridional Mode (PMM) and tropical cyclone (TC) activity in the western North Pacific (WNP). It is found that the positive PMM phase favors the occurrence of TCs in the WNP while the negative PMM phase inhibits the occurrence of TCs there. Observed relationships are consistent with those from a long-term pre-industrial control experiment (1000 years) of a high-resolution TC-resolving Geophysical Fluid Dynamics Laboratory (GFDL) Forecast-oriented Low Ocean Resolution (FLOR) coupled climate model. The diagnostic relationship between the PMM and TCs in observations and the model is further supported by sensitivity experiments with FLOR. The modulation of TC genesis by the PMM is primarily through the anomalous zonal vertical wind shear (ZVWS) changes in the WNP, especially in the southeastern WNP. The anomalous ZVWS can be attributed to the responses of the atmosphere to the anomalous warming in the northwestern part of the PMM pattern during the positive PMM phase, which resembles a classic Matsuno-Gill pattern. Such influences on TC genesis are strengthened by a cyclonic flow over the WNP. The significant relationship between TCs and the PMM identified here may provide a useful reference for seasonal forecasting of TCs and interpreting changes in TC activity in the WNP.
Walsh, Kevin J., J McBride, Philip J Klotzbach, S Balachandran, Suzana J Camargo, G Holland, Thomas R Knutson, James Kossin, Tsz-Cheung Lee, Adam H Sobel, and M Sugi, January 2016: Tropical cyclones and climate change. Wiley Interdisciplinary Reviews: Climate Change, 7(1), DOI:10.1002/wcc.371. Abstract
Recent research has strengthened the understanding of the links between climate and tropical cyclones (TCs) on various timescales. Geological records of past climates have shown century-long variations in TC numbers. While no significant trends have been identified in the Atlantic since the late 19th century, significant observed trends in TC numbers and intensities have occurred in this basin over the past few decades, and trends in other basins are increasingly being identified. However, understanding of the causes of these trends is incomplete, and confidence in these trends continues to be hampered by a lack of consistent observations in some basins. A theoretical basis for maximum TC intensity appears now to be well established, but a climate theory of TC formation remains elusive. Climate models mostly continue to predict future decreases in global TC numbers, projected increases in the intensities of the strongest storms and increased rainfall rates. Sea level rise will likely contribute toward increased storm surge risk. Against the background of global climate change and sea level rise, it is important to carry out quantitative assessments on the potential risk of TC-induced storm surge and flooding to densely populated cities and river deltas. Several climate models are now able to generate a good distribution of both TC numbers and intensities in the current climate. Inconsistent TC projection results emerge from modeling studies due to different downscaling methodologies and warming scenarios, inconsistencies in projected changes of large-scale conditions, and differences in model physics and tracking algorithms.
Ogata, Tomomichi, Ryo Mizuta, Yukimasa Adachi, Hiroyuki Murakami, and Tomoaki Ose, December 2015: Effect of air-sea coupling on the frequency distribution of intense tropical cyclones over the northwestern Pacific. Geophysical Research Letters, 42(23), DOI:10.1002/2015GL066774. Abstract
Effect of air-sea coupling on the frequency distribution of intense tropical cyclones (TCs) over the northwestern Pacific (NWP) region is investigated using an atmosphere and ocean coupled general circulation model (AOGCM). Monthly varying flux adjustment enables AOGCM to simulate both subseasonal air-sea interaction and realistic seasonal to interannual sea surface temperature (SST) variability. The maximum of intense TC distribution around 20–30°N in the AGCM shifts equatorward in the AOGCM due to the air-sea coupling. Hence, AOGCM reduces northward intense TC distribution bias seen in AGCM. Over the NWP, AOGCM-simulated SST variability is large around 20–30°N where the warm mixed layer becomes shallower rapidly. Active entrainment from subsurface water over this region causes stronger SST cooling, and hence, TC intensity decreases. These results suggest that air-sea coupling characterized by subsurface oceanic condition causes more realistic distribution of intense TCs over the NWP.
Retrospective seasonal forecasts of North Atlantic tropical cyclone (TC) activity over the period 1980–2014 are conducted using a GFDL high-resolution coupled climate model [Forecast-Oriented Low Ocean Resolution (FLOR)]. The focus is on basin-total TC and U.S. landfall frequency. The correlations between observed and model predicted basin-total TC counts range from 0.4 to 0.6 depending on the month of the initial forecast. The correlation values for U.S. landfalling activity based on individual TCs tracked from the model are smaller and between 0.1 and 0.4. Given the limited skill from the model, statistical methods are used to complement the dynamical seasonal TC prediction from the FLOR model. Observed and predicted TC tracks were classified into four groups using fuzzy c-mean clustering to evaluate the model’s predictability in observed classification of TC tracks. Analyses revealed that the FLOR model has the highest skill in predicting TC frequency for the cluster of TCs that tracks through the Caribbean and the Gulf of Mexico.
New hybrid models are developed to improve the prediction of observed basin-total TC and landfall TC frequencies. These models use large-scale climate predictors from the FLOR model as predictors for generalized linear models. The hybrid models show considerable improvements in the skill in predicting the basin-total TC frequencies relative to the dynamical model. The new hybrid model shows correlation coefficients as high as 0.75 for basinwide TC counts from the first two lead months and retains values around 0.50 even at the 6-month lead forecast. The hybrid model also shows comparable or higher skill in forecasting U.S. landfalling TCs relative to the dynamical predictions. The correlation coefficient is about 0.5 for the 2–5-month lead times.
Zhang, Wei, Gabriele Villarini, Gabriel A Vecchi, Hiroyuki Murakami, and Richard G Gudgel, June 2016: Statistical-dynamical seasonal forecast of western North Pacific and East Asia landfalling tropical cyclones using the high-resolution GFDL FLOR coupled model. Journal of Advances in Modeling Earth Systems, 8(2), DOI:10.1002/2015MS000607. Abstract
This study examines the seasonal prediction of western North Pacific [WNP) and East Asia landfalling tropical cyclones (TCs) using the Geophysical Fluid Dynamics Laboratory(GFDL) Forecast-oriented Low Ocean Resolution version of CM2.5 with Flux Adjustment (FLOR-FA) and finite-mixture-model (FMM)-based statistical cluster analysis. Using the FMM-based cluster analysis, seven clusters are identified from the historical and FLOR-FA-predicted TC tracks for the period 1980–2013. FLOR-FA has significant skill in predicting year-to-year variations in the frequency of TCs within clusters 1 (recurving TCs) and 5 (straight-moving TCs). By building Poisson regression models for each cluster using key predictors (i.e., sea surface temperature, 500 hPa geopotential height, and zonal vertical wind shear), the predictive skill for almost all the clusters at all initialization months improves with respect to the dynamic prediction. The prediction of total WNP TC frequency made by combining hybrid predictions for each of the seven clusters in the hybrid model shows skill higher than what achieved using the TC frequency directly from FLOR-FA initialized from March to July. However, the hybrid predictions for total WNP TC frequency initialized from January to February exhibit lower skill than FLOR-FA. The prediction of TC landfall over East Asia made by combining the hybrid models of TC frequency in each cluster and its landfall rate over East Asia also outperforms FLOR-FA for all initialization months January through July.
Han, R, H Wang, Zeng-Zhen Hu, Arun Kumar, W Li, L Long, J-K E Schemm, P Peng, W Wang, D Si, X Jia, Ming Zhao, and Gabriel A Vecchi, et al., September 2016: An assessment of multi-model simulations for the variability of western North Pacific tropical cyclones and its association with ENSO. Journal of Climate, 29(18), DOI:10.1175/JCLI-D-15-0720.1. Abstract
An assessment of simulations of the interannual variability of tropical cyclones (TCs) over the western North Pacific (WNP) and its association with El Niño–Southern Oscillation (ENSO), as well as a subsequent diagnosis for possible causes of model biases generated from simulated large scale climate conditions, are documented in the paper. The model experiments are carried out by the Hurricane Work Group under the U.S. Climate Variability and Predictability Research Program (CLIVAR) using five global climate models (GCMs) with a total of 16 ensemble members forced by the observed sea surface temperature, and spanning the 28-yr period from 1982 to 2009. The results show GISS and GFDL model ensemble means best simulate the interannual variability of TCs and the multi-model ensemble mean (MME) follows. Also, the MME has the closest climate mean annual number of WNP TCs and the smallest root-mean-square error to the observation.
Most GCMs can simulate the interannual variability of WNP TCs well, with stronger TC activities during two types of El Niño, namely eastern Pacific (EP) and central Pacific (CP) El Niño, and weaker activity during La Niña. However, none of the models capture the differences in TC activity between EP and CP El Niño as shown in observations. The inability of models to distinguish the differences in TC activities between the two types of El Niño events may be due to the bias of the models in response to the shift of tropical heating associated with CP El Niño.
Kossin, James, Kerry A Emanuel, and Gabriel A Vecchi, June 2016: Comment on 'Roles of interbasin frequency changes in the poleward shifts of the maximum intensity location of tropical cyclones'. Environmental Research Letters, 11(6), DOI:10.1088/1748-9326/11/6/068001.
This study aims to assess the connections between the El Niño Southern Oscillation (ENSO) and tropical cyclones near Guam (GuamTC) using the state-of-the-art Geophysical Fluid Dynamics Laboratory (GFDL) Forecast-oriented Low Ocean Resolution Version of CM2.5 (FLOR). In observations, more (less) GuamTCs occur in El Niño (La Niña) years and the ENSO-GuamTC connections arise from TC genesis locations in ENSO phases. The observed ENSO-GuamTC connections are realistically simulated in the two control experiments that use two versions of FLOR, the standard version and another with flux adjustments (FLOR-FA). The ENSO-GuamTC connections in FLOR-FA are closer to observations than those in FLOR because of a better representation of TC genesis during ENSO phases. The physical mechanisms underlying the observed ENSO-GuamTC connections are further supported in the long-term control experiments with FLOR/FLOR-FA. The ENSO-GuamTC connections in sea surface temperature (SST)- and sea surface salinity (SSS)-restoring experiments with FLOR 1990 strongly resemble the observations, suggesting the ENSO-GuamTC connections arise substantially from the forcing of SST. The prediction skill of FLOR-FA for GuamTC frequency is quite promising in terms of correlation and root mean square error and is higher than that of FLOR for the period 1980-2014. This study shows the capability of global climate models (FLOR/FLOR-FA) in simulating the linkage between ENSO and TC activity near a highly localized region (i.e., Guam) and in predicting the frequency of TCs at the sub-basin scale.
Luitel, B, Gabriele Villarini, and Gabriel A Vecchi, January 2018: Verification of the skill of numerical weather prediction models in forecasting rainfall from U.S. landfalling tropical cyclones. Journal of Hydrology, 556, DOI:10.1016/j.jhydrol.2016.09.019. Abstract
The goal of this study is the evaluation of the skill of five state-of-the-art numerical weather prediction (NWP) systems [European Centre for Medium-Range Weather Forecasts (ECMWF), UK Met Office (UKMO), National Centers for Environmental Prediction (NCEP), China Meteorological Administration (CMA), and Canadian Meteorological Center (CMC)] in forecasting rainfall from North Atlantic tropical cyclones (TCs). Analyses focus on 15 North Atlantic TCs that made landfall along the U.S. coast over the 2007–2012 period. As reference data we use gridded rainfall provided by the Climate Prediction Center (CPC). We consider forecast lead-times up to five days. To benchmark the skill of these models, we consider rainfall estimates from one radar-based (Stage IV) and four satellite-based [Tropical Rainfall Measuring Mission - Multi-satellite Precipitation Analysis (TMPA, both real-time and research version); Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN); the CPC MORPHing Technique (CMORPH)] rainfall products. Daily and storm total rainfall fields from each of these remote sensing products are compared to the reference data to obtain information about the range of errors we can expect from “observational data.” The skill of the NWP models is quantified: (1) by visual examination of the distribution of the errors in storm total rainfall for the different lead-times, and numerical examination of the first three moments of the error distribution; (2) relative to climatology at the daily scale. Considering these skill metrics, we conclude that the NWP models can provide skillful forecasts of TC rainfall with lead-times up to 48 h, without a consistently best or worst NWP model.
Recent modeling studies have consistently shown that the global frequency of tropical cyclones will decrease but that of very intense tropical cyclones may increase in the future warmer climate. It has been noted, however, that the uncertainty in the projected changes in the frequency of very intense tropical cyclones, particularly the changes in the regional frequency, is very large. Here we present a projection of the changes in the frequency of intense tropical cyclones estimated by a statistical downscaling of ensemble of many high-resolution global model experiments. The results indicate that the changes in the frequency of very intense (category 4 and 5) tropical cyclones are not uniform on the globe. The frequency will increase in most regions but decrease in the south western part of Northwest Pacific, the South Pacific, and eastern part of the South Indian Ocean.
Khouakhi, A, Gabriele Villarini, and Gabriel A Vecchi, January 2017: Contribution of tropical cyclones to rainfall at the global scale. Journal of Climate, 30(1), DOI:10.1175/JCLI-D-16-0298.1. Abstract
This study quantifies the relative contribution of tropical cyclones (TCs) to annual, seasonal and extreme rainfall, and examines the connection between El Niño–Southern Oscillation (ENSO) and the occurrence of extreme TC-induced rainfall across the globe. We use historical six-hour best track TC datasets and daily precipitation data from 18607 global rain gauges with at least 25 complete years of data between 1970 and 2014. The highest TC-induced rainfall totals occur in eastern Asia (>400 mm/year) and northeastern Australia (>200mm/year), followed by the southeastern United States and along the coast of the Gulf of Mexico (100 to 150 mm/year). Fractionally, TCs account for 35% to 50% of the mean annual rainfall in northwestern Australia, southeastern China, the northern Philippines and Baja California, Mexico. Seasonally, between 40% and 50% of TC-induced rain is recorded along the western coast of Australia and in islands of the south Indian Ocean in the austral summer, and in eastern Asia and Mexico in boreal summer and fall. In terms of extremes, using annual maximum and peak-over-threshold approaches, we find the highest proportions of TC-induced rainfall in East Asia, followed by Australia and North-Central America, with fractional contributions generally decreasing as one moves inland from the coast. The relationship between TC-induced extreme rainfall and ENSO reveals that TC-induced extreme rainfall tends to occur more frequently in Australia and along the U.S. East Coast during La Niña, and along eastern Asia and the northwestern Pacific islands during El Niño.
Tropical cyclones are studied under the idealized framework of rotating radiative-convective equilibrium, achieved in a large doubly-periodic f -plane by coupling the column physics of a global atmospheric model to rotating hydrostatic dynamics. Unlike previous studies which prescribe uniform sea surface temperature (SST) over the domain, SSTs are now predicted by coupling the atmosphere to a simple slab ocean model. With coupling, SSTs under the eyewall region of tropical cyclones (TCs) become cooler than the environment. However, the domain still fills up with multiple long-lived TCs in all cases examined, including at the limit of very small depth of the slab. The cooling of SSTs under the eyewall increases as the depth of the slab ocean layer decreases but levels off at roughly 6.5 K as the depth approaches zero. At the eyewall, the storm interior is decoupled from the cooler surface and moist entropy is no longer well-mixed along the angular momentum surface in the boundary layer. TC intensity is reduced from the potential intensity computed without the cooling, but the intensity reduction is smaller than that estimated by a potential intensity taking into account the cooling and assuming that moist entropy is well mixed along angular momentum surfaces within the atmospheric boundary layer.
This study attempts to improve the prediction of western North Pacific (WNP) and East Asia (EA) landfalling tropical cyclones (TCs) using modes of large-scale climate variability [e.g., the Pacific meridional mode (PMM), the Atlantic meridional mode (AMM), and North Atlantic sea surface temperature anomalies (NASST)] as predictors in a hybrid statistical–dynamical scheme, based on dynamical model forecasts with the GFDL Forecast-Oriented Low Ocean Resolution version of CM2.5 with flux adjustments (FLOR-FA). Overall, the predictive skill of the hybrid model for the WNP TC frequency increases from lead month 5 (initialized in January) to lead month 0 (initialized in June) in terms of correlation coefficient and root-mean-square error (RMSE). The hybrid model outperforms FLOR-FA in predicting WNP TC frequency for all lead months. The predictive skill of the hybrid model improves as the forecast lead time decreases, with values of the correlation coefficient increasing from 0.56 for forecasts initialized in January to 0.69 in June. The hybrid models for landfalling TCs over the entire East Asian (EEA) coast and its three subregions [i.e., southern EA (SEA), middle EA (MEA), and northern EA (NEA)] dramatically outperform FLOR-FA. The correlation coefficient between predicted and observed TC landfall over SEA increases from 0.52 for forecasts initialized in January to 0.64 in June. The hybrid models substantially reduce the RMSE of landfalling TCs over SEA and EEA compared with FLOR-FA. This study suggests that the PMM and NASST/AMM can be used to improve statistical/hybrid forecast models for the frequencies of WNP or East Asia landfalling TCs.
This study examines the impacts of the Pacific Meridional Mode (PMM) on North Atlantic tropical cyclones (TCs) making landfall along the coastal US, Caribbean Islands and Mexico, and provides insights on the underlying physical mechanisms using observations and model simulations. There is a statistically significant time-lagged association between spring PMM and the August–October US and Caribbean landfalling TCs. Specifically, the positive (negative) spring PMM events tend to be followed by fewer (more) TCs affecting the coastal US (especially over the Gulf of Mexico and Florida) and the Caribbean Islands. This lagged association is mainly caused by the lagged impacts of PMM on the El Niño Southern Oscillation (ENSO), and the subsequent impacts of ENSO on TC frequency and landfalls. Positive (negative) PMM events are largely followed by El Niño (La Niña) events, which lead to less (more) TC geneses close to the US coast (i.e., the Gulf of Mexico and the Caribbean Sea); this also leads to easterly (westerly) steering flow in the vicinity of the US and Caribbean coast, which is unfavorable (favorable) to TC landfall across the Gulf of Mexico, Florida and Caribbean Islands. Perturbation simulations with the state-of-the-art Geophysical Fluid Dynamics Laboratory Forecast-oriented Low Ocean Resolution Version of CM2.5 (FLOR) support the linkage between PMM and TC landfall activity. The time-lagged impacts of spring PMM on TC landfalling activity results in a new predictor to forecast seasonal TC landfall activity along the US (especially over the Gulf of Mexico and Florida) and Caribbean coastal regions.
This study proposes a set of process-oriented diagnostics with the aim of understanding how model physics and numerics control the representation of tropical cyclones (TCs), especially their intensity distribution, in GCMs. Three simulations are made using two 50-km GCMs developed at NOAA’s Geophysical Fluid Dynamics Laboratory. The two models are forced with fixed sea surface temperature (AM2.5 and HiRAM), and in the third simulation the AM2.5 model is coupled to an ocean GCM (FLOR).
The frequency distributions of maximum surface wind near TC centers show that HiRAM tends to develop stronger TCs than the other models do. Large-scale environmental parameters, such as potential intensity, do not explain the differences between HiRAM and the other models. It is found that HiRAM produces a greater amount of precipitation near the TC center, suggesting that associated greater diabatic heating enables TCs to become stronger in HiRAM. HiRAM also shows a greater contrast in relative humidity and surface latent heat flux between the inner and outer regions of TCs.
Various fields are composited on precipitation percentiles to reveal the essential character of the interaction among convection, moisture, and surface heat flux. Results show that the moisture sensitivity of convection is higher in HiRAM than in the other model simulations. HiRAM also exhibits a stronger feedback from surface latent heat flux to convection via near-surface wind speed in heavy rain rate regimes. The results emphasize that the moisture-convection coupling and the surface heat flux feedback are critical processes that affect the intensity of TCs in GCMs.
Gualtieri, L, Suzana J Camargo, and Salvatore Pascale, et al., February 2018: The persistent signature of tropical cyclones in ambient seismic noise. Earth and Planetary Science Letters, 484, DOI:10.1016/j.epsl.2017.12.026. Abstract
The spectrum of ambient seismic noise shows strong signals associated with tropical cyclones, yet a detailed understanding of these signals and the relationship between them and the storms is currently lacking. Through the analysis of more than a decade of seismic data recorded at several stations located in and adjacent to the northwest Pacific Ocean, here we show that there is a persistent and frequency-dependent signature of tropical cyclones in ambient seismic noise that depends on characteristics of the storm and on the detailed location of the station relative to the storm. An adaptive statistical model shows that the spectral amplitude of ambient seismic noise, and notably of the short-period secondary microseisms, has a strong relationship with tropical cyclone intensity and can be employed to extract information on the tropical cyclones.
This study examines the performance of the Geophysical Fluid Dynamics Laboratory Forecast-Oriented Low Ocean Resolution version of CM2.5 (FLOR; ~ 50-km mesh) and high-resolution FLOR (HiFLOR; ~ 25-km mesh) in reproducing the climatology and interannual variability in rainfall associated with tropical cyclones (TCs) in both sea surface temperature (SST)-nudging and seasonal-forecast experiments. Overall, HiFLOR outperforms FLOR in capturing the observed climatology of TC rainfall, particularly in East Asia, North America and Australia. In general, both FLOR and HiFLOR underestimate the observed TC rainfall in the coastal regions along the Bay of Bengal, connected to their failure to accurately simulate the bimodal structure of the TC genesis seasonality. A crucial factor in capturing the climatology of TC rainfall by the models is the simulation of the climatology of spatial TC density. Overall, while HiFLOR leads to a better characterization of the areas affected by TC rainfall, the SST-nudging and seasonal-forecast experiments with both models show limited skill in reproducing the year-to-year variation in TC rainfall. Ensemble-based estimates from these models indicate low potential skill for year-to-year variations in TC rainfall, yet the models show lower skill than this. Therefore, the low skill for interannual TC rainfall in these models reflects both a fundamental limit on predictability/reproducibility of seasonal TC rainfall as well as shortcomings in the models.
Li, Weiwei, Z Wang, Gan Zhang, M S Peng, S G Benjamin, and Ming Zhao, December 2018: Subseasonal Variability of Rossby Wave Breaking and Impacts on Tropical Cyclones during the North Atlantic Warm Season. Journal of Climate, 31(23), DOI:10.1175/JCLI-D-17-0880.1. Abstract
This study investigates the subseasonal variability of anticyclonic Rossby wave breaking (AWB) and its impacts on atmospheric circulations and tropical cyclones (TCs) over the North Atlantic in the warm season from 1985 to 2013. Significant anomalies in sea level pressure, tropospheric wind and humidity fields are found over the tropical-subtropical Atlantic within 8 days of an AWB activity peak. Such anomalies may lead to suppressed TC activity on the subseasonal timescale, but a significant negative correlation between the subseasonal variability of AWB and Atlantic basin-wide TC activity does not exist every year, likely due to the modulation of TCs by other factors. It is also found that AWB occurrence may be modulated by the Madden-Julian Oscillation (MJO). In particular, AWB occurrence over the tropical-subtropical West Atlantic is reduced in phases 2 and 3 and enhanced in phases 6 and 7 based on the Real-time Multivariate MJO Index (RMM).
The impacts of AWB on the predictive skill of Atlantic TCs are examined using the Global Ensemble Forecasting System (GEFS) reforecasts with the forecast lead time up to 2 weeks. The hit rate of tropical cyclogenesis during active AWB episodes is lower than the long-term mean hit rate, and the GEFS is less skillful in capturing the variations of weekly TC activity during the years of enhanced AWB activity. The lower predictability of TCs is consistent with the lower predictability of environmental variables (such as vertical wind shear, moisture, and low-level vorticity) under the extratropical influence.
Zhang, L, and Leo Oey, January 2019: Young ocean waves favor the rapid intensification of tropical cyclones - a global observational analysis. Monthly Weather Review, 147(1), DOI:10.1175/MWR-D-18-0214.1. Abstract
Identifying the condition(s) of how tropical cyclones intensify, in particular rapid intensification, is challenging, due to the complexity of the problem involving internal dynamics, environments and mutual interactions; yet the benefit to improved forecasts may be rewarding. To make the analysis more tractable, an attempt is made here focusing near the sea surface, by examining 23-year global observations comprising over 16,000 cases of tropical cyclone intensity change, together with upper-ocean features, surface waves and low-level atmospheric moisture convergence. Contrary to the popular misconception, we found no statistically significant evidence that thicker upper-ocean layers and/or warmer temperatures are conducive to rapid intensification. Instead, we found in storms undergoing rapid intensification significantly higher coincidence of low-level moisture convergence and a dimensionless air-sea exchange coefficient closely related to the youth of the surface waves under the storm. This finding is consistent with the previous modeling results, verified here using ensemble experiments, that higher coincidence of moisture and surface fluxes tends to correlate with intensification, through greater precipitation and heat release. The young waves grow to saturation in the right-front quadrant due to trapped-wave resonance for a group of Goldilocks cyclones that translate neither too slowly nor too quickly, which 70% of rapidly-intensifying storms belong. Young waves in rapidly-intensifying storms also produce relatively less (compared to the wind input) Stokes-induced mixing and cooling in the cyclone core. A reinforcing coupling between tropical cyclone wind and waves leading to rapid intensification is proposed.
Keller, J H., C M Grams, M Riemer, and Heather M Archambault, et al., April 2019: The Extratropical Transition of Tropical Cyclones Part II: Interaction with the midlatitude flow, downstream impacts, and implications for predictability. Monthly Weather Review, 147(4), DOI:10.1175/MWR-D-17-0329.1. Abstract
The extratropical transition (ET) of tropical cyclones often has an important impact on the nature and predictability of the midlatitude flow. This review synthesizes the current understanding of the dynamical and physical processes that govern this impact and highlights the relationship of downstream development during ET to high-impact weather, with a focus on downstream regions. It updates a previous review from 2003 and identifies new and emerging challenges, and future research needs. First, the mechanisms through which the transitioning cyclone impacts the midlatitude flow in its immediate vicinity is discussed. This ‘direct impact’ manifests in the formation of a jet streak and the amplification of a ridge directly downstream of the cyclone. This initial flow modification triggers or amplifies a midlatitude Rossby wave packet, which disperses the impact of ET into downstream regions (‘downstream impact’) and may contribute to the formation of high-impact weather. Details are provided concerning the impact of ET on forecast uncertainty in downstream regions and on the impact of observations on forecast skill. The sources and characteristics of the following key features and processes that may determine the manifestation of the impact of ET on the midlatitude flow are discussed: the upper-tropospheric divergent outflow, mainly associated with latent heat release in the troposphere below, and the phasing between the transitioning cyclone and the midlatitude wave pattern. Improving the representation of diabatic processes during ET in models, and a climatological assessment of the ET’s impact on downstream high-impact weather are examples for future research directions.
Riboldi, J, C M Grams, M Riemer, and Heather M Archambault, February 2019: A phase-locking perspective on Rossby wave amplification and atmospheric blocking downstream of recurving western North Pacific tropical cyclones. Monthly Weather Review, 147(2), DOI:10.1175/MWR-D-18-0271.1. Abstract
The extratropical transition (ET) of tropical cyclones (TCs) can significantly influence the evolution of the midlatitude flow. However, the interaction between recurving TCs and upstream upper-level troughs features a large and partly unexplained case-to-case variability. In this study a synoptic, feature-based climatology of TC-trough interactions is constructed to discriminate recurving TCs that interact with decelerating and accelerating troughs. Upper-level troughs reducing their eastward propagation speed during the interaction with recurving TCs exhibit phase-locking with lower-level temperature anomalies and are linked to pronounced downstream Rossby wave amplification. Conversely, accelerating troughs do not exhibit phase-locking and are associated with a nonsignificant downstream impact. Irrotational outflow near the tropopause associated with latent heat release in regions of heavy precipitation near the transitioning storm can promote phase-locking (via enhancement of trough deceleration) and further enhance the downstream impact (via advection of air with low potential vorticity in the direction of the waveguide). These different impacts affect the probability of atmospheric blocking at the end of the Pacific storm track, which is generally higher if a TC-trough interaction occurs in the western North Pacific. Blocking in the eastern North Pacific is up to three times more likely than climatology if an interaction between a TC and a decelerating trough occurs upstream, whereas no statistical deviation with respect to climatology is observed for accelerating troughs. The outlined results support the hypothesis that differences in phase-locking can explain the observed variability in the downstream impact of ET.
Catalano, A J., Anthony J Broccoli, Sarah B Kapnick, and Tyler P Janoski, April 2019: High-Impact Extratropical Cyclones along the Northeast Coast of the United States in a Long Coupled Climate Model Simulation. Journal of Climate, 32(7), DOI:10.1175/JCLI-D-18-0376.1. Abstract
High-impact extratropical cyclones (ETCs) cause considerable damage along the Northeast coast of the United States through strong winds and inundation, but these relatively rare events are difficult to analyze owing to limited historical records. Using a 1505-year simulation from the GFDL FLOR coupled model, statistical analyses of extreme events are performed including exceedance probability computations to compare estimates from shorter segments to estimates that could be obtained from a record of considerable length. The most extreme events possess characteristics including exceptionally low central pressure, hurricane-force winds, and a large surge potential, which would greatly impact nearby regions. Return level estimates of metrics of ETC intensity using shorter, historical-length segments of the FLOR simulation are underestimated compared to levels determined using the full simulation. This indicates that if the underlying distributions of observed ETC metrics are similar to those of the 1505-year FLOR distributions, the actual frequency of extreme ETC events could also be underestimated.
Comparisons between FLOR and reanalysis products suggest that not all features of simulated high-impact ETCs are representative of observations. Spatial track densities are similar, but FLOR exhibits a negative bias in central pressure and a positive bias in wind speed, particularly for more intense events. Although the existence of these model biases precludes the quantitative use of model-derived return statistics as a substitute for those derived from shorter observational records, this work suggests that statistics from future models of higher fidelity could be used to better constrain the probability of extreme ETC events and their impacts.
Wang, D, T Kukulka, Brandon G Reichl, Tetsu Hara, and Isaac Ginis, et al., September 2018: Interaction of Langmuir Turbulence and Inertial Currents in the Ocean Surface Boundary Layer under Tropical Cyclones. Journal of Physical Oceanography, 48(9), DOI:10.1175/JPO-D-17-0258.1. Abstract
Based on a large-eddy simulation approach, this study investigates the response of the ocean surface boundary layer (OSBL) and Langmuir turbulence (LT) to extreme wind and complex wave forcing under tropical cyclones (TCs). The Stokes drift vector that drives LT is determined from spectral wave simulations. During maximum TC winds, LT substantially enhances the entrainment of cool water, causing rapid OSBL deepening. This coincides with relatively strong wave forcing, weak inertial currents, and shallow OSBL depth , measured by smaller ratios of , where denotes a Stokes drift decay length scale. LT directly affects a near-surface layer whose depth is estimated from enhanced anisotropy ratios of velocity variances. During rapid OSBL deepening, is proportional to , and LT efficiently transports momentum in coherent structures, locally enhancing shear instabilities in a deeper shear-driven layer, which is controlled by LT. After the TC passes, inertial currents are stronger and is greater while is shallower and proportional to . During this time, the LT-affected surface layer is too shallow to directly influence the deeper shear-driven layer, so that both layers are weakly coupled. At the same time, LT reduces surface currents that play a key role in the surface energy input at a later stage. These two factors contribute to relatively small TKE levels and entrainment rates after TC passage. Therefore, our study illustrates that inertial currents need to be taken into account for a complete understanding of LT and its effects on OSBL dynamics in TC conditions.
Knutson, Thomas R., et al., October 2019: Tropical Cyclones and Climate Change Assessment: Part I. Detection and Attribution. Bulletin of the American Meteorological Society, 100(10), DOI:10.1175/BAMS-D-18-0189.1. Abstract
We assess whether detectable changes in tropical cyclone activity have been identified in observations and whether any changes can be attributed to anthropogenic climate change.
An assessment was made of whether detectable changes in tropical cyclone (TC) activity are identifiable in observations and whether any changes can be attributed to anthropogenic climate change. Overall, historical data suggest detectable TC activity changes in some regions associated with TC track changes, while data quality and quantity issues create greater challenges for analyses based on TC intensity and frequency.
A number of specific published conclusions (case studies) about possible detectable anthropogenic influence on TCs were assessed using the conventional approach of preferentially avoiding Type I errors (i.e., overstating anthropogenic influence or detection). We conclude there is at least low-to-medium confidence that the observed poleward migration of the latitude of maximum intensity in the western North Pacific is detectable, or highly unusual compared to expected natural variability. Opinion on the author team was divided on whether any observed TC changes demonstrate discernible anthropogenic influence, or whether any other observed changes represent detectable changes.
The issue was then reframed by assessing evidence for detectable anthropogenic influence while seeking to reduce the chance of Type II errors (i.e., missing or understating anthropogenic influence or detection). For this purpose, we used a much weaker “balance of evidence” criterion for assessment. This leads to a number of more speculative TC detection and/or attribution statements, which we recognize have substantial potential for being false alarms (i.e., overstating anthropogenic influence or detection) but which may be useful for risk assessment. Several examples of these alternative statements, derived using this approach, are presented in the report.
Purpose of Review:
Tropical cyclones (TCs) are strongly influenced by the large-scale environment of the tropics and will, therefore, be modified by climate changes. Numerical simulations designed to understand the sensitivities of TCs to environmental changes have typically followed one of two approaches: single-storm domain sizes with convection-permitting resolution and uniform thermal boundary conditions or comprehensive global high-resolution (about 50 km in the horizontal) atmospheric general circulation model (GCM) simulations. The approaches reviewed here rest between these two and are an important component of hierarchical modelling of the atmosphere: aquaplanet TC simulations.
Recent Findings:
Idealized model configurations have revealed controls on equilibrium TC size in large-domain simulations of rotating radiative-convective equilibrium. Simulations that include differential rotation (spherical geometry) but retain uniform thermal forcing have revealed a new mechanism of TC propagation change via storm-scale dynamics and show a poleward shift in genesis in response to warming. Simulations with Earth-like meridional thermal forcing gradients have isolated competing influences on TC genesis via shifts in the atmospheric general circulation and the temperature dependence of TC genesis in the absence of mean circulation changes.
Summary:
Aquaplanet simulations of TCs with variants that include or inhibit certain processes have recently emerged as a research methodology that has advanced the understanding of the climatic controls on TC activity. Looking forward, idealized boundary condition model configurations can be used as a bridge between GCM resolution and convection-permitting resolution models and as a tool for identifying additional mechanisms through which climate changes influence TC activity.
Knutson, Thomas R., et al., March 2020: Tropical Cyclones and Climate Change Assessment: Part II. Projected Response to Anthropogenic Warming. Bulletin of the American Meteorological Society, 101(3), DOI:10.1175/BAMS-D-18-0194.1. Abstract
We assess model-projected changes in tropical cyclone activity for a 2°C anthropogenic warming. Medium-to-high confidence projections include increased tropical cyclone rainfall rates, intensity, and proportion of storms that reach Category 4-5 intensity globally.
Model projections of tropical cyclone (TC) activity response to anthropogenic warming in climate models are assessed. Observations, theory, and models, with increasing robustness, indicate rising global TC risk for some metrics -- that are projected to impact multiple regions.
A 2°C anthropogenic global warming is projected to impact TC activity as follows: i) The most confident TC-related projection is that sea level rise accompanying the warming will lead to higher storm inundation levels, assuming all other factors are unchanged. ii) For TC precipitation rates, there is at least medium-to-high confidence in an increase globally, with a median projected increase of 14%, or close to the rate of tropical water vapor increase with warming, at constant relative humidity. iii) For TC intensity, ten of 11 authors had at least medium-to-high confidence that the global average will increase. The median projected increase in lifetime maximum surface wind speeds is about 5% (range 1–10%) in available higher resolution studies. iv) For the global proportion (as opposed to frequency) of TCs that reach very intense (Category 4–5) levels, there is at least medium-to-high confidence in an increase, with a median projected change of +13%. Author opinion was more mixed and confidence levels lower for the following projections: v) a further poleward expansion of the latitude of maximum TC intensity in the western North Pacific; vi) a decrease of global TC frequency, as projected in most studies; vii) an increase in global very intense TC frequency (Category 4–5), seen most prominently in higher resolution models; and viii) a slowdown in TC translation speed.
Strong, Jeffrey D., Gabriel A Vecchi, and Paul Ginoux, May 2018: The Climatological Effect of Saharan Dust on Global Tropical Cyclones in a Fully Coupled GCM. Journal of Geophysical Research: Atmospheres, 123(10), DOI:10.1029/2017JD027808. Abstract
Climate in the tropical North Atlantic and West Africa is known to be sensitive to both the atmospheric burden and optical properties of aerosolized mineral dust. We investigate the global climatic response to an idealized perturbation in atmospheric burden of Saharan‐born mineral dust, comparable to the observed changes between the 1960's and 1980's, using simulations with the high resolution, fully coupled GFDL Climate Model 2.5, Forecast‐oriented Low Ocean Resolution version, across a range of realistic optical properties, with a specific focus on tropical cyclones. The direct radiative response at the top of the atmosphere (ToA) and at the surface along with regional hydrologic and thermodynamic responses are in agreement with previous studies, depending largely on the amount of aerosol absorption versus scattering. In all simulations, dust causes a decrease in tropical cyclone activity across the North Atlantic Ocean, as determined by a tropical cyclone tracking scheme, with the largest response occurring in the most absorbing and scattering optical regimes. These changes are partially corroborated by common local genesis potential indices. However, no clear‐cut explanation can be developed upon inspection of their constituent variables. There are also non‐negligible anomalies in the North Pacific and Indian Oceans in these simulations. A relationship between accumulated cyclone energy and ToA radiative flux anomalies is used to explain the North Atlantic anomalies, while analogy to known climate variations can help us understand the far‐field response to the dust forcing.
