High-resolution atmospheric models are powerful tools for hurricane track and intensity predictions. Although using high resolution contributes to better representation of hurricane structure and intensity, its value in the prediction of steering flow and storm tracks is uncertain. Here we present experiments suggesting that biases in the predicted North Atlantic hurricane tracks in a high-resolution (approximately 3 km grid-spacing) model originates from the model's explicit simulation of deep convection. Differing behavior of explicit convection leads to changes in the synoptic-scale pattern and thereby to the steering flow. Our results suggest that optimizing small-scale convection activity, for example, through the model's horizontal advection scheme, can lead to significantly improved hurricane track prediction (∼10% reduction of mean track error) at lead times beyond 72 hr. This work calls attention to the behavior of explicit convection in high-resolution models, and its often overlooked role in affecting larger-scale circulations and hurricane track 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.
In this paper, U.S. landfalling tropical cyclone (TC) activity is projected for the late twenty-first century using a two-step dynamical downscaling framework. A regional atmospheric model, is run for 27 seasons, to generate tropical storm cases. Each storm case is -resimulated (up to 15 days) using the higher-resolution Geophysical Fluid Dynamics Laboratory hurricane model. Thirteen CMIP3 or CMIP5 climate change scenarios are explored. Robustness of projections is assessed using statistical significance tests and comparing changes across models. The proportion of TCs making U.S. landfall increases for the warming scenarios, due, in part, to an increases in the percentage of TC genesis near the U.S. coast and a change in climatological steering flows favoring more U.S. landfall events. The increases in U.S. landfall proportion leads to an increase in U.S. landfalling category 4–5 hurricane frequency, averaging about + 400% across the models; 10 of 13 models/ensembles project an increase (which is statistically significant in three of 13 models). We have only tentative confidence in this latter increase, which occurs despite a robust decrease in Atlantic basin category 1–5 hurricane frequency, no robust change in Atlantic basin category 4–5 and U.S. landfalling category 1–5 hurricane frequency, and no robust change in U.S. landfalling hurricane intensities. Rainfall rates, averaged within a 100-km radius of the storms, are projected to increase by about 18% for U.S. landfalling TCs. Important caveats to the study include low correlation (skill) for interannual variability of modeled vs. observed U.S. TC landfall frequency and model bias of excessive TC genesis near and east of the U.S. east coast in present-day simulations.
We investigate the sensitivity of hurricane intensity and structure to the horizontal tracer advection in the Geophysical Fluid Dynamics Laboratory (GFDL) Finite-Volume Cubed-Sphere Dynamical Core (FV3). We compare two schemes, a monotonic scheme and a less diffusive positive-definite scheme. The positive-definite scheme leads to significant improvement in the intensity prediction relative to the monotonic scheme in a suite of 5-day forecasts that mostly consist of rapidly intensifying hurricanes. Notable storm structural differences are present: the radius of maximum wind (RMW) is smaller and eyewall convection occurs farther inside the RMW when the positive-definite scheme is used. Moreover, we find that the horizontal tracer advection scheme affects the eyewall convection location by affecting the moisture distribution in the inner-core region. This study highlights the importance of dynamical core algorithms in hurricane intensity prediction.
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.
We present the System for High‐resolution prediction on Earth‐to‐Local Domains (SHiELD), an atmosphere model developed by the Geophysical Fluid Dynamics Laboratory (GFDL) coupling the nonhydrostatic FV3 Dynamical Core to a physics suite originally taken from the Global Forecast System. SHiELD is designed to demonstrate new capabilities within its components, explore new model applications, and to answer scientific questions through these new functionalities. A variety of configurations are presented, including short‐to‐medium‐range and subseasonal‐to‐seasonal prediction, global‐to‐regional convective‐scale hurricane and contiguous U.S. precipitation forecasts, and global cloud‐resolving modeling. Advances within SHiELD can be seamlessly transitioned into other Unified Forecast System or FV3‐based models, including operational implementations of the Unified Forecast System. Continued development of SHiELD has shown improvement upon existing models. The flagship 13‐km SHiELD demonstrates steadily improved large‐scale prediction skill and precipitation prediction skill. SHiELD and the coarser‐resolution S‐SHiELD demonstrate a superior diurnal cycle compared to existing climate models; the latter also demonstrates 28 days of useful prediction skill for the Madden‐Julian Oscillation. The global‐to‐regional nested configurations T‐SHiELD (tropical Atlantic) and C‐SHiELD (contiguous United States) show significant improvement in hurricane structure from a new tracer advection scheme and promise for medium‐range prediction of convective storms.
Successful collaborations played a pivotal role in transitioning the GFDL hurricane research model into a long-standing state-of-the-art operational system that provided critical guidance for over 20 years.
The hurricane project at the NOAA Geophysical Fluid Dynamics Laboratory (GFDL) was established in 1970. By the mid 1970s pioneering research had led to the development of a new hurricane model. As the reputation of the model grew, GFDL was approached in 1986 by the director of the National Meteorological Center about establishing collaboration between the two Federal organizations to transition the model into an operational modeling system. After a multi-year effort by GFDL scientists to develop a system that could support rigorous requirements of operations, and multi-year testing had demonstrated its superior performance compared to existing guidance products, operational implementation was made in 1995. Through collaboration between GFDL and the US Navy, the model was also made operational at Fleet Numerical Meteorology and Oceanography Center in 1996. GFDL scientists continued to support and improve the model during the next two decades by collaborating with other scientists at GFDL, the NCEP Environmental Modeling Center (EMC), the National Hurricane Center, the US Navy, the University of Rhode Island (URI), Old Dominion University, and the NOAA Hurricane Research Division. Scientists at GFDL, URI, and EMC collaborated to transfer key components of the GFDL model to the NWS new Hurricane Weather and Research Forecast (HWRF) model that became operational in 2007. The purpose of the article is to highlight the critical role of these collaborations. It is hoped that the experiences of the authors will serve as an example of how such collaboration can benefit the nation with improved weather guidance products.
We use the fvGFS model developed at the Geophysical Fluid Dynamics Laboratory (GFDL) to demonstrate the potential of the upcoming United States Next Generation Global Prediction System for hurricane prediction. The fvGFS retrospective forecasts initialized with the European Centre for Medium‐Range Weather Forecasts (ECMWF) data showed much‐improved track forecasts for the 2017 Atlantic hurricane season compared to the best performing ECMWF operational model. The fvGFS greatly improved the ECMWF's poor track forecast for Hurricane Maria (2017). For Hurricane Irma (2017), a well‐predicted case by the ECMWF model, the fvGFS produced even lower 5‐day track forecast errors. The fvGFS also showed better intensity prediction than both the United States and the ECMWF operational models, indicating the robustness of its numerical algorithms.
A new global model using the GFDL nonhydrostatic Finite-Volume Cubed-Sphere Dynamical Core (FV3) coupled to physical parameterizations from the National Centers for Environmental Prediction's Global Forecast System (NCEP/GFS) was built at GFDL, named fvGFS. The modern dynamical core, FV3, has been selected for National Oceanic and Atmospheric Administration’s Next Generation Global Prediction System (NGGPS) due to its accuracy, adaptability, and computational efficiency, which brings a great opportunity for the unification of weather and climate prediction systems.
The performance of tropical cyclone (TC) forecasts in the 13-km fvGFS is evaluated globally based on 363 daily cases of 10-day forecasts in 2015. Track and intensity errors of TCs in fvGFS are compared to those in the operational GFS. The fvGFS outperforms the GFS in TC intensity prediction for all basins. For TC track prediction, the fvGFS forecasts are substantially better over the northern Atlantic basin and the northern Pacific Ocean than the GFS forecasts. An updated version of the fvGFS with the GFDL 6-category cloud microphysics scheme is also investigated based on the same 363 cases. With this upgraded microphysics scheme, fvGFS shows much improvement in TC intensity prediction over the operational GFS. Besides track and intensity forecasts, the performance of TC genesis forecast is also compared between the fvGFS and operational GFS. In addition to evaluating the hit/false alarm ratios, a novel method is developed to investigate the lengths of TC genesis lead times in the forecasts. Both versions of fvGFS show higher hit ratios, lower false alarm ratios and longer genesis lead times than those of the GFS model in most of the TC basins.
Heming, J T., F Prates, and Morris A Bender, et al., December 2019: Review of Recent Progress in Tropical Cyclone Track Forecasting and Expression of Uncertainties. Tropical Cyclone Research and Review, 8(4), DOI:10.1016/j.tcrr.2020.01.001. Abstract
The Ninth International Workshop on Tropical Cyclones (IWTC-9) took place in Hawaii, USA in December 2018. This review paper was presented at the Workshop under the Tropical Cyclone Track topic. The forecasting of tropical cyclone (TC) track has seen significant improvements in recent decades both by numerical weather prediction models and by regional warning centres who issue forecasts having made use of these models and other forecasting techniques. Heming and Goerss (2010) gave an overview of forecasting techniques and models available for TC forecasting, including evidence of the improvement in performance over the years. However, the models and techniques used for TC forecasting have continued to develop in the last decade. This presentation gives an updated overview of many of the numerical weather prediction models and other techniques used for TC track prediction. It includes recent performance statistics both by the models and the regional warning centres.
The 2017 Atlantic hurricane season had several high-impact tropical cyclones (TCs), including multiple cases of rapid intensification (RI). A high-resolution nested version of the GFDL fvGFS model (HifvGFS) was used to conduct hindcasts of all Atlantic TCs between August 7 and October 15.
HifvGFS showed promising track forecast performance, with similar error patterns and skill compared to the operational GFS and HWRF models. Some of the larger track forecast errors were associated with the erratic tracks of Jose and Lee. A case study of Maria found that although the track forecasts were generally skillful, a right-of-track bias was noted in some cases associated with initialization and prediction of ridging north of the storm.
The intensity forecasts showed large improvement over the GFS and global fvGFS models, but were somewhat less skillful than HWRF. The largest negative intensity forecast errors were associated with the RI of Irma, Lee, and Maria, while the largest positive errors were found with recurving cases that were generally weakening. The structure forecasts were also compared with observations, and HifvGFS was found to generally have wind radii larger than observations. Detailed examination of the forecasts of Hurricanes Harvey and Maria showed that HifvGFS was able to predict the structural evolution leading to RI in some cases, but was not as skillful with other RI cases. One case study of Maria suggested that inclusion of ocean coupling could significantly reduce the positive bias seen during and after recurvature.
The impact of storm size on the forecast of tropical cyclone storm track and intensity is investigated using the 2016 version of the operational GFDL hurricane model. Evaluation was made for 1,529 forecasts in the Atlantic, eastern Pacific, and western North Pacific basins, during the 2014 and 2015 seasons. The track and intensity errors were computed from forecasts in which the 34-knot wind radii obtained from the operational TC-Vitals that are used to initialize TCs in the GFDL model were replaced with wind radii estimates derived using an equally-weighted average of six objective estimates. It was found that modifying the radius of 34-knot winds had a significant positive impact on the intensity forecasts in the 1-2 day lead times. For example, at 48h, the intensity error was reduced 10%, 5% and 4% in the Atlantic, eastern Pacific, and western North Pacific, respectively. The largest improvements in intensity forecasts were for those tropical cyclones undergoing rapid intensification, with a maximum error reduction in the 1-2 day forecast lead time of 14 and 17% in the eastern and western North Pacific, respectively. The large negative intensity biases in the eastern and western North Pacific were also reduced 25% and 75% in the 12 to 72h forecast lead times. Although the overall impact on the average track error was neutral, forecasts of recurving storms were improved and tracks of non-recurving storms degraded. Results also suggest that objective specification of storm size may impact intensity forecasts in other high resolution numerical models, particularly for tropical cyclones entering a rapid intensification phase.
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.
Zhang, Banglin, R S Lindzen, Vijay Tallapragada, Fuzhong Weng, Q Liu, J A Sippel, Zaizhong Ma, and Morris A Bender, October 2016: Increasing vertical resolution in US models to improve track forecasts of Hurricane Joaquin with HWRF as an example. Proceedings of the National Academy of Sciences, 113(42), DOI:10.1073/pnas.1613800113. Abstract
The atmosphere−ocean coupled Hurricane Weather Research and Forecast model (HWRF) developed at the National Centers for Environmental Prediction (NCEP) is used as an example to illustrate the impact of model vertical resolution on track forecasts of tropical cyclones. A number of HWRF forecasting experiments were carried out at different vertical resolutions for Hurricane Joaquin, which occurred from September 27 to October 8, 2015, in the Atlantic Basin. The results show that the track prediction for Hurricane Joaquin is much more accurate with higher vertical resolution. The positive impacts of higher vertical resolution on hurricane track forecasts suggest that National Oceanic and Atmospheric Administration/NCEP should upgrade both HWRF and the Global Forecast System to have more vertical levels.
Global projections of intense tropical cyclone activity are derived from the Geophysical Fluid Dynamics Laboratory (GFDL) HiRAM (50 km grid) atmospheric model and the GFDL Hurricane Model using a two-stage downscaling procedure. First, tropical cyclone genesis is simulated globally using the HiRAM atmospheric model. Each storm is then downscaled into the GFDL Hurricane Model, with horizontal grid-spacing near the storm of 6 km, and including ocean coupling (e.g., ‘cold wake’ generation). Simulations are performed using observed sea surface temperatures (SSTs) (1980-2008); for a “control run” with 20 repeating seasonal cycles; and for a late 21st century projection using an altered SST seasonal cycle obtained from a CMIP5/RCP4.5 multi-model ensemble. In general agreement with most previous studies, projections with this framework indicate fewer tropical cyclones globally in a warmer late-21st-century climate, but also an increase in average cyclone intensity, precipitation rates, and in the number and occurrence-days of very intense category 4-5 storms. While these changes are apparent in the globally averaged tropical cyclone statistics, they are not necessarily present in each individual basin. The inter-basin variation of changes in most of the tropical cyclone metrics we examined is directly correlated to the variation in magnitude of SST increases between the basins. Finally, the framework is shown capable of reproducing both the observed global distribution of outer storm size--albeit with a slight high bias--and its inter-basin variability. Projected median size is found to remain nearly constant globally, with increases in most basins offset by decreases in the Northwest Pacific.
In this extended abstract, we report on progress in two areas of research at GFDL relating to Indian Ocean regional climate and climate change. The first topic is an assessment of regional surface temperature trends in the Indian Ocean and surrounding region. Here we illustrate the use of a multi-model approach (CMIP3 or CMIP5 model ensembles) to assess whether an anthropogenic warming signal has emerged in the historical data, including identification of where the observed trends are consistent or not with current climate models. Trends that are consistent with All Forcing runs but inconsistent with Natural Forcing Only runs are ones which we can attribute, at least in part, to anthropogenic forcing.
Twenty-first-century projections of Atlantic climate change are downscaled to explore the robustness of potential changes in hurricane activity. Multimodel ensembles using the phase 3 of the Coupled Model Intercomparison Project (CMIP3)/Special Report on Emissions Scenarios A1B (SRES A1B; late-twenty-first century) and phase 5 of the Coupled Model Intercomparison Project (CMIP5)/representative concentration pathway 4.5 (RCP4.5; early- and late-twenty-first century) scenarios are examined. Ten individual CMIP3 models are downscaled to assess the spread of results among the CMIP3 (but not the CMIP5) models. Downscaling simulations are compared for 18-km grid regional and 50-km grid global models. Storm cases from the regional model are further downscaled into the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model (9-km inner grid spacing, with ocean coupling) to simulate intense hurricanes at a finer resolution.
A significant reduction in tropical storm frequency is projected for the CMIP3 (−27%), CMIP5-early (−20%) and CMIP5-late (−23%) ensembles and for 5 of the 10 individual CMIP3 models. Lifetime maximum hurricane intensity increases significantly in the high-resolution experiments—by 4%–6% for CMIP3 and CMIP5 ensembles. A significant increase (+87%) in the frequency of very intense (categories 4 and 5) hurricanes (winds ≥ 59 m s−1) is projected using CMIP3, but smaller, only marginally significant increases are projected (+45% and +39%) for the CMIP5-early and CMIP5-late scenarios. Hurricane rainfall rates increase robustly for the CMIP3 and CMIP5 scenarios. For the late-twenty-first century, this increase amounts to +20% to +30% in the model hurricane’s inner core, with a smaller increase (~10%) for averaging radii of 200 km or larger. The fractional increase in precipitation at large radii (200–400 km) approximates that expected from environmental water vapor content scaling, while increases for the inner core exceed this level.
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.
The past decade has been marked by significant advancements in numerical weather prediction of hurricanes, which have greatly contributed to the steady decline in forecast track error. Since its operational implementation by the U.S. National Weather Service (NWS) in 1995, the best-track model performer has been NOAA’s regional hurricane model developed at the Geophysical Fluid Dynamics Laboratory (GFDL). The purpose of this paper is to summarize the major upgrades to the GFDL hurricane forecast system since 1998. These include coupling the atmospheric component with the Princeton Ocean Model, which became operational in 2001, major physics upgrades implemented in 2003 and 2006, and increases in both the vertical resolution in 2003 and the horizontal resolution in 2002 and 2005. The paper will also report on the GFDL model performance for both track and intensity, focusing particularly on the 2003 through 2006 hurricane seasons. During this period, the GFDL track errors were the lowest of all the dynamical model guidance available to the NWS Tropical Prediction Center in both the Atlantic and eastern Pacific basins. It will also be shown that the GFDL model has exhibited a steady reduction in its intensity errors during the past 5 yr, and can now provide skillful intensity forecasts. Tests of 153 forecasts from the 2004 and 2005 Atlantic hurricane seasons and 75 forecasts from the 2005 eastern Pacific season have demonstrated a positive impact on both track and intensity prediction in the 2006 GFDL model upgrade, through introduction of a cloud microphysics package and an improved air–sea momentum flux parameterization. In addition, the large positive intensity bias in sheared environments observed in previous versions of the model is significantly reduced. This led to the significant improvement in the model’s reliability and skill for forecasting intensity that occurred in 2006.
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, Morris A Bender, and Yoshio Kurihara, 2000: Typhoon forecast with the GFDL Hurricane Model: Forecast skill and comparison of predictions using AVN and NOGAPS global analyses. Journal of the Meteorological Society of Japan, 78(6), 777-788. Abstract PDF
A hurricane model developed at GFDL, NOAA, was combined with each of AVN and NOGAPS global analyses to construct typhoon prediction systems GFDS and GFDN, respectively. The GFDS system performed 125 (178) forecast experiments for 16 (24) storms in the western North Pacific basin during 1995 (1996). It exhibited considerable skill in the forecast of tropical cyclone tracks. The average forecast position errors as 12, 24, 36, 48 and 72 h in 1995 (1996) were 95 (108), 146 (178), 193 (227), 249 (280), and 465 (480) km. The improvement with GFDS in the typhoon position forecast over CLIPER was roughly 30%. The reduction of position errors in both average and standard deviations indicates superior forecast accuracy and consistency of GFDS, although there existed systematic northward bias in the forecast motion at low latitudes. On the other hand, intensity forecast was not satisfactory, showing a tendency to overpredict weak storms and underpredict strong storms, similar to the tendency in the Atlantic
Two sets of forecasts performed in the 1996 season, the one by GFDS and the other by GFDN, were compared with each other. Forecast skills of the storm position with the two systems were comparable. However, the two forecast positions tended to be systematically biased toward different directions. As a result, when the two forecasts were averaged, the mean error was 10% smaller than that of each forecast. Also, overall improvement in track forecast was obtained in supplemental experiments in which individual forecasts were corrected for systematic biases. Though systematic biases is not steady, there may be ways to utilize it for improvement of tropical cyclone forecasts.
Kurihara, Yoshio, Robert E Tuleya, and Morris A Bender, 1998: Application and improvement of the GFDL Hurricane Prediction System In Research Activities in Atmospheric and Oceanic Modelling, WMO/TD No. 865, Geneva, Switzerland, World Meteorological Organization, 5.31.
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.
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.
Bender, Morris A., C-C Wu, M A Rennick, and Yoshio Kurihara, 1997: Comparison of the GFDL Hurricane Model prediction in the Western Pacific using the NOGAPS and AVN Global Analysis In 22nd Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 615-616.
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, Robert E Tuleya, and Morris A Bender, 1997: Improvement of the GFDL Hurricane Prediction System In CAS/JSC Working Group on Numerical Experimentation - Research Activities in Atmospheric & Oceanic Modelling, Report No. 25, WMO/TD-No. 792, Geneva, Switzerland, World Meteorological Organization, 5.22.
Bender, Morris A., Robert E Tuleya, Yoshio Kurihara, and S J Lord, 1996: Results of the operational GFDL hurricane model at NCEP In 11th Conference on Numerical Weather Prediction, Boston, MA, American Meteorological Society, 393-395.
Ginis, Isaac, and Morris A Bender, 1996: Coupled tropical cyclone-ocean modeling 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, 9.10-9.11.
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.
Tuleya, Robert E., Morris A Bender, and Yoshio Kurihara, 1996: Prediction of hurricane landfall using the GFDL model In 11th Conference on Numerical Weather Prediction, Boston, MA, American Meteorological Society, 407-408.
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.
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.
The hurricane model initialization scheme developed at GFDL was modified to improve the representation of the environmental fields in the initial condition. The filter domain defining the extent of the tropical cyclone in the global analysis is determined from the distribution of the low-level disturbance winds. The shape of the domain is generally not circular in order to minimize the removal of important nonhurricane features near the storm region. An optimum interpolation technique is used to determine the environmental fields within the filter domain. Outside of the domain, the environmental fields are identical to the original global analysis. The generation process of the realistic and mode-compatible vortex has also undergone some minor modifications so that reasonable vortices are produced for various data conditions. The upgraded hurricane prediction system was tested for a number of cases and compared against the previous version and yielded an overall improvement in the forecasts of storm track. The system was run in an automated semioperational mode during the 1993 hurricane season for 36 cases in the Atlantic and 36 cases in the eastern Pacific basin. It demonstrated satisfactory skill in the storm track forecasts in many cases, including the abrupt recurvature of Hurricane Emily in the Atlantic and the landfall of Hurricane Lidia onto the Pacific coast of Mexico.
Bender, Morris A., Isaac Ginis, and Yoshio Kurihara, 1993: Numerical simulations of hurricane-ocean interaction with a high-resolution coupled model In 20th Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 563-566. PDF
Bender, Morris A., Isaac Ginis, and Yoshio Kurihara, 1993: Numerical simulations of tropical cyclone-ocean interaction with a high-resolution coupled model. Journal of Geophysical Research, 98(D12), 23,245-23,263. Abstract PDF
The tropical cyclone-ocean interaction was investigated using a high-resolution tropical cyclone ocean coupled model. The model design consisted of the NOAA Geophysical Fluid Dynamics Laboratory tropical cyclone prediction model which was coupled with a multilayer primitive equation ocean model. Coupling between the hurricane and the ocean models was carried out by passing into the ocean model the wind stress, heat, and moisture fluxes computed in the hurricane model. The new sea surface temperature (SST) calculated by the ocean model was then used in the tropical cyclone model. A set of idealized numerical experiments were performed in which a tropical cyclone vortex was embedded in both easterly and westerly basic flows of 2.5, 5, and 7.5 m s-1 with a fourth experiment run with no basic flow specified initially. The profile of the trangential wind for Hurricane Gloria at 1200 UTC 22, September 1985 was used as the initial condition of the tropical cyclone for each of the experiments. The model ocean was initially horizontally homogenous and quiescent. To clarify the impact of the ocean response to the hurricane's behavior, analogous experiments were also carried out with the SST kept constant (control cases). The experiments indicated that the cooling of the sea surface induced by the tropical cyclone resulted in a significant impact on the ultimate storm intensity due to the reduction of total heat flux directed into the tropical cyclone above the regions of decreased SST. The sea surface cooling produced by the tropical cyclones was found to be larger when the storms moved slower. In the experiments run without an initial basic flow,the maximum SST anomaly was about -5.6°C with a resulting difference in the minimum sea level pressure and maximum surface winds of 16.4 hPa and -7 m s-1, respectively. In contrast, in the experiments run with the 7.5 ms-1 basic flow, the maximum SST anomalies ranged from about 2.6° to 3.0°C with a difference in the minimum sea level pressure and maximum surface winds of about 7.3 hPa and -2.7 m s-1. The tropical cyclone-ocean coupling significantly influenced the storm track only for the case with no basic flow and the 2.5 m s-1 easterly flow. In these cases the storm with the ocean interaction turned more to the north and east (no basic flow) or the north (2.5 m s-1 easterly flow) of the experiments with constant SST. In the first case, the storm by 72 hours was located over 70 km to the east-southeast of the control case. A possible explanation for this track deviation is related to a systematic weakening of the mean tangential flow at all radii of the storm due to the interaction with the ocean and resulting alteration of the beta drift.
The initialization scheme designed at GFDL to specify a more realistic initial storm structure of tropical cyclones was tested on four real data cases using the GFDL high-resolution multiply nested movable mesh hurricane model. Three of the test cases involved Hurricane Gloria (1985) in the Atlantic basin; the fourth involved Hurricane Gilbert (1988) in the Gulf of Mexico. The initialization scheme produced an initial vortex that was well adapted to the forecast model and was much more realiztic in size and intensity than the storm structure obtained from the NMC T80 global analysis. As a result, the erratic storm motion seen in previous intergrations of the GFDL model has been nearly eliminated with dramatic improvements in track forecasts during the first 48 h of the prediction. Using the new scheme, the average 24-h and 48-h forecast error for the four test cases was 58 and 94 km, respectively, compared with 143 and 191 km for the noninitialized forecasts starting from the global analysis. The average National Hurricane Center operational forecast error at 24 and 48 h was 118 and 212 km for the same four cases. After 48 h the difference in the average track error became small between the integrations starting from the global analysis and the forecasts starting from the fields obtained by the initialization scheme
With accurate specification of the initial vortex structure, changes in the storm intensity were also well predicted in these cases. The model correctly forecasted the rapid intensification of Hurricane Gloria just after the system was first upgraded to a hurricane. The model storm intensification also ceased at approximately the same time as observed, with gradual weakening as the storm moved north and approached the east coast of the United States. In the forecast of Hurricane Gilbert, the model storm initially weakened as it moved over the Yucatan Peninsula and underwent only moderate reintensification after moving over the Gulf of Mexico, in good agreement with observations
Finally, in the case where the track of Hurricane Gloria was well forecast, the distribution of the maximum low-level winds was accurately predicted as the storm moved up the east coast of the United States. During this period the model successfully reproduced many observed features such as large asymmetries in the wind field, with strongest winds occurring well east of the storm center, and a sharp decrease of the wind speed at the coast. Although asymmetry in the wind distribution was reproduced to a first order in the forecast starting with the global analysis, the agreement with observations was much better with the specified vortex, primarily due to a more realistic radius of maximum wind and storm intensity.
Kurihara, Yoshio, Morris A Bender, and R Ross, 1993: An initialization scheme of hurricane models by vortex specification. Monthly Weather Review, 121(7), 2030-2045. Abstract PDF
A scheme is presented to improve the representation of a tropical cyclone in the initial condition of a high-resolution hurricane model. In the proposed method, a crudely resolved tropical cyclone in the large-scale analysis is replaced by a vortex that is properly specified for use in the prediction model.
Appropriate filters are used to remove the vortex from the large-scale analysis so that a smooth environmental field remains. The new specified bogus vortex takes the form of a deviation from the environmental field so that it can be easily merged with the latter field at the correct position. The specified vortex consists of both axisymmetric and asymmetric components. The symmetric component is generated by the time integration of an axisymmetric version of the hurricane prediction model. This ensures dynamical and thermodynamical consistency in the vortex structure, including the moisture field, and also compatibility of the vortex with the resolution and physics of the hurricane model. In the course of the integration of the axisymmetric model, the tangential wind component is gradually forced to a target wind profile determined from observational information and empirical knowledge. This makes the symmetric vortex a good approximation to the corresponding real tropical cyclone. The symmetric flow thus produced is used to generate an asymmetric wind field by the time integration of a simplified barotropic vorticity equation, including the beta effect. The asymmetric wind field, which can make a significant contribution to the vortex motion, is then added to the symmetric flow. After merging the specified vortex with the environmental flow, the mass field is diagnosed from the divergence equation with an appropriately controlled time tendency. The wind field remains unchanged at this step of initialization.
Since the vortex specified by the proposed method is well adapted to the hurricane prediction model, problems of initial adjustment and false spinup of the model vortex, a long-standing difficulty in the dynamical prediction of tropical cyclones, are alleviated. It is anticipated that the improvement of the initial conditions can reduce the error in hurricane track forecasting and extend the feasibility of tropical cyclone forecasting to intensify change.
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.
Kurihara, Yoshio, Morris A Bender, Robert E Tuleya, and R Ross, 1993: Hurricane forecasting with the GFDL automated prediction system In 20th Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 323-326.
Bender, Morris A., R Ross, Yoshio Kurihara, and Robert E Tuleya, 1991: Improvements in tropical cyclone track and intensity forecasts using a bogus vortex In 19th Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 324-325. PDF
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.
The prediction capability of the GFDL triply nested, movable mesh model, with finest grid resolution of 1/6 degree, was investigated using several case studies of Hurricane Gloria (1985) during the period that the storm approached and moved up the east coast of the United States. The initial conditions for these experiments were interpolated from an NMC T80 global analysis at 0000 UTC 25 September and 1200 UTC 22 September. The integrations starting from 0000 UTC 25 September were run 72 h, while those starting on 1200 UTC 22 September were run 132 h. The lateral boundary conditions were obtained from either an integration of the NMC T80 forecast model or the T80 global analysis, or were fixed to the initial value.
The model's predicted track of Gloria for each integration was compared against the best track determined by the National Hurricane Center (NHC). For the case starting from 0000 UTC 25 September using a forecasted boundary condition, the model successfully forecasted significant acceleration of the storm's movement after 48 h. The 72 h forecast error was about 191 km, compared to 480 km for the official track forecast made by the NHC.
To examine the model's skill in simulating the storm structure, distributions of the low level maximum wind and total storm rainfall during passage of the model storm are shown and compared with observed values. The model successfully reproduced many observed features such as the occurrence of strong winds well east of the storm center, with an abrupt decrease of the wind field along the coastline. When the storm track was accurately forecasted, the total storm rainfall amounts agreed well with the observed values. In both the model integration and observations, a significant structural change took place as the storm accelerated toward the north with little significant precipitation occurring south of the storm center and heavy precipitation spreading well north of the storm. It appears that the gross features of the structure of the storm's outer region resulted from the interaction of the vortex with its environment.
Sensitivity of the model forecast to the lateral boundary condition and the horizontal resolution was also investigated. The storm's track error was greatly affected after the boundary error propagated by advection to the storm region. The impact of the horizontal resolution on the forecast was such that the model with one degree resolution produced a fairly good track forecast up to 48 h, but failed to simulate some of the main structural features.
In the experiments starting from the 0000 UTC September 25 initial field, the interior storm structure did not develop, and the storm exhibited too large a radius of maximum wind throughout the integration. However, the integrations starting from 1200 UTC September 22 developed a more intense storm, with a more realistic radius of maximum wind. These differences were due to the spinup time necessary for the storm to develop in the model when starting from a coarse resolution global analysis which did not adequately resolve the fine structure of the storm interior. This indicates the importance of proper specification of the storm in the initial field.
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.
Kurihara, Yoshio, Christopher Kerr, and Morris A Bender, 1989: An improved numerical scheme to treat the open lateral boundary of a regional model. Monthly Weather Review, 117(12), 2714-2722. Abstract PDF
A numerical scheme proposed by Kurihara and Bender is modified so as to improve the behavior of open lateral boundaries of a regional model. In the new scheme, both the local values and the gradients of fields from a larger model are used to define the time-dependent reference values toward which the boundary gridpoint values of the regional model prediction are relaxed at each step of the model integration. Use of the gradients in the boundary forcing imposes constraints on the vorticity, divergence and baroclinicity fields for the regional model. The relaxation time of forcing is set to be short for the normal component of wind. For other variables, the relaxation time at a given boundary gridpoint depends on the wind direction at that gridpoint, with a minimum at a point of normal inflow and a maximum at a point of normal outflow. The forcing strength is reduced in the planetary boundary layer so that the boundary layer structure is determined mainly by the surface condition of the regional model. Also, a simple method to control the total mass in the regional model is described. Numerical results from 96-hour integrations with the improved scheme are compared with those from the previous scheme for the cases of the propagations of a wave and a vortex. The behavior of the model at the lateral boundary was noticeably improved with the use of the new scheme, while the solution in the interior domain was little affected by the scheme modification.
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.
Bender, Morris A., Robert E Tuleya, and Yoshio Kurihara, 1985: A numerical study of the effect of a mountain range on a landfalling tropical cyclone In 16th Conference on Hurricanes & Tropical Meteorology, Boston. MA, American Meteorological Society, 146-147.
A triply-nested, movable mesh model was used to study the effects of a mountain range on a landfalling tropical cyclone embedded in an easterly flow of ~ 10 m s-1. The integration domain consisted of a 37 degree wide and 45 degree long channel, with an innermost mesh resolution of 1/6 degrees. An idealized mountain range with maximum height of ~ 958 meters was placed parallel to the shoreline. The mountain range, which spanned 19 degrees in the north-south direction and 5 degrees in the east-west direction, was centered in the middle of the channel. Results obtained were compared with a previous landfall simulation, performed without the effect of the mountain range included. In particular, comparison was made of the total storm rainfall, maximum wind distribution and storm decay rate. It was found that the storm filled much more rapidly in the simulation run with the mountain included. The mountain range affected the decay rate through reduction in the supply of latent and kinetic energy into the storm circulation during, as well as after, passage of the storm over the mountain. It was found that a low-level, warm and dry region was produced where the storm winds descended the mountain slope.
In order to better isolate the effect of the mountain on the basic easterly flow, a supplemental integration was performed for the flow without the storm. It revealed that the mountain range caused a significant change in the basic flow over the mountain as well as up to several hundred kilometers downstream and extending considerably above the mountain top. A low-level southerly jet was observed to the west of the mountain base.
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.
Bender, Morris A., and Yoshio Kurihara, 1983: The energy budgets for the eye and eye wall of a numerically simulated tropical cyclone. Journal of the Meteorological Society of Japan, 61(2), 239-244. Abstract PDF
Energy budgets are analyzed for a tropical cyclone simulated previously in a quadruply nested mesh model (Kurihara and Bender, 1982). It will be shown that the eddy kinetic energy within the eye is comparable in magnitude to that of the mean kinetic energy. It is supplied by import from the eye wall regions as well as by the conversion from total potential energy. At the same time it is converted to the kinetic energy of the mean flow and also lost by the dissipation. The influx of mean kinetic energy from the outer radii to the eye wall region and the export of potential energy both to the outer radii and to the eye region play important roles in the energetics of the eye wall region. Many obtained features agree well with those of a coarser resolution model (Tuleya and Kurihara, 1975) in which the eye of the vortex could not be resolved. This suggests that the eye structure has little impact on the energetics to the eye wall and outer regions of a tropical cyclone.
Kurihara, Yoshio, and Morris A Bender, 1983: A numerical scheme to treat the open lateral boundary of a limited area model. Monthly Weather Review, 111(3), 445-454. Abstract PDF
A numerical scheme to treat the open lateral boundary of a limited-area primitive equation model was formulated. Although overspecification of the boundary condition is inevitable in the pointwise boundary setting, the scheme was designed to keep the overspecification to a minimum degree. To impose the boundary conditions, a damping technique was used. Special care was taken to deal with the boundary layer winds at the lateral boundary. The above scheme is most suitable when gravity waves do not prevail in the vicinity of the open boundary.
The scheme was tested in the numerical integrations of prognostic equations for a Haurwitz-type wave. Experimental results are presented which indicate the utility of the proposed method.
Tuleya, Robert E., Morris A Bender, and Yoshio Kurihara, 1983: A numerical study of simulated hurricane landfall In Sixth Conference on Numerical Weather Prediction, Boston, MA, American Meteorological Society, 323-324.
Kurihara, Yoshio, and Morris A Bender, 1982: Structure and analysis of the eye of a numerically simulated tropical cyclone. Journal of the Meteorological Society of Japan, 60(1), 381-395. Abstract PDF
A tropical cyclone has been simulated in a quadruply nested mesh model with finest grid resolution of about 5 km. At the center of the vortex, a compact eye was maintained.
Azimuthal means as well as asymmetry of the eye and the eye wall structure are described. The asymmetric features within the eye wall moved cyclonically at a much smaller rotation rate than the cyclonic wind within the eye wall. Roles of the mean radial-vertical circulation, the eddy motions and the diffusion effect in the maintenance of the mean structure are analyzed. In the analysis, attention is given to the balance between the wind and pressure fields and also to the budgets of relative angular momentum, heat and water vapor. The eddy motions caused a cooling and moistening effect in the eye which counterbalanced a warming and drying effect due to the mean sinking motion.
The mesh nesting strategy proposed by Kurihara et al. (1979) was used to construct a movable, nested-mesh, 11-level primitive equation model. The framework of the model is described in detail.
With the use of a triply nested mesh system with 1 degree, 1/3 degree and 1/6 degree longitude-latitude resolution, a small intense dry vortex in a zonal flow of 10 m s-1 was successfully advected for 48 h. The shape of the vortex was well preserved during the time integration which involved over 50 movements of the innermost mesh. The noise, which was excited when a mesh moved, was suppressed in ~4 minutes after the movement. For comparison, the results from similar experiments performed with reduced inner mesh resolutions are also presented.
Kurihara, Yoshio, and Morris A Bender, 1979: Supplementary note on "A scheme of dynamic initialization of the boundary layer in a primitive equation model". Monthly Weather Review, 107(9), 1219-1221. Abstract PDF
A scheme is presented for improving the previously proposed method of dynamic initialization of the boundary layer in a primitive equation model (Kurihara and Tuleya, 1978). Performance of the revised scheme is shown for the case of a strong vortex superposed on a zonal flow.
A numerical scheme to construct a two-way, movable, nested-mesh primitive equation model is proposed. Dynamical coupling in a two-way nesting system is performed at a dynamical interface which is separated from a mesh interface by two coarse-grid intervals. Dynamical interaction is achieved by a method which conserves mass, momentum and internal energy of the system. During the course of integration, the nested mesh moves so that the central position of the disturbance contained in the fine-mesh area never deviates from the center of the nest by more than one coarse-mesh interval. New grid data near the leading and trailing edges of the moving nest are obtained by an interpolation method which has a conservation property. The proposed methods of dynamical coupling and mesh movement were extensively tested by a one-dimensional shallow water equation model. Numerical results of these experiments are presented.
