Projections of future tropical cyclone frequency are uncertain, ranging from a slight increase to a considerable decrease according to climate models. Estimation of how much the Earth’s surface temperature warms in response to greenhouse gas increase, quantified by effective climate sensitivity, is also uncertain. These two uncertainties have historically been studied independently as they concern different scales: One quantifies the extreme weather and the other the mean climate. Here, we show that these two uncertainties are not independent and are both influenced by the response of tropical clouds to warming. Across climate models, we show an anticorrelation between shortwave cloud radiative feedback and changes in the frequency of seed vortices, a prevalent type of tropical cyclone precursors. We further show an anticorrelation between effective climate sensitivity and tropical cyclone frequency changes, suggesting that global tropical cyclone frequency tends to decrease more substantially in models with larger temperature increase.
When evaluating the effect of carbon dioxide (CO2) changes on Earth’s climate, it is widely assumed that instantaneous radiative forcing from a doubling of a given CO2 concentration (IRF2×CO2) is constant and that variances in climate sensitivity arise from differences in radiative feedbacks or dependence of these feedbacks on the climatological base state. Here, we show that the IRF2×CO2 is not constant, but rather depends on the climatological base state, increasing by about 25% for every doubling of CO2, and has increased by about 10% since the preindustrial era primarily due to the cooling within the upper stratosphere, implying a proportionate increase in climate sensitivity. This base-state dependence also explains about half of the intermodel spread in IRF2×CO2, a problem that has persisted among climate models for nearly three decades.
The response of tropical cyclone (TC) frequency to sea surface warming is uncertain in climate models. We hypothesize that one source of uncertainty is the anomalies of large-scale atmospheric radiation in response to climate change, and whose influence on TC frequency is investigated. Given two atmospheric models with opposite TC frequency responses to uniform sea surface warming, we interchange their atmospheric radiation anomalies in experiments with prescribed radiative heating rates. The largest model discrepancy occurs in the western North Pacific, where the TC frequency tends to increase with anomalous large-scale ascent caused by prescribed positive radiation anomalies, while the TC frequency tends to decrease with anomalous large-scale descent caused by prescribed negative radiation anomalies. The model spread in TC frequency response is approximated by the model spread in the frequency response of pre-TC vortices (seeds), which is explained by changes in the large-scale circulation using a downscaling formula known as the seed propensity index. We further generalize the index to predict the influence of large-scale radiation anomalies on TC seed frequency. The results show that model spread in TC and seed frequency response can be reduced when constraining the large-scale radiation anomalies.
Zhang, Bosong, Brian J Soden, and Gabriel A Vecchi, February 2023: A vertically resolved analysis of radiative feedbacks on moist static energy variance in tropical cyclones. Journal of Climate, 36(4), DOI:10.1175/JCLI-D-22-0199.11125-1141. Abstract
A vertically resolved moist static energy (MSE) variance budget framework is used to diagnose processes associated with the development of tropical cyclones (TCs) in a general circulation model (GCM) under realistic boundary conditions. Previous studies have shown that interactions between radiation and MSE promote TC development. Here, we examine the vertical contributions of radiation and its interactions with MSE by performing several mechanism-denial experiments in which synoptic-scale radiative interactions are suppressed either in the boundary layer or in the free troposphere. Partly suppressing radiative interactions results in a reduction in global TC frequency. However, the magnitude of reduction and structure of the feedback depend on the intensity and structure of the TCs in these mechanism-denial experiments, indicating that both the magnitude and the vertical location of radiative interactions can impact global TC frequency. Using instantaneous 6-hourly outputs, an explicit computation reveals distinct spatial patterns of the advection term: the vertical component is positive in the mid- to upper troposphere, which reflects an upward transport of MSE by deep convection, whereas the horizontal component is positive in the boundary layer. These results illustrate the impact of the vertical distribution of radiative interactions and vertically varied contribution of the advection term in the development of TCs.
This study investigates how climate sensitivity depends upon the spatial pattern of radiative forcing. Sensitivity experiments using a coupled ocean-atmosphere model were conducted by adding anomalous incoming solar radiation over the entire globe, Northern Hemisphere mid-latitudes, Southern Ocean, and tropics. The varied forcing patterns led to highly divergent climate sensitivities. Specifically, the climate is nearly twice as sensitive to Southern Ocean forcing as tropical forcing. Strong coupling between the surface and free troposphere in the tropics increases the inversion strength, leading to smaller cloud feedback in the tropical forcing experiments. In contrast, the extratropics exhibit weaker coupling, a decrease or near-zero change in the inversion strength, and strong positive cloud feedback. These results contrast with the conventional SST-pattern effect in which tropical surface temperature changes regulate climate sensitivity. They also have important implications for other potentially asymmetric forcings, such as those from geoengineering, volcanic eruptions, and paleoclimatic changes.
Wang, Chenggong, Brian J Soden, Wenchang Yang, and Gabriel A Vecchi, February 2021: Compensation between cloud feedback and aerosol-cloud interaction in CMIP6 models. Geophysical Research Letters, 48(4), DOI:10.1029/2020GL091024. Abstract
The most recent generation of climate models (the 6th Phase of the Coupled Model Intercomparison Project) yields estimates of effective climate sensitivity (ECS) that are much higher than past generations due to a stronger amplification from cloud feedback. If plausible, these models require substantially larger greenhouse gas reductions to meet global warming targets. We show that models with a more positive cloud feedback also have a stronger cooling effect from aerosol-cloud interactions. These two effects offset each other during the historical period when both aerosols and greenhouse gases increase, allowing either more positive or neutral cloud feedback models to reproduce the observed global-mean temperature change. Since anthropogenic aerosols primarily concentrate in the Northern Hemisphere, strong aerosol-cloud interaction models produce an interhemispheric asymmetric warming. We show that the observed warming asymmetry during the mid to late 20th century is more consistent with low ECS (weak aerosol indirect effect) models.
Zhang, Bosong, Brian J Soden, Gabriel A Vecchi, and Wenchang Yang, November 2021: Investigating the causes and impacts of convective aggregation in a high resolution atmospheric GCM. Journal of Advances in Modeling Earth Systems, 13(11), DOI:10.1029/2021MS002675. Abstract
A ∼50 km resolution atmospheric general circulation model (GCM) is used to investigate the impact of radiative interactions on spatial organization of convection, the model's mean state, and extreme precipitation events in the presence of realistic boundary conditions. Mechanism-denial experiments are performed in which synoptic-scale feedbacks between radiation and dynamics are suppressed by overwriting the model-generated atmospheric radiative cooling rates with its monthly varying climatological values. When synoptic-scale radiative interactions are disabled, the annual mean circulation and precipitation remain almost unchanged, however tropical convection becomes less aggregated, with an increase in cloud fraction and relative humidity in the free troposphere but a decrease in both variables in the boundary layer. Changes in cloud fraction and relative humidity in the boundary layer exhibit more sensitivity to the presence of radiative interactions than variations in the degree of aggregation. The less aggregated state is associated with a decrease in the frequency of extreme precipitation events, coincident with a decrease in the dynamical contribution to the magnitude of extreme precipitation. At regional scales, the spatial contrast in radiative cooling between dry and moist regions diminishes when radiative interactions are suppressed, reducing the upgradient transport of energy, degree of aggregation, and frequency of extreme precipitation events. However, the mean width of the tropical rain belt remains almost unaffected when radiative interactions are disabled. These results offer insights into how radiation-circulation coupling affects the spatial organization of convection, distributions of clouds and humidity, and weather extremes.
The past few years have seen a growing investment in the development of global eddy‐resolving ocean models, but the impact of incorporating such high ocean resolution on precipitation responses to CO2 forcing has yet to be investigated. This study analyzes precipitation changes from a suite of GFDL models incorporating eddy‐resolving (0.1o), eddy‐permitting (0.25o) and eddy‐parameterizing (1o) ocean models. The incorporation of eddy resolution does not challenge the large‐scale structure of precipitation changes but results in substantial regional differences, particularly over ocean. These oceanic differences are primarily driven by the pattern of SST changes with greater sensitivity in lower latitudes. The largest impact of ocean resolution on SST changes occurs in eddy rich regions (e.g., boundary currents and the Southern Ocean), where impact on precipitation changes is also found to various degrees. In the Gulf Stream region where previous studies found considerable impact of eddy resolution on the simulation of climatological precipitation, we do not find such impact from the GFDL models but we do find substantial impact on precipitation changes. The eddy‐parameterizing model projects a banded structure common to the CMIP5 models, whereas the higher‐resolution models project a poleward shift of precipitation maxima associated with an enhanced Gulf Stream warming. Over land, precipitation changes are generally not very sensitive to ocean resolution. In eastern North America adjacent to the Gulf Stream region, moderate differences are found between resolutions. We discuss the mechanisms of land differences, which arise through the simulation of both climatological SST and SST changes.
He, Jie, and Brian J Soden, January 2017: A re-examination of the projected subtropical precipitation decline. Nature Climate Change, 7(1), DOI:10.1038/nclimate3157. Abstract
A large-scale precipitation decline in the subtropics is a widely accepted projection of future climate change1, 2, 3, but its causes and implications are uncertain. Two mechanisms are commonly used to explain the large-scale subtropical precipitation decline: an amplification of moisture export due to the increase in moisture4 and a poleward shift of subtropical subsidence associated with the poleward expansion of the Hadley cell5, 6. In an idealized experiment with abrupt CO2 increase, we find that the subtropical precipitation decline forms primarily in the fast adjustment to CO2 forcing during which neither of the two proposed mechanisms exists. Permitting the increase in moisture and the Hadley cell expansion does not substantially change the characteristics of the large-scale subtropical precipitation decline. This precipitation change should be interpreted as a response to the land–sea warming contrast, the direct radiative forcing of CO2 and, in certain regions, the pattern of SST changes. Moreover, the subtropical precipitation decline is projected predominately over oceans. Over subtropical land regions, the precipitation decline is muted or even reversed by the land–sea warming contrast.
He, Jie, Clara Deser, and Brian J Soden, May 2017: Atmospheric and Oceanic Origins of Tropical Precipitation Variability. Journal of Climate, 30(9), DOI:10.1175/JCLI-D-16-0714.1. Abstract
The intrinsic atmospheric and ocean-induced tropical precipitation variability is studied using millennial control simulations with various degrees of ocean coupling. A comparison between the coupled simulation and the atmosphere-only simulation with climatological sea surface temperatures (SSTs) shows that a substantial amount of tropical precipitation variability is generated without oceanic influence. This intrinsic atmospheric variability features a red noise spectrum from daily to monthly time scales and a white noise spectrum beyond the monthly time scale. The oceanic impact is inappreciable for submonthly time scales but important at interannual and longer time scales. For time scales longer than a year, it enhances precipitation variability throughout much of the tropical oceans and suppresses it in some subtropical areas, preferentially in the summer hemisphere. The sign of the ocean-induced precipitation variability can be inferred from the local precipitation–SST relationship, which largely reflects the local feedbacks between the two, although nonlocal forcing associated with El Niño–Southern Oscillation also plays a role. The thermodynamic and dynamic nature of the ocean-induced precipitation variability is studied by comparing the fully coupled and slab ocean simulations. For time scales longer than a year, equatorial precipitation variability is almost entirely driven by ocean circulation, except in the Atlantic Ocean. In the rest of the tropics, ocean-induced precipitation variability is dominated by mixed layer thermodynamics. Additional analyses indicate that both dynamic and thermodynamic oceanic processes are important for establishing the leading modes of large-scale tropical precipitation variability. On the other hand, ocean dynamics likely dampens tropical Pacific variability at multidecadal time scales and beyond.
We examine the change in tropical cyclone (TC) tracks that result from projected changes in the large-scale steering flow and genesis location due to increasing greenhouse gases. Tracks are first simulated using a Beta and Advection Model (BAM) and NCEP-NCAR Reanalysis winds for all TCs that formed in the North Atlantic main development region (MDR) for the period 1950-2010. Changes in genesis location and large-scale steering flow are then estimated from an ensemble mean of 17 CMIP3 models for the A1b emissions scenario. The BAM simulations are then repeated with these changes to estimate how the TC tracks would respond to increased greenhouse gases. As the climate warms, the models project a weakening of the subtropical easterlies as well as an eastward shift in genesis location. This results in a statistically significant decrease in straight-moving (westward) storm tracks of 5.5% and an increase in recurving (open ocean) tracks of 5.5%. These track changes decrease TC counts over the Southern Gulf of Mexico and Caribbean by 1-1.5 per decade and increase TC counts over the central Atlantic by 1-1.5 per decade. Changes in the large-scale steering flow account for a vast majority of the projected changes in TC trajectories.
The response of the Walker circulation to Last Glacial Maximum (LGM) forcing
is analyzed using an ensemble of six coordinated coupled climate model experiments.
The tropical atmospheric overturning circulation strengthens in all models in a manner
that is dictated by the response of the hydrological cycle to tropical cooling. This
response arises from the same mechanism that has been found to explain the weakening
of the tropical circulation in response to anthropogenic global warming, but with opposite
sign. Analysis of the model differences shows that the ascending branch of the Walker
circulation strengthens via this mechanism, but vertical motion also weakens over areas
of the Maritime Continent exposed due to lower sea level. Each model exhibits a
different balance between these two mechanisms, and the result is a Pacific Walker
circulation response that is not robust. Further, even those models that simulate a stronger
Walker circulation during the LGM do not simulate clear patterns of surface cooling,
such as La Niña-like cooling or enhanced equatorial cooling, as proposed by previous
studies. In contrast, the changes in the Walker circulation have a robust and distinctive
signature on the tilt of the equatorial thermocline, as expected from zonal momentum
balance. The changes in the Walker circulation also have a clear signature on the spatial
pattern of the precipitation changes. A reduction of the east-west salinity contrast in the
Indian Ocean is related to the precipitation changes resulting from a weakening of the
Indian Walker circulation. These results indicate that proxies of thermocline depth and
sea surface salinity can be used to detect actual LGM changes in the Pacific and Indian
Walker circulations, respectively and help constrain the sensitivity of the Walker
circulation to tropical cooling.
We assess the vertical distribution of cloud feedbacks in coupled climate models, taking care to distinguish between cloud feedbacks and a change in cloud forcing. We show that the effect of cloud changes on the longwave fluxes provides a strong positive feedback that is broadly consistent across models. In contrast, the effect of cloud changes on the shortwave fluxes ranges from a modest negative to a strong positive feedback, and is responsible for most of the intermodel spread in net cloud feedback. The feedback from high clouds is positive in all models, and is consistent with that anticipated by the Proportionately Higher Anvil Temperature hypothesis over the tropics. In contrast, low cloud cover is responsible for roughly three-quarters of the difference in global mean net cloud feedback among models, with the largest contributions from regions associated with low-level subtropical marine cloud systems.
The climate response of the equatorial Pacific to increased greenhouse gases is investigated using numerical experiments from 11 climate models participating in the Intergovernmental Panel on Climate Change’s Fourth Assessment Report. Multimodel mean climate responses to CO2 doubling are identified and related to changes in the heat budget of the surface layer. Weaker ocean surface currents driven by a slowing down of the Walker circulation reduce ocean dynamical cooling throughout the equatorial Pacific. The combined anomalous ocean dynamical plus radiative heating from CO2 is balanced by different processes in the western and eastern basins: Cloud cover feedbacks and evaporation balance the heating over the warm pool, while increased cooling by ocean vertical heat transport balances the warming over the cold tongue. This increased cooling by vertical ocean heat transport arises from increased near-surface thermal stratification, despite a reduction in vertical velocity. The stratification response is found to be a permanent feature of the equilibrium climate potentially linked to both thermodynamical and dynamical changes within the equatorial Pacific. Briefly stated, ocean dynamical changes act to reduce (enhance) the net heating in the east (west). This explains why the models simulate enhanced equatorial warming, rather than El Niño–like warming, in response to a weaker Walker circulation. To conclude, the implications for detecting these signals in the modern observational record are discussed.
The extent to which the climate will change due to an external forcing depends largely on radiative feedbacks, which act to amplify or damp the surface temperature response. There are a variety of issues that complicate the analysis of radiative feedbacks in global climate models, resulting in some confusion regarding their strengths and distributions. In this paper, the authors present a method for quantifying climate feedbacks based on “radiative kernels” that describe the differential response of the top-of-atmosphere radiative fluxes to incremental changes in the feedback variables. The use of radiative kernels enables one to decompose the feedback into one factor that depends on the radiative transfer algorithm and the unperturbed climate state and a second factor that arises from the climate response of the feedback variables. Such decomposition facilitates an understanding of the spatial characteristics of the feedbacks and the causes of intermodel differences. This technique provides a simple and accurate way to compare feedbacks across different models using a consistent methodology. Cloud feedbacks cannot be evaluated directly from a cloud radiative kernel because of strong nonlinearities, but they can be estimated from the change in cloud forcing and the difference between the full-sky and clear-sky kernels. The authors construct maps to illustrate the regional structure of the feedbacks and compare results obtained using three different model kernels to demonstrate the robustness of the methodology. The results confirm that models typically generate globally averaged cloud feedbacks that are substantially positive or near neutral, unlike the change in cloud forcing itself, which is as often negative as positive
Vecchi, Gabriel A., A C Clement, and Brian J Soden, February 2008: Examining the tropical Pacific's response to global warming. EOS, 89(9), 81, 83. PDF
Alternative interpretations of the relationship between sea surface temperature and hurricane activity imply vastly different future Atlantic hurricane activity.
Huang, Yi, V Ramaswamy, and Brian J Soden, March 2007: An investigation of the sensitivity of the clear-sky outgoing longwave radiation to atmospheric temperature and water vapor. Journal of Geophysical Research, 112, D05104, DOI:10.1029/2005JD006906. Abstract
The rate at which the outgoing longwave radiation (OLR) responds to perturbations in temperature and moisture plays a fundamental role in determining climate sensitivity. This study examines the clear-sky OLR sensitivities to temperature and water vapor, as quantified by its partial derivatives (radiative Jacobians). The Jacobians, as computed by the Geophysical Fluid Dynamics Laboratory (GFDL)'s line-by-line (LBL) radiative transfer model are used to verify the results from the parameterized GFDL GCM (general circulation model) radiation code. The results show that the (1) Jacobians of OLR due to incremental changes in temperature and water vapor are insensitive to different formulations of water vapor continuum absorption and (2) Jacobians of OLR are properly captured by the GCM longwave band approximation. Simulations with the GCM demonstrate that uncertainties in the formulation of continuum absorption have little impact on the climate model simulation of clear-sky OLR changes in response to prescribed sea surface temperature (SST) perturbation. The numerically computed Jacobians of OLR are used to reconstruct the tropical annual mean OLR from the variations of temperature and water vapor over the period 1980–1999. The reconstructed OLR anomaly time series agrees well with that computed explicitly by the GCM. On the basis of this result, it becomes possible to separate out the temperature and water vapor contributions to the OLR variation. The results show that the temperature contribution dominates the water vapor contribution in the lower and middle troposphere, while in the upper troposphere the two contributions largely offset each other.
To help understand possible impacts of anthropogenic greenhouse warming on hurricane activity, we assess model-projected changes in large-scale environmental factors tied to variations in hurricane statistics. This study focuses on vertical wind shear (Vs) over the tropical Atlantic during hurricane season, the increase of which has been historically associated with diminished hurricane activity and intensity. A suite of state-of-the-art global climate model experiments is used to project changes in Vs over the 21st century. Substantial increases in tropical Atlantic and East Pacific shear are robust features of these experiments, and are shown to be connected to the model-projected decrease in the Pacific Walker circulation. The relative changes in shear are found to be comparable to those of other large-scale environmental parameters associated with Atlantic hurricane activity. The influence of these Vs changes should be incorporated into projections of long-term hurricane activity.
This study examines the response of the tropical atmospheric and oceanic circulation to increasing
greenhouse gases using a coordinated set of twenty-first-century climate model experiments performed for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The strength
of the atmospheric overturning circulation decreases as the climate warms in all IPCC AR4 models, in a
manner consistent with the thermodynamic scaling arguments of Held and Soden. The weakening occurs
preferentially in the zonally asymmetric (i.e., Walker) rather than zonal-mean (i.e., Hadley) component of
the tropical circulation and is shown to induce substantial changes to the thermal structure and circulation
of the tropical oceans. Evidence suggests that the overall circulation weakens by decreasing the frequency
of strong updrafts and increasing the frequency of weak updrafts, although the robustness of this behavior
across all models cannot be confirmed because of the lack of data. As the climate warms, changes in both
the atmospheric and ocean circulation over the tropical Pacific Ocean resemble “El Niño–like” conditions;
however, the mechanisms are shown to be distinct from those of El Niño and are reproduced in both mixed
layer and full ocean dynamics coupled climate models. The character of the Indian Ocean response to global
warming resembles that of Indian Ocean dipole mode events. The consensus of model results presented
here is also consistent with recently detected changes in sea level pressure since the mid–nineteenth century.
The
response of tropical cyclone activity to global warming is widely debated.
It is often assumed that warmer sea surface temperatures provide a more
favourable environment for the development and intensification of tropical
cyclones, but cyclone genesis and intensity are also affected by the
vertical thermodynamic properties of the atmosphere. Here we use climate
models and observational reconstructions to explore the relationship between
changes in sea surface temperature and tropical cyclone 'potential
intensity'—a measure that provides an upper bound on cyclone intensity and
can also reflect the likelihood of cyclone development. We find that changes
in local sea surface temperature are inadequate for characterizing even the
sign of changes in potential intensity, but that long-term changes in
potential intensity are closely related to the regional structure of
warming; regions that warm more than the tropical average are characterized
by increased potential intensity, and vice versa. We use this relationship
to reconstruct changes in potential intensity over the twentieth century
from observational reconstructions of sea surface temperature. We find that,
even though tropical Atlantic sea surface temperatures are currently at a
historical high, Atlantic potential intensity probably peaked in the 1930s
and 1950s, and recent values are near the historical average. Our results
indicate that—per unit local sea surface temperature change—the response of
tropical cyclone activity to natural climate variations, which tend to
involve localized changes in sea surface temperature, may be larger than the
response to the more uniform patterns of greenhouse-gas-induced warming.
Using the climate change experiments generated for the Fourth Assessment of the Intergovernmental Panel on Climate Change, this study examines some aspects of the changes in the hydrological cycle that are robust across the models. These responses include the decrease in convective mass fluxes, the increase in horizontal moisture transport, the associated enhancement of the pattern of evaporation minus precipitation and its temporal variance, and the decrease in the horizontal sensible heat transport in the extratropics. A surprising finding is that a robust decrease in extratropical sensible heat transport is found only in the equilibrium climate response, as estimated in slab ocean responses to the doubling of CO2 , and not in transient climate change scenarios. All of these robust responses are consequences of the increase in lower-tropospheric water vapor.
Mace, G G., M Deng, Brian J Soden, and E Zipser, 2006: Association of Tropical Cirrus in the 10–15-km Layer with Deep Convective Sources: An Observational Study Combining Millimeter Radar Data and Satellite-Derived Trajectories. Journal of the Atmospheric Sciences, 63(2), DOI:10.1175/JAS3627.1. Abstract
In this paper, millimeter cloud radar (MMCR) and Geosynchronous Meteorological Satellite (GMS) data are combined to study the properties of tropical cirrus that are common in the 10–15-km layer of the tropical troposphere in the western Pacific. Millimeter cloud radar observations collected by the Atmospheric Radiation Measurement program on the islands of Manus and Nauru in the western and central equatorial Pacific during a 12-month period spanning 1999 and 2000 show differences in cirrus properties: over Manus, where clouds above 7 km are observed 48% of the time, the cirrus are thicker and warmer on average and the radar reflectivity and Doppler velocity are larger; over Nauru clouds above 7 km are observed 23% of time. To explain the differences in cloud properties, the relationship between tropical cirrus and deep convection is examined by combining the radar observations with GMS satellite-derived back trajectories. Using a data record of 1 yr, it is found that 47% of the cirrus observed over Manus can be traced to a deep convective source within the past 12 h while just 16% of the cirrus observed over Nauru appear to have a convective source within the previous 12 h. Of the cirrus that can be traced to deep convection, the evolution of the radar-observed cloud properties is examined as a function of apparent cloud age. The radar Doppler moments and ice water path of the observed cirrus at both sites generally decrease as the cirrus age increase. At Manus, it is found that cirrus during boreal winter typically advect over the site from the southeast from convection associated with the winter monsoon, while during boreal summer, the trajectories are mainly from the northeast. The properties of these two populations of cirrus are found to be different, with the winter cirrus having higher concentrations of smaller particles. Examining statistics of the regional convection using Tropical Rainfall Measuring Mission (TRMM), it is found that the properties of the winter monsoon convection in the cirrus source region are consistent with more intense convection compared to the convection in the summer source region.
Observations reveal that the substantial cooling of the global lower stratosphere over 1979–2003 occurred in two pronounced steplike transitions. These arose in the aftermath of two major volcanic eruptions, with each cooling transition being followed by a period of relatively steady temperatures. Climate model simulations indicate that the space-time structure of the observed cooling is largely attributable to the combined effect of changes in both anthropogenic factors (ozone depletion and increases in well-mixed greenhouse gases) and natural factors (solar irradiance variation and volcanic aerosols). The anthropogenic factors drove the overall cooling during the period, and the natural ones modulated the evolution of the cooling.
The climate feedbacks in coupled ocean–atmosphere models are compared using a coordinated set of twenty-first-century climate change experiments. Water vapor is found to provide the largest positive feedback in all models and its strength is consistent with that expected from constant relative humidity changes in the water vapor mixing ratio. The feedbacks from clouds and surface albedo are also found to be positive in all models, while the only stabilizing (negative) feedback comes from the temperature response. Large intermodel differences in the lapse rate feedback are observed and shown to be associated with differing regional patterns of surface warming. Consistent with previous studies, it is found that the vertical changes in temperature and water vapor are tightly coupled in all models and, importantly, demonstrate that intermodel differences in the sum of lapse rate and water vapor feedbacks are small. In contrast, intermodel differences in cloud feedback are found to provide the largest source of uncertainty in current predictions of climate sensitivity.
Soden, Brian J., 2006: Water-vapor observations In Frontiers of Climate Modeling, Kiehl, J T, V. Ramanathan, eds., UK, Cambridge University Press, 285-311.
The climate response to idealized changes in the atmospheric CO2 concentration by the new GFDL climate model (CM2) is documented. This new model is very different from earlier GFDL models in its parameterizations of subgrid-scale physical processes, numerical algorithms, and resolution. The model was constructed to be useful for both seasonal-to-interannual predictions and climate change research. Unlike previous versions of the global coupled GFDL climate models, CM2 does not use flux adjustments to maintain a stable control climate. Results from two model versions, Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), are presented.
Two atmosphere–mixed layer ocean or slab models, Slab Model versions 2.0 (SM2.0) and 2.1 (SM2.1), are constructed corresponding to CM2.0 and CM2.1. Using the SM2 models to estimate the climate sensitivity, it is found that the equilibrium globally averaged surface air temperature increases 2.9 (SM2.0) and 3.4 K (SM2.1) for a doubling of the atmospheric CO2 concentration. When forced by a 1% per year CO2 increase, the surface air temperature difference around the time of CO2 doubling [transient climate response (TCR)] is about 1.6 K for both coupled model versions (CM2.0 and CM2.1). The simulated warming is near the median of the responses documented for the climate models used in the 2001 Intergovernmental Panel on Climate Change (IPCC) Working Group I Third Assessment Report (TAR).
The thermohaline circulation (THC) weakened in response to increasing atmospheric CO2. By the time of CO2 doubling, the weakening in CM2.1 is larger than that found in CM2.0: 7 and 4 Sv (1 Sv 106 m3 s−1), respectively. However, the THC in the control integration of CM2.1 is stronger than in CM2.0, so that the percentage change in the THC between the two versions is more similar. The average THC change for the models presented in the TAR is about 3 or 4 Sv; however, the range across the model results is very large, varying from a slight increase (+2 Sv) to a large decrease (−10 Sv).
Since the mid-nineteenth century the Earth's surface has warmed1, 2, 3, and models indicate that human activities have caused part of the warming by altering the radiative balance of the atmosphere1, 3. Simple theories suggest that global warming will reduce the strength of the mean tropical atmospheric circulation4, 5. An important aspect of this tropical circulation is a large-scale zonal (east–west) overturning of air across the equatorial Pacific Ocean—driven by convection to the west and subsidence to the east—known as the Walker circulation6. Here we explore changes in tropical Pacific circulation since the mid-nineteenth century using observations and a suite of global climate model experiments. Observed Indo-Pacific sea level pressure reveals a weakening of the Walker circulation. The size of this trend is consistent with theoretical predictions, is accurately reproduced by climate model simulations and, within the climate models, is largely due to anthropogenic forcing. The climate model indicates that the weakened surface winds have altered the thermal structure and circulation of the tropical Pacific Ocean. These results support model projections of further weakening of tropical atmospheric circulation during the twenty-first century4, 5, 7.
Webb, M J., Catherine A Senior, D M H Sexton, W J Ingram, K D Williams, M A Ringer, B McAveney, R Colman, Brian J Soden, Richard G Gudgel, Thomas R Knutson, S Emori, T Ogura, Y Tsushima, N Andronova, B Li, I Musat, Sandrine Bony, and Karl E Taylor, 2006: On the contribution of local feedback mechanisms to the range of climate sensitivity in two GCM ensembles. Climate Dynamics, 27(1), DOI:10.1007/s00382-006-0111-2. Abstract
Global and local feedback analysis techniques have been applied to two ensembles of mixed layer equilibrium CO2 doubling climate change experiments, from the CFMIP (Cloud Feedback Model Intercomparison Project) and QUMP (Quantifying Uncertainty in Model Predictions) projects. Neither of these new ensembles shows evidence of a statistically significant change in the ensemble mean or variance in global mean climate sensitivity when compared with the results from the mixed layer models quoted in the Third Assessment Report of the IPCC. Global mean feedback analysis of these two ensembles confirms the large contribution made by inter-model differences in cloud feedbacks to those in climate sensitivity in earlier studies; net cloud feedbacks are responsible for 66% of the inter-model variance in the total feedback in the CFMIP ensemble and 85% in the QUMP ensemble. The ensemble mean global feedback components are all statistically indistinguishable between the two ensembles, except for the clear-sky shortwave feedback which is stronger in the CFMIP ensemble. While ensemble variances of the shortwave cloud feedback and both clear-sky feedback terms are larger in CFMIP, there is considerable overlap in the cloud feedback ranges; QUMP spans 80% or more of the CFMIP ranges in longwave and shortwave cloud feedback. We introduce a local cloud feedback classification system which distinguishes different types of cloud feedbacks on the basis of the relative strengths of their longwave and shortwave components, and interpret these in terms of responses of different cloud types diagnosed by the International Satellite Cloud Climatology Project simulator. In the CFMIP ensemble, areas where low-top cloud changes constitute the largest cloud response are responsible for 59% of the contribution from cloud feedback to the variance in the total feedback. A similar figure is found for the QUMP ensemble. Areas of positive low cloud feedback (associated with reductions in low level cloud amount) contribute most to this figure in the CFMIP ensemble, while areas of negative cloud feedback (associated with increases in low level cloud amount and optical thickness) contribute most in QUMP. Classes associated with high-top cloud feedbacks are responsible for 33 and 20% of the cloud feedback contribution in CFMIP and QUMP, respectively, while classes where no particular cloud type stands out are responsible for 8 and 21%.
Wyant, M C., Christopher S Bretherton, Julio T Bacmeister, J T Kiehl, Isaac M Held, Ming Zhao, Stephen A Klein, and Brian J Soden, 2006: A comparison of low-latitude cloud properties and their response to climate change in three AGCMs sorted into regimes using mid-tropospheric vertical velocity. Climate Dynamics, 27(2-3), DOI:10.1007/s00382-006-0138-4. Abstract
Low-latitude cloud distributions and cloud responses to climate perturbations are compared in near-current versions of three leading U.S. AGCMs, the NCAR CAM 3.0, the GFDL AM2.12b, and the NASA GMAO NSIPP-2 model. The analysis technique of Bony et al. (Clim Dyn 22:71–86, 2004) is used to sort cloud variables by dynamical regime using the monthly mean pressure velocity ω at 500 hPa from 30S to 30N. All models simulate the climatological monthly mean top-of-atmosphere longwave and shortwave cloud radiative forcing (CRF) adequately in all ω-regimes. However, they disagree with each other and with ISCCP satellite observations in regime-sorted cloud fraction, condensate amount, and cloud-top height. All models have too little cloud with tops in the middle troposphere and too much thin cirrus in ascent regimes. In subsidence regimes one model simulates cloud condensate to be too near the surface, while another generates condensate over an excessively deep layer of the lower troposphere. Standardized climate perturbation experiments of the three models are also compared, including uniform SST increase, patterned SST increase, and doubled CO2 over a mixed layer ocean. The regime-sorted cloud and CRF perturbations are very different between models, and show lesser, but still significant, differences between the same model simulating different types of imposed climate perturbation. There is a negative correlation across all general circulation models (GCMs) and climate perturbations between changes in tropical low cloud cover and changes in net CRF, suggesting a dominant role for boundary layer cloud in these changes. For some of the cases presented, upper-level clouds in deep convection regimes are also important, and changes in such regimes can either reinforce or partially cancel the net CRF response from the boundary layer cloud in subsidence regimes. This study highlights the continuing uncertainty in both low and high cloud feedbacks simulated by GCMs.
A key disagreement exists between global climate model (GCM) simulations and satellite observations of the decadal variability in the tropical-mean radiation budget. Measurements from the Earth Radiation Budget Experiment (ERBE) over the period 1984-2001 indicate a trend of increasing longwave emission and decreasing shortwave reflection that no GCM can currently reproduce. Motivated by these results, a series of model sensitivity experiments is performed to investigate hypotheses that have been advanced to explain this discrepancy. Specifically, the extent to which a strengthening of the Hadley circulation or a change in convective precipitation efficiency can alter the tropical-mean radiation budget is assessed. Results from both model sensitivity experiments and an empirical analysis of ERBE observations suggest that the tropical-mean radiation budget is remarkably insensitive to changes in the tropical circulation. The empirical estimate suggests that it would require at least a doubling in strength of the Hadley circulation in order to generate the observed decadal radiative flux changes. In contrast, rather small changes in a model's convective precipitation efficiency can generate changes comparable to those observed, provided that the precipitation efficiency lies near the upper end of its possible range. If, however, the precipitation efficiency of tropical convective systems is more moderate, the model experiments suggest that the climate would be rather insensitive to changes in its value. Further observations are necessary to constrain the potential effects of microphysics on the top-of-atmosphere radiation budget.
Huang, X, Brian J Soden, and D L Jackson, 2005: Interannual co-variability of tropical temperature and humidity: A comparison of model, reanalysis data and satellite observation. Geophysical Research Letters, 32, L17808, DOI:10.1029/2005GL023375. Abstract
We use a 20-year record of HIRS radiance measurements to evaluate the fidelity of interannual co-variability of tropical humidity and temperature in the reanalyses and GFDL AM2 simulations. Large inconsistencies between the NCEP and ECMWF reanalyses are found as are disagreements between the reanalyses and AM2 simulations. The largest discrepancies occur in the middle and upper troposphere where the NCEP and ECMWF tropical-mean relative humidity anomalies are found to be negatively correlated. When compared to HIRS dataset, NCEP is found to have unrealistically large interannual variablity in both the upper (6.7 µm) and middle (7.3 µm) tropospheric humidity channels. The radiance anomalies simulated from AM2 model output are shown to agree well with those observed by HIRS. These results support the validity of the strong coupling between temperature and humidity variations simulated in the GFDL AM2 and highlight the need to improve the representation of interannual variations of humidity in the reanalyses.
Climate models predict that the concentration of water vapor in the upper troposphere could double by the end of the century as a result of increases in greenhouse gases. Such moistening plays a key role in amplifying the rate at which the climate warms in response to anthropogenic activities, but has been difficult to detect because of deficiencies in conventional observing systems. We use satellite measurements to highlight a distinct radiative signature of upper tropospheric moistening over the period 1982 to 2004. The observed moistening is accurately captured by climate model simulations and lends further credence to model projections of future global warming.
High-resolution (0.1‹ ~ 0.1‹) geostationary satellite infrared radiances at 11 ƒÊm in combination with gridded (2.5‹ ~ 2.0‹) hourly surface precipitation observations are employed to document the spatial structure of the diurnal cycle of summertime deep convection and associated precipitation over North America. Comparison of the diurnal cycle pattern between the satellite retrieval and surface observations demonstrates the reliability of satellite radiances for inferring the diurnal cycle of precipitation, especially the diurnal phase. On the basis of the satellite radiances, we find that over most land regions, deep convection peaks in the late afternoon and early evening, a few hours later than the peak of land surface temperature. However, strong regional variations exist in both the diurnal phase and amplitude, implying that topography, land-sea contrast, and coastline curvature play an important role in modulating the diurnal cycle. Examples of such effects are highlighted over Florida, the Great Plains, and the North American monsoon region.
for climate research developed at the Geophysical Fluid Dynamics Laboratory (GFDL) are presented. The atmosphere model, known as AM2, includes a new gridpoint dynamical core, a prognostic cloud scheme, and a multispecies aerosol climatology, as well as components from previous models used at GFDL. The land model, known as LM2, includes soil sensible and latent heat storage, groundwater storage, and stomatal resistance. The performance of the coupled model AM2–LM2 is evaluated with a series of prescribed sea surface temperature (SST) simulations. Particular focus is given to the model's climatology and the characteristics of interannual variability related to E1 Niño– Southern Oscillation (ENSO).
One AM2–LM2 integration was performed according to the prescriptions of the second Atmospheric Model Intercomparison Project (AMIP II) and data were submitted to the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Particular strengths of AM2–LM2, as judged by comparison to other models participating in AMIP II, include its circulation and distributions of precipitation. Prominent problems of AM2– LM2 include a cold bias to surface and tropospheric temperatures, weak tropical cyclone activity, and weak tropical intraseasonal activity associated with the Madden–Julian oscillation.
An ensemble of 10 AM2–LM2 integrations with observed SSTs for the second half of the twentieth century permits a statistically reliable assessment of the model's response to ENSO. In general, AM2–LM2 produces a realistic simulation of the anomalies in tropical precipitation and extratropical circulation that are associated with ENSO.
Soden, Brian J., 2004: The impact of tropical convection and cirrus on upper tropospheric humidity: A Lagrangian analysis of satellite measurements. Geophysical Research Letters, 31(20), L20104, DOI:10.1029/2004GL020980. Abstract
Geostationary satellite observations are used in conjunction with an objective pattern-tracking algorithm to describe the Lagrangian evolution of convection, clouds and water vapor in the tropical upper troposphere. This analysis reveals that larger convective events within a Lagrangian air mass are associated with larger and longer-lived cirrus anvil shields. Convective systems which generate larger cirrus shields are, in turn, associated with higher downstream humidity levels following the anvil's dissipation. In the absence of cirrus, the clear-sky upper troposphere is shown to dry at a rate consistent with radiatively-driven subsidence. The presence of cirrus anvils following a convective event is shown to reduce the rate of drying and for large anvils can even change its sign. Analysis of the Lagrangian tendencies suggests that this moistening effect is not attributable to the evaporation of cirrus condensate, but instead results from the same dynamical mechanisms responsible for the formation and maintenance of the cirrus anvil.
Uncertainty in cloud feedback is the leading cause of discrepancy in model predictions of climate change. The use of observed or model-simulated radiative fluxes to diagnose the effect of clouds on climate sensitivity requires an accurate understanding of the distinction between a change in cloud radiative forcing and a cloud feedback. This study compares simulations from different versions of the GFDL Atmospheric Model 2 (AM2) that have widely varying strengths of cloud feedback to illustrate the differences between the two and highlight the potential for changes in cloud radiative forcing to be misinterpreted.
Soden, Brian J., D D Turner, B M Lesht, and L M Miloshevich, 2004: An analysis of satellite, radiosonde, and lidar observations of upper tropospheric water vapor from the Atmospheric Radiation Measurement Program. Journal of Geophysical Research, 109, D04105, DOI:10.1029/2003JD003828. Abstract PDF
To improve our understanding of the distribution and radiative effects of water vapor, the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program has conducted a series of coordinated water vapor Intensive Observation Periods (IOPs). This study uses observations collected from four ARM IOPs to accomplish two goals: First we compare radiosonde and Raman lidar observations of upper tropospheric water vapor with colocated geostationary satellite radiances at 6.7 μm. During all four IOPs we find excellent agreement between the satellite and Raman lidar observations of upper tropospheric humidity with systematic differences of ~10%. In contrast, radiosondes equipped with Vaisala sensors are shown to be systematically drier in the upper troposphere by ~40% relative to both the lidar and satellite measurements. Second, we assess the performance of various “correction” strategies designed to rectify known deficiencies in the radiosonde measurements. It is shown that existing methods for correcting the radiosonde dry bias, while effective in the lower troposphere, offer little improvement in the upper troposphere. An alternative method based on variational assimilation of satellite radiances is presented and, when applied to the radiosonde measurements, is shown to significantly improve their agreement with coincident Raman lidar observations. It is suggested that a similar strategy could be used to improve the quality of the global historical record of radiosonde water vapor observations during the satellite era.
Tian, B, Brian J Soden, and X Wu, 2004: Diurnal cycle of convection, clouds, and water vapor in the tropical upper troposphere: Satellites versus a general circulation model. Journal of Geophysical Research, 109, D10101, DOI:10.1029/2003JD004117. Abstract
Global high-resolution (3-hourly, 0.1° × 0.1° longitude-latitude) water vapor (6.7 μm) and window (11 μm) radiances from multiple geostationary satellites are used to document the diurnal cycle of upper tropospheric relative humidity (UTH) and its relationship to deep convection and high clouds in the whole tropics and to evaluate the ability of the new Geophysical Fluid Dynamics Laboratory (GFDL) global atmosphere and land model (AM2/LM2) to simulate these diurnal variations. Similar to the diurnal cycle of deep convection and high clouds, coherent diurnal variations in UTH are also observed over the deep convective regions, where the daily mean UTH is high. In addition, the diurnal cycle in UTH also features a land-sea contrast: stronger over land but weaker over ocean. UTH tends to peak around midnight over ocean in contrast to 0300 LST over land. Furthermore, UTH is observed to lag high cloud cover by ~6 hours, and the latter further lags deep convection, implying that deep convection serves to moisten the upper troposphere through the evaporation of the cirrus anvil clouds generated by deep convection. Compared to the satellite observations, AM2/LM2 can roughly capture the diurnal phases of deep convection, high cloud cover, and UTH over land; however, the magnitudes are noticeably weaker in the model. Over the oceans the AM2/LM2 has difficulty in simulating both the diurnal phase and amplitude of these quantities. These results reveal some important deficiencies in the model's convection and cloud parameterization schemes and suggest the lack of a diurnal cycle in SST may be a shortcoming in the boundary forcing for atmospheric models
Zhang, C, B E Mapes, and Brian J Soden, October 2003: Bimodality in tropical water vapour. Quarterly Journal of the Royal Meteorological Society, 129(594), DOI:10.1256/qj.02.166. Abstract
Probability distribution functions of tropospheric water vapour in the tropics are shown to be commonly bimodal. This bimodality implies sharp gradients between dry and moist regimes in space and time. A method of testing for and quantifying bimodality is introduced. Using this method, the bimodality of water vapour is surveyed in satellite and in situ observations, as well as in global model re-analysis data and simulations. The bimodality suggests that the radiative drying time after an injection of moisture by convection is short (1-2 days) compared to a homogenizing time, whether physical (mixing) or mathematical (averaging). It is shown that the local bimodality found in cloud-model simulations and in situ point measurements disappears with modest time averaging (18 h and 200 km), but then reappears on the global-scale, where dry and moist regions are separated so widely that synoptic- and large-scale mixing times exceed the drying time-scale. Large discrepancies exist in the ability to reproduce the global-scale bimodality by global model re-analysis and simulations.
Free, M, I Durre, John R Lanzante, Stephen A Klein, and Brian J Soden, et al., 2002: Creating Climate Reference Datasets: CARDS Workshop on Adjusting Radiosonde Temperature Data for Climate Monitoring. Bulletin of the American Meteorological Society, 83(6), 891-899. Abstract PDF
Homogeneous upper-air temperature time series are necessary for climate change detection and attribution. About 20 participants met at the National Climatic Data Center in Asheville, North Carolina on 11-12 October 2000 to discuss methods of adjusting radiosonde data for inhomogeneities arising from instrument and other changes. Representatives of several research groups described their methods for identifying change points and adjusting temperature time series and compared the results of applying these methods to data from 12 radiosonde stations. The limited agreement among these results and the potential impact of these adjustments on upperair trends estimates indicate a need for further work in this area and for greater attention to homogeneity issues in planning future changes in radiosonde observations.
This paper presents a quantitative methodology for evaluating air-sea fluxes related to ENSO from different atmospheric products. A statistical model of the fluxes from each atmospheric product is coupled to an ocean general circulation model (GCM). Four different products are evaluated: reanalyses from the National Centers for Environmental Prediction (NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF), satellite-derived data from the Special Sensor Microwave/Imaging (SSM/I) platform and the International Satellite Cloud Climatology Project (ISCCP), and an atmospheric GCM developed at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Atmospheric Model Intercomparison Project (AMIP) II. For this study, comparisons between the datasets are restricted to the dominant air-sea mode. #The stability of a coupled model using only the dominant mode and the associated predictive skill of the model are strongly dependent on which atmospheric product is used. The model is unstable and oscillatory for the ECMWF product, damped and ocillatory for the NCEP and GFDL products, and unstable (nonoscillatory) for the satellite product. The ocean model is coupled with patterns of wind stress as well as heat fluxes. This distinguishes the present approach from the existing paradigm for ENSO models where surface heat fluxes are parameterized as a local damping term in the sea surface temperature (SST) equation.
Penner, Joyce, S Zhang, Mian Chin, C C Chuang, J Feichter, Y Feng, I V Geogdzhayev, Paul Ginoux, Michael Herzog, and Brian J Soden, et al., 2002: A comparison of model- and satellite-derived aerosol optical depth and reflectivity. Journal of the Atmospheric Sciences, 59(3), 441-460. Abstract PDF
The determination of an accurate quantitative understanding of the role of tropospheric aerosols in the earth's radiation budget is extremely important because forcing by anthropogenic aerosols presently represents one of the most uncertain aspects of climate models. Here the authors present a systematic comparison of three different analyses of satellite-retrieved aerosol optical depth based on the Advanced Very High Resolution Radiometer (AVHRR)-measured radiances with optical depths derived from six different models. Also compared are the model-derived clear-sky reflected shortwave radiation with satellite-measured reflectivities derived from the Earth Radiation Budget Experiment (ERBE) satellite.
The three different satellite-derived optical depths differ by between -0.10 and 0.07 optical depth units in comparison to the average of the three analyses depending on latitude and month, but the general features of the retrievals are similar. The models differ by between -0.09 and +0.16 optical depth units from the average of the models. Differences between the average of the models and the average of the satellite analyses range over -0.11 to +0.05 optical depth units. These differences are significant since the annual average clear-sky radiative forcing associated with the difference between the average of the models and the average of the satellite analyses ranges between -3.9 and 0.7 W m-2 depending on latitude and is -1.7 W m-2 on a global average annual basis. Variations in the source strengths of dimethylsulfide-derived aerosols and sea salt aerosols can explain differences between the models, and between the models and satellite retrievals of up to 0.2 optical depth units.
The comparison of model-generated reflected shortwave radiation and ERBE-measured shortwave radiation is similar in character as a function of latitude to the analysis of modeled and satellite-retrieved optical depths, but the differences between the modeled clear-sky reflected flux and the ERBE clear-sky reflected flux is generally larger than that inferred from the difference between the models and the AVHRR optical depths, especially at high latitudes. The difference between the mean of the models and the ERBE-analyzed clear-sky flux is 1.6 W m-2.
The overall comparison indicates that the model-generated aerosol optical depth is systematically lower than that inferred from measurements between the latitudes of 10° and 30°S. It is not likely that the shortfall is due to small values of the sea salt optical depth because increases in this component would create modeled optical depths that are larger than those from satellites in the region north of 30°N and near 50°S. Instead, the source strengths for DMS and biomass aerosols in the models may be too low. Firm conclusions, however, will require better retrieval procedures for the satellites, including better cloud screening procedures, further improvement of the model's treatment of aerosol transport and removal, and a better determination of aerosol source strengths.
Soden, Brian J., Richard T Wetherald, Georgiy Stenchikov, and A Robock, 2002: Global cooling after the eruption of Mount Pinatubo: A test of climate feedback by water vapor. Science, 296(5568), 727-730. Abstract PDF
The sensitivity of Earth's climate to an external radiative forcing depends critically on the response of water vapor. We use the global cooling and drying of the atmosphere that was observed after the eruption of Mount Pinatubo to test model predictions of the climate feedback from water vapor. Here, we first highlight the success of the model in reproducing the observed drying after the volcanic eruption. Then, by comparing model simulations with and without water vapor feedback, we demonstrate the importance of the atmospheric drying in amplifying the temperature change and show that, without the strong positive feedback from water vapor, the model is unable to reproduce the observed cooling. These results provide quantitative evidence of the reliability of water vapor feedback in current climate models, which is crucial to their use for global warming projections.
Wielicki, B A., T Wong, Richard P Allan, A Slingo, J T Kiehl, Brian J Soden, C Tony Gordon, Arthur J Miller, S-K Yang, David A Randall, F Robertson, J Susskind, and H Jacobowitz, 2002: Evidence for large decadal variability in the tropical mean radiative energy budget. Science, 295(5556), 841-844. Abstract PDF
It is widely assumed that variations in Earth's radiative energy budget at large time and space scales are small. We present new evidence from a compilation of over two decades of accurate satellite data that the top-of-atmosphere (TOA) tropical radiative energy budget is much more dynamic and variable than previously thought. Results indicate that the radiation budget changes are caused by changes in tropical mean cloudiness. The results of several current climate model simulations fail to predict this large observed variation in tropical energy budget. The missing variability in the models highlights the critical need to improve cloud modeling in the tropics so that prediction of tropical climate on interannual and decadal time scales can be improved.
Soden, Brian J., C S Velden, and Robert E Tuleya, 2001: The impact of satellite winds on experimental GFDL hurricane model forecasts. Monthly Weather Review, 129(4), 835-852. Abstract PDF
A series of experimental forecasts are performed to evaluate the impact of enhanced satellite-derived winds on numerical hurricane track predictions. The winds are derived from Geostationary Operational Environmental Satellite-8 (GOES-8) multispectral radiance observations by tracking cloud and water vapor patterns from successive satellite images. A three-dimensional optimum interpolation method is developed to assimilate the satellite winds directly into the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane prediction system. A series of parallel forecasts are then performed, both with and without the assimilation of GOES winds. Except for the assimilation of the satellite winds, the model integrations are identical in all other respects. A strength of this study is the large number of experiments performed. Over 100 cases are examined from 11 different storms covering three seasons (1996–98), enabling the authors to account for and examine the case-to-case variability in the forecast results when performing the assessment. On average, assimilation of the GOES winds leads to statistically significant improvements for all forecast periods, with the relative reductions in track error ranging from ~5% at 12 h to ~12% at 36 h. The percentage of improved forecasts increases following the assimilation of the satellite winds, with roughly three improved forecasts for every two degraded ones. Inclusion of the satellite winds also dramatically reduces the westward bias that has been a persistent feature of the GFDL model forecasts, implying that much of this bias may be related to errors in the initial conditions rather than a deficiency in the model itself. Finally, a composite analysis of the deep-layer flow fields suggests that the reduction in track error may be associated with the ability of the GOES winds to more accurately depict the strength of vorticity gyres in the environmental flow. These results offer compelling evidence that the assimilation of satellite winds can significantly improve the accuracy of hurricane track forecasts.
Stocker, T F., Thomas L Delworth, Stephen M Griffies, Isaac M Held, V Ramaswamy, and Brian J Soden, et al., 2001: Physical climate processes and feedbacks In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 418-470.
Water vapor is the dominant greenhouse gas, the most important gaseous source of infrared opacity in the atmosphere. As the concentrations of other greenhouse gases, particularly carbon dioxide, increase because of human activity, it is centrally important to predict how the water vapor distribution will be affected. To the extent that water vapor concentrations increase in a warmer world, the climatic effects of the other greenhouse gases will be amplified. Models of the Earth's climate indicate that this is an important positive feedback that increases the sensitivity of surface temperatures to carbon dioxide by a factor of two when considered in isolation from other feedbacks, and possibly by as much as a factor of three or more when interactions with other feedbacks are considered. Critics of this consensus have attempted to provide reasons why modeling results are overestimating the strength of this feedback.
Our uncertainty concerning climate sensitivity is disturbing. The range most often quoted for the equilibrium global mean surface temperature response to a doubling of CO2 concentrations in the atmosphere is 1.5°C to 4.5°C. If the Earth lies near the upper bound of this sensitivity range, climate changes in the twenty-first century will be profound. The range in sensitivity is primarily due to differing assumptions about how the Earth's cloud distribution is maintained; all the models on which these estimates are based possess strong water vapor feedback. If this feedback is, in fact, substantially weaker than predicted in current models, sensitivities in the upper half of this range would be much less likely, a conclusion that would clearly have important policy implications. In this review, we describe the background behind the prevailing view on water vapor feedback and some of the arguments raised by its critics, and attempt to explain why these arguments have not modified the consensus within the climate research community.
Soden, Brian J., 2000: The diurnal cycle of convection, clouds, and water vapor in the tropical upper troposphere. Geophysical Research Letters, 27(15), 2173-2176. Abstract PDF
Hourly observations of the 6.7 µm water vapor radiances from geostationary satellites are used to document the diurnal cycle in upper tropospheric water vapor and its relationship to cloud cover and convection. A coherent diurnal cycle in tropical water vapor is observed which lags the variations in cloud cover by approximately 2 hours. The variations in upper tropospheric cloud and water vapor occur (roughly) in phase with changes in deep convection over land, but nearly 12 hours out of phase with those over ocean. This feature is shown to be associated with differences in the vertical structure of land and ocean convection and offers a useful test of convective parameterizations in atmospheric models.
Soden, Brian J., 2000: Enlightening water vapour. Nature, 406(6793), 247-248. PDF
Soden, Brian J., 2000: The sensitivity of the tropical hydrological cycle to ENSO. Journal of Climate, 13(3), 538-549. Abstract PDF
Satellite observations of temperature, water vapor, precipitation and longwave radiation are used to characterize the variation of the tropical hydrologic and energy budgets associated with the El Niño-Southern Oscillation (ENSO). As the tropical oceans warm during an El Niño event, the precipitation intensity, water vapor mass, and temperature of the tropical atmosphere are observed to increase, reflecting a more vigorous hydrologic cycle. The enhanced latent heat release and resultant atmospheric warming lead to an increase in the emission of longwave radiation. Atmospheric global climate models, forced with observed sea surface temperatures (SSTs), accurately reproduce the observed tropospheric temperature, water vapor, and outgoing longwave radiation changes. However, the predicted variations in tropical-mean precipitation rate and surface longwave radiation are substantially smaller than observed. The comparison suggests that either (i) the sensitivity of the tropical hydrological cycle to ENSO-driven changes in SST is substantially underpredicted in existing climate models or (ii) that current satellite observations are inadequate to accurately monitor ENSO-related changes in the tropical-mean precipitation. Either conclusion has important implications for current efforts to monitor and predict changes in the intensity of the hydrological cycle.
Soden, Brian J., and S R Schroeder, 2000: Decadal variations in tropical water vapor: a comparison of observations and a model simulation. Journal of Climate, 13(19), 3337-3341. Abstract PDF
Multiple satellite records of tropical-mean water vapor are compared with a general circulation model (GCM) simulation to assess the ability to monitor and to predict low-frequency changes in total precipitable water. Particular attention is focused on the drying between 1979 and 1995 recorded by a TOVS statistical retrieval that is calibrated to radiosondes. Both a GCM integrated with observed SSTs and microwave and TOVS physical retrievals that overlap the drying period show no sustained drying. This discrepancy is consistent with the suggestion by Ross and Gaffen that the TOVS statistical algorithm is vulnerable to radiosonde instrumentation changes over this period that introduce an artificial drying trend into the retrieval.
Soden, Brian J., S Tjernkes, J Schmetz, R Saunders, J Bates, B Ellingson, R Engelen, and M Daniel Schwarzkopf, et al., 2000: An intercomparison of radiation codes for retrieving upper-tropospheric humidity in the 6.3-:m band: A report from the first GVaP Workshop. Bulletin of the American Meteorological Society, 81(4), 797-808. Abstract PDF
An intercomparison of radiation codes used in retrieving upper-tropospheric humidity (UTH) from observations in the <2 (6.3 :m) water vapor absorption band was performed. This intercomparison is one part of a coordinated effort within the Global Energy and Water Cycle Experiment Water Vapor Project to assess our ability to monitor the distribution and variations of upper-tropospheric moisture from spaceborne sensors. A total of 23 different codes, ranging from detailed line-by-line (LBL) models, to coarser-resolution narrowband (NB) models, to highly parameterized single-band (SB) models participated in the study. Forward calculations were performed using a carefully selected set of temperature and moisture profiles chosen to be representative of a wide range of atmospheric conditions. The LBL model calculations exhibited the greatest consistency with each other, typically agreeing to within 0.5 K in terms of the equivalent blackbody brightness temperature (Tb). The majority of NB and SB models agreed to within ±1 K of the LBL models, although a few older models exhibited systematic Tb biases in excess of 2 K. A discussion of the discrepancies between various models, their association with differences in model physics (e.g., continuum absorption), and their implications for UTH retrieval and radiance assimilation is presented.
Haywood, Jim M., V Ramaswamy, and Brian J Soden, 1999: Tropospheric aerosol climate forcing in clear-sky satellite observations over the oceans. Science, 283(5406), 1299-1303. Abstract PDF
Tropospheric aerosols affect the radiative forcing of Earth's climate, but their variable concentrations complicate an understanding of their global influence. Model-based estimates of aerosol distributions helped reveal spatial patterns indicative of the presence of tropospheric aerosols in the satellite-observed clear-sky solar radiation budget over the world's oceans. The results show that, although geographical signatures due to both natural and anthropogenic aerosols are manifest in the satellite observations, the naturally occurring sea-salt is the leading aerosol contributor to the global-mean clear-sky radiation balance over oceans.
Klein, Stephen A., Brian J Soden, and Ngar-Cheung Lau, 1999: Remote sea surface temperature variations during ENSO: Evidence for a tropical atmospheric bridge. Journal of Climate, 12(4), 917-932. Abstract PDF
In an El Niño event, positive SST anomalies usually appear in remote ocean basins such as the South China Sea, the Indian Ocean, and the tropical North Atlantic approximately 3 to 6 months after SST anomalies peak in the tropical Pacific. Ship data from 1952 to 1992 and satellite data from the 1980s both demonstrate that changes in atmospheric circulation accompanying El Niño induce changes in cloud cover and evaporation which, in turn, increase the net heat flux entering these remote oceans. It is postulated that this increased heat flux is responsible for the surface warming of these oceans. Specifically, over the eastern Indian Ocean and South China Sea, enhanced subsidence during El Niño reduces cloud cover and increases the solar radiation absorbed by the ocean, thereby leading to enhanced SSTs. In the tropical North Atlantic, a weakening of the trade winds during El Niño reduces surface evaporation and increases SSTs. These relationships fit the concept of an "atmospheric bridge" that connects SST anomalies in the central equatorial Pacific to those in remote tropical oceans.
Soden, Brian J., 1999: How well can we monitor and predict an intensification of the hydrological cycle?GEWEX News, 9(3), 1, 4-5.
Soden, Brian J., 1998: Tracking upper tropospheric water vapor radiances: A satellite perspective. Journal of Geophysical Research, 103(D14), 17,069-17,081. Abstract PDF
Hourly observations of 6.7 µm "water vapor" radiances from geostationary satellites are used in conjunction with an objective pattern-tracking algorithm to trace upper tropospheric water vapor features from sequential images. Analysis of measurements covering the 3 month period of June-August 1987 illustrates a close relationship between the upper tropospheric moisture and the tropical circulation. Over humid tropical regions the movement of water vapor patterns reveals a diverging upper level flow away from centers of deep convection. Likewise, arid subtropical regions are characterized by converging upper level water vapor patterns indicating sinking air. The evolution of upper tropospheric moisture is also explored from a Lagrangian perspective by considering the change in moisture of a pattern as it is tracked from one image to the next. This analysis reveals that the clear-sky upper troposphere in both tropical and subtropical regions becomes increasingly drier with time reflecting the impact of large-scale subsidence in drying the troposphere. When separated according to cloud amount, water vapor patterns associated with clouds are observed to dry substantially slower than those without clouds, presumably due to the evaporation of cloud condensate. Trajectories of upper tropospheric moisture are also constructed by iteratively tracking water vapor patterns from successive satellite images. The trajectories reveal two distinct paths for water vapor patterns entering the dry subsidence region of the subtropical South Pacific. One path highlights the southward propagation of convective outflow from the tropics while the other path reflects the injection of moisture from eastward propagating subtropical disturbances. The existence of two distinct source regions suggests that a complete picture of the processes regulating the driest and most radiatively transparent regions of the subtropics will require an understanding of both types of convective systems.
Ice clouds associated with large-scale atmospheric processes are studied using the SKYHI general circulation model (GCM) and parameterizations for their microphysical and radiative properties. The ice source is deposition from vapor, and the ice sinks are gravitational settling and sublimation. Effective particle sizes for ice distributions are related empirically to temperature. Radiative properties are evaluated as functions of ice path and effective size using approximations to detailed radiative-transfer solutions (Mie theory and geometric ray tracing). The distributions of atmospheric ice and their impact on climate and climate sensitivity are evaluated by integrating the SKYHI GCM (developed at the Geophysical Fluid Dynamics Laboratory) for six model months. Most of the major climatological cirrus regions revealed by satellite observations appear in the GCM. The radiative forcing associated with ice clouds acts to warm the Earth-atmosphere system. Relative to a SKYHI integration without these clouds, zonally averaged temperatures are warmer in the upper tropical troposphere with ice clouds. The presence of ice produced small net changes in the sensitivity of SKYHI climate to radiative perturbations, but this represents an intricate balance among changes in clear-, cloud-, solar-, and longwave-sensitivity components. Deficiencies in the representation of ice clouds are identified as results of biases in the large-scale GCM fields which drive the parameterization and neglect of subgrid variations in these fields, as well as parameterization simplifications of complex microphysical and radiative processes.
Soden, Brian J., 1997: Variations in tropical greenhouse forcing during El Niño In Eighth Symposium on Global Change Studies, Boston, MA, American Meteorological Society, J29-J30.
Soden, Brian J., 1997: Variations in the tropical greenhouse effect during El Niño. Journal of Climate, 10(5), 1050-1055. Abstract PDF
Observations of the clear-sky outgoing longwave radiation and sea surface temperature are combined to examine the evolution of the tropical greenhouse effect from colder La Niña conditions in early 1985 to warmer El Niño conditions in late 1987. Although comparison of individual months can suggest a decrease in greenhouse trapping from cold to warm conditions, when the entire 4-yr record is considered a distinct increase in tropical-mean greenhouse trapping of ~2 W m-2 is observed in conjunction with a ~0.4 K increase in tropical-mean sea surface temperature. This observed increase compares favorably with GCM simulations of the change in the clear-sky greenhouse effect during El Niño-Southern Oscillation (ENSO). Superimposed on top of the SST-driven change in greenhouse trapping are dynamically induced changes in tropical moisture apparently associated with a redistribution of SST during ENSO. The GCM simulations also successfully reproduce this feature, providing reassurance in the ability of GCMs to predict both dynamically and thermodynamically driven changes in greenhouse trapping.
Spangenberg, D A., G G Mace, T P Ackerman, N L Seaman, and Brian J Soden, 1997: Evaluation of model-simulated upper troposphere humidity using 6.7 µm satellite observations. Journal of Geophysical Research, 102(D22), 25,737-25,749. Abstract PDF
Use of mesoscale models to simulate details of upper tropospheric relative humidity (UTRH) fields represents an important step toward understanding the evolution of small-scale water vapor structures that are responsible for cirrus growth and dissipation. Because mesoscale model UTRH simulations require initialization and verification and since radiosonde measurements of relative humidity are unreliable in the upper troposphere, we use GOES 6.7 µm water vapor observations to validate the Pennsylvania State University/National Center for Atmospheric Research nonhydrostatic mesoscale model (MM5) simulations of UTRH. To accomplish this task, MM5 temperature and moisture profiles are used in a forward calculation of the clear-sky 6.7 µm brightness temperature (T6.7), which is converted into UTRH. A statistical analysis is done to evaluate MM5 simulations ot T6.7 and UTRH against the GOES 7 observations. For the simulations, an average correlation coefficient of 0.80 was found with a dry bias of 1.6 K. In terms of UTRH, the average correlation coefficient was 0.65 with a dry bias of 3.3%. We also found that MM5 fails to simulate accurately extrema in the UTRH field.
Chen, C-T, E Roeckner, and Brian J Soden, 1996: A comparison of satellite observations and model simulations of column-integrated moisture and upper-tropospheric humidity. Journal of Climate, 9(7), 1561-1585. Abstract PDF
Water vapor distributions obtained from the fourth generation ECHAM general circulation model are compared with satellite observations of total precipitable water (TPW) from the Special Sensor Microwave/Imager (SSM/I) and upper-tropospheric relative humidity (UTH) from TIROS-N Operational Vertical Sounder (TOVS). In general, the model simulations agree well with satellite observations of the climatological mean, seasonal variation, and interannual variation of moisture. There are, however, biases in the details. Underestimates in TPW and UTH are found off the west coast of continents, especially in the boreal summer over the eastern subtropical Pacific. These biases are related to both enhanced dry advection due to an excessively strong subtropical high and greater large-scale subsidence. A more intense tropical circulation in ECHAM4 is evidenced by the broadening of the high TPW and UTH zone that coincides with the equatorial convective regions. Additionally, interannual anomalies in equatorial UTH and TPW simulated by the model are found to be more sensitive to tropical SST anomalies than are the satellite data. The impact of changes in physical parameterizations upon the moisture distribution is also examined by comparing the simulations from the previous ECHAM3 and the current ECHAM4 models. The dry bias at the equator in ECHAM3 is related to the closure assumption used for deep convection, while the dry bias in UTH over the high-latitude winter hemisphere in ECHAM3 is a result of negative specific humidities produced by the spectral vapor advection scheme. With the new semi-Lagrangian advection scheme in ECHAM4, the simulated UTH over the same region becomes moister than TOVS observations suggest. The impact of discrepancies in the simulated water vapor distributions upon the radiation budget and cloud distribution in the model are also described.
Donner, Leo J., Brian J Soden, and Charles J Seman, 1996: Use of ISCCP data to evaluate a GCM parameterization for ice clouds In International Workshop on Research Uses of ISCCP Datasets, World Climate Research Programme, WCRP-97, WMO/TD No. 790, World Meteorological Organization, 11.39.
Frey, R A., A S Ackerman, and Brian J Soden, 1996: Climate parameters from satellite spectral measurements. Part I: Collocated AVHRR and HIRS/2 observations of spectral greenhouse parameter. Journal of Climate, 9(2), 327-344. Abstract PDF
An automated method of monitoring various climate parameters using collocated Advanced Very High Resolution Radiometer (AVHRR) and High-Resolution Infrared Sounder-2 (HIRS/2) observations has been developed. The method, referred to as CHAPS (collocated HIRS/2 and AVHRR products) was implemented during the months of July 1993 and January and July 1994. This paper presents the oceanic cloud screening method and analysis of the spectral greenhouse parameter for July 1993 and January 1994. In addition, the CHAPS derived clear-sky parameters are compared to the NESDIS historical dataset. There is agreement between NESDIS and CHAPS for the g6.7 and g7.3. The NESDIS 8.2 µ-m radiance distribution, particularly for regions with extensive low-level cloud cover.
It is shown that the spectral greenhouse parameter at wavelengths sensitive to middle and upper atmospheric water vapor content is dependent on SST via its connection to large-scale atmospheric circulation patterns. It is also shown that the variability of the spectral greenhouse parameter is strongly a function of latitude at these wavelengths, as well as in spectral regions sensitive to lower-level water vapor. Standard deviations are largest in the Tropics and generally decrease poleward. In contrast, variability in the spectral regions sensitive to upper-tropospheric temperature peaks in the middle latitudes and has its minimum in tropical latitudes.
Variability in the relationship between g and SST shows only a weak dependence on season for channels sensitive to water vapor content. A strong seasonal dependence is found in the g14.2 for the middle-latitude regions associated with changes in the temperature structure of the upper troposphere.
The relationship between the spectral greenhouse parameter and the broadband greenhouse parameter is presented and discussed. It is found that the range in broadband g for warm tropical SSTs is driven by spectral changes at wavelengths sensitive to upper-tropospheric water vapor. For cooler SSTs associated with the middle latitudes, the range in g is a function of the spectral greenhouse parameter sensitive to the temperature structure of the upper troposphere.
Soden, Brian J., and F P Bretherton, 1996: Interpretation of TOVS water vapor radiances in terms of layer-average relative humidities: Method and climatology for the upper, middle, and lower troposphere. Journal of Geophysical Research, 101(D5), 9333-9343. Abstract PDF
This study presents an analytical expression, derived from radiative theory, for relating water vapor radiances to layer-average relative humidities. This "radiance-to-humidity transformation" provides a simple, yet reliable, means of interpreting satellite observations of the upwelling radiation in the 6.3-µm water vapor absorption band in terms of a more familiar water vapor quantity. Despite its simplicity, when compared to detailed radiative transfer calculations of the upper (6.7 µm) tropospheric water vapor radiance, the transformation is demonstrated to be accurate within ~ 1 K. Similar levels of accuracy are found when the transformation is compared to detailed calculations of the middle (7.3 µm) and lower (8.3 µm) tropospheric water vapor radiance, provided that the emission from the underlying surface is taken into account. On the basis of these results, the radiance-to-humidity transformation is used to interpret TIROS operational vertical sounder observed water vapor radiances in terms of the relative humidity averaged over deep layers of the upper, middle, and lower troposphere. We then present near-global maps of the geographic distribution and climatological variations of upper, middle, and lower-tropospheric humidity for the period 1981-1991. These maps clearly depict the role of the large-scale circulation in regulating the location and temporal variation of tropospheric water vapor.
Soden, Brian J., and John R Lanzante, 1996: An assessment of satellite and radiosonde climatologies of upper-tropospheric water vapor. Journal of Climate, 9(6), 1235-1250. Abstract PDF
This study compares radiosonde and satellite climatologies of upper-tropospheric water vapor for the period 1979-1991. Comparison of the two climatologies reveals significant differences in the regional distribution of upper-tropospheric relative humidity. These discrepancies exhibit a distinct geopolitical dependence that is demonstrated to result from international differences in radiosonde instrumentation. Specifically, radiosondes equipped with goldbeater's skin humidity sensors (found primarily in the former Soviet Union, China, and eastern Europe) report a systematically moister upper troposphere relative to the satellite observations, whereas radiosondes equipped with capacitive or carbon hygristor sensors (found at most other locations) report a systematically drier upper troposphere. The bias between humidity sensors is roughly 15%-20% in terms of the relative humidity, being slightly greater during summer than during winter and greater in the upper troposphere than in the midtroposphere. However, once the instrumentation bias is accounted for, regional variations of satellite and radiosonde upper-tropospheric relative humidity are shown to be in good agreement. Additionally, temporal variations in radiosonde upper-tropospheric humidity agree reasonably well with the satellite observations and exhibit much less dependence upon instrumentation.
The impact that the limited spatial coverage of the radiosonde network has upon the moisture climatology is also examined and found to introduce systematic errors of 10%-20% relative humidity over data-sparse regions of the Tropics. It is further suggested that the present radiosonde network lacks sufficient coverage in the eastern tropical Pacific to adequately capture ENSO-related variations in upper-tropospheric moisture. Finally, we investigate the impact of the clear-sky sampling restriction upon the satellite moisture climatology. Comparison of clear-sky and total-sky radiosonde observations suggests the clear-sky sampling limitation introduces a modest dry bias (<10% relative humidity) in the satellite climatology.
Soden, Brian J., Robert E Tuleya, and C S Velden, 1996: Improving hurricane forecasts through the assimilation of satellite-derived winds In 15th Conference on Weather Analysis and Forecasting, Boston, MA, American Meteorological Society, 505.
Westphal, D L., and Brian J Soden, et al., 1996: Initialization and validation of a simulation of Cirrus using FIRE-II data. Journal of the Atmospheric Sciences, 53(23), 3397-3429. Abstract PDF
Observations from a wide variety of instruments and platforms are used to validate many different aspects of a three-dimensional mesoscale simulation of the dynamics, cloud microphysics, and radiative transfer of a cirrus cloud system observed on 26 November 1991 during the second cirrus field program of the First International Satellite Cloud Climatology Program (ISCCP) Regional Experiment (FIRE-II) located in southeastern Kansas. The simulation was made with a mesoscale dynamical model utilizing a simplified bulk water cloud scheme and a spectral model of radiative transfer. Expressions for cirrus optical properties for solar and infrared wavelength intervals as functions of ice water content and effective particle radius are modified for the midlatitude cirrus observed during FIRE-II and are shown to compare favorably with explicit size-resolving calculations of the optical properties. Rawinsonde, Raman lidar, and satellite data are evaluated and combined to produce a time-height cross section of humidity at the central FIRE-II site for model verification. Due to the wide spacing of rawinsondes and their infrequent release, important moisture features go undetected and are absent in the conventional analyses. The upper-tropospheric humidities used for the initial conditions were generally less than 50% of those inferred from satellite data, yet over the course of a 24-h simulation the model produced a distribution that closely resembles the large-scale features of the satellite analysis. The simulated distribution and concentration of ice compares favorably with data from radar, lidar, satellite, and aircraft. Direct comparison is made between the radiative transfer simulation and data from broadband and spectral sensors and inferred quantities such as cloud albedo, optical depth, and top-of-the-atmosphere 11-Mu m brightness temperature, and the 6.7 um brightness temperature. Comparison is also made with theoretical heating rates calculated using the rawinsonde data and measured ice water size distributions near the central site. For this case study, and perhaps for most other mesoscale applications, the differences between the observed and simulated radiative quantities are due more to errors in the prediction of ice water content, than to errors in the optical properties or the radiative transfer solution technique.
Soden, Brian J., and R Fu, 1995: A satellite analysis of deep convection, upper-tropospheric humidity, and the greenhouse effect. Journal of Climate, 8(10), 2333-2351. Abstract PDF
This paper combines satellite measurements of the upwelling 6.7-µm radiance from TOVS with cloud-property information from ISCCP and outgoing longwave radiative fluxes from ERBE to analyze the climatological interactions between deep convection, upper-tropospheric humidity, and atmospheric greenhouse trapping. The satellite instruments provide unmatched spatial and temporal coverage, enabling detailed examination of regional, seasonal, and interannual variations between these quantities. The present analysis demonstrates that enhanced tropical convection is associated with increased upper-tropospheric relative humidity. The positive relationship between deep convection and upper-tropospheric humidity is observed for both regional and temporal variations, and is also demonstrated to occur over a wide range of space and time scales. Analysis of ERBE outgoing longwave radiation measurements indicates that regions or periods of increased upper-tropospheric moisture are strongly correlated with an enhanced greenhouse trapping, although the effects of lower-tropospheric moisture and temperature lapse rate are also observed to be important. The combined results for the Tropics provide a picture consistent with a positive interrelationship between deep convection, upper-tropospheric humidity, and the greenhouse effect. In extratropical regions, temporal variations in upper-tropospheric humidity exhibit little relationship to variations in deep convection, suggesting the importance of other dynamical processes in determining changes in upper-tropospheric moisture for this region. Comparison of the observed relationships between convection, upper-tropospheric moisture,and greenhouse trapping with climate model simulations indicates that the Geophysical Fluid Dynamics Laboratory (GFDL) GCM is qualitatively successful in capturing the observed relationship between these quantities. This evidence supports the ability of the GFDL GCM to predict upper-tropospheric water vapor feedback, despite the model's relatively simplified treatment of moist convective processes.
Wetherald, Richard T., and Brian J Soden, 1995: General simulation of atmospheric temperature and moisture in the GFDL AMIP Experiment In Proceedings of the First International AMIP Scientific Conference, WCRP-92, WMO/TD No. 732, Geneva, Switzerland, World Meteorological Organization, 97-100.
Soden, Brian J., A S Ackerman, D O'C Starr, S H Melfi, and R A Ferrare, 1994: Comparison of upper tropospheric water vapor from GOES, Raman lidar, and cross-chain loran atmospheric sounding system measurements. Journal of Geophysical Research, 99(D10), 21,005-21,016. Abstract PDF
Observations of upper tropospheric relative humidity obtained from Raman lidar and CLASS sonde instruments obtained during the FIRE Cirrus-II field program are compared with satellite measurements from the GOES 6.7-µm channel. The 6.7-µm channel is sensitive to water vapor integrated over a broad layer in the upper troposphere (roughly 500-200 mbar). Instantaneous measurements of the upper tropospheric relative humidity from GOES are shown to agree to within roughly 6% of the nearest lidar observations and 9% of the nearest CLASS observations. The CLASS data exhibit a slight yet systematic dry bias in upper tropospheric humidity, a result which is consistent with previous radiosonde intercomparisons. Temporal stratification of the CLASS data indicates that the magnitude of the bias is dependent upon the time of day, suggesting a solar heating effect in the radiosonde sensor. Using CLASS profiles, the impact of vertical variability in relative humidity upon the GOES upper tropospheric humidity measurements is also examined. The upper tropospheric humidity inferred from the GOES 6.7-µm channel is demonstrated to agree to within roughly 5% of the relative humidity vertically averaged over the depth of atmosphere to which the 6.7-µm channel is sensitive. The results of this study encourage the use of satellite measurements in the 6.7-µm channel to quantitatively describe the distribution and temporal evolution of the upper tropospheric humidity field.
Soden, Brian J., and F P Bretherton, 1994: Evaluation of water vapor distribution in general circulation models using satellite observations. Journal of Geophysical Research, 99(D1), 1187-1210. Abstract PDF
This paper presents a comparison of the water vapor distribution obtained from two general circulation models, the European Centre for Medium-Range Weather Forecasts (ECMWF) model and the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM), with satellite observations of total precipitable water (TPW) from SSM/I and upper tropospheric relative humidity (UTH) from GOES. Overall, both models are successful in capturing the primary features of the observed water vapor distribution and its seasonal variation. For the ECMWF model, however, a systematic moist bias in TPW is noted over well-known stratocumulus regions in the eastern subtropical oceans. Comparison with radiosonde profiles suggests that this problem is attributable to difficulties in modeling the shallowness of the boundary layer and large vertical water vapor gradients which characterize these regions. In comparison, the CCM is more successful in capturing the low values of TPW in the stratocumulus regions, although it tends to exhibit a dry bias over the eastern half of the subtropical oceans and a corresponding moist bias in the western half. The CCM also significantly overestimates the daily variability of the moisture fields in convective regions, suggesting a problem in simulating the temporal nature of moisture transport by deep convection. Comparison of the monthly mean UTH distribution indicates generally larger discrepancies than were noted for TPW owing to the greater influence of large-scale dynamical processes in determining the distribution of UTH. In particular, the ECMWF model exhibits a distinct dry bias along the ITCZ and a moist bias over the subtropical descending branches of the Hadley cell, suggesting an underprediction in the strength of the Hadley circulation. The CCM, on the other hand, demonstrates greater discrepancies in UTH than are observed for the ECMWF model, but none that are as clearly correlated with well-known features of the large-scale circulation.
Soden, Brian J., and Leo J Donner, 1994: Evaluation of a GCM cirrus parameterization using satellite observations. Journal of Geophysical Research, 99(D7), 14,401-14,413. Abstract PDF
This study applies a simple yet effective methodology to validate a general circulation model parameterization of cirrus ice water path. The methodology combines large-scale dynamic and thermodynamic fields from operational analyses with prescribed occurrence of cirrus clouds from satellite observations to simulate a global distribution of ice water path. The predicted cloud properties are then compared with the corresponding satellite measurements of visible optical depth and infrared cloud emissivity to evaluate the reliability of the parameterization. This methodology enables the validation to focus strictly on the water loading side of the parameterization by eliminating uncertainties involved in predicting the occurrence of cirrus internally within the parameterization. Overall, the parameterization performs remarkably well in capturing the observed spatial patterns of cirrus optical properties. Spatial correlations between the observed and the predicted optical depths are typically greater than 0.7 for the tropics and northern hemisphere midlatitudes. The good spatial agreement largely stems from the strong dependence of the ice water path upon the temperature of the environment in which the clouds form. Poorer correlation (r ~ 0.3) are noted over the southern hemisphere midlatitudes, suggesting that additional processes not accounted for by the parameterization may be important there. Quantitative evaluation of the parameterization is hindered by the present uncertainty in the size distribution of cirrus ice particles. Consequently, it is difficult to determine if discrepancies between the observed and the predicted optical properties are attributable to errors in the parameterized ice water path or to geographic variations in effective radii.
Soden, Brian J., and F P Bretherton, 1993: Upper troposphere relative humidity from the GOES 6.7 µm Channel: Method and climatology for July 1987. Journal of Geophysical Research, 98(D9), 16,669-16,688. Abstract
This paper presents an analysis of upper tropospheric relative humidity and clouds determined from geostationary satellite observations of the upwelling infrared radiation. The 6.7-µm channel is located near the center of a strong water vapor absorption band and under clear sky conditions is primarily sensitive to the relative humidity averaged over a depth of atmosphere extending from 200 to 500 mbar. Estimates of the clear sky radiance at 6.7-µm are obtained by utilizing the local spatial structure of the radiance field at 11-µm and the correlation between 11-µm and 6.7-µm, to discriminate between clear and cloudy pixels. This approach is demonstrated to be more reliable than cloud clearance based solely upon the local spatial structure of the 6.7-µm channel alone and provides estimates of the clear sky 6.7-µm brightness temperature for areas 2° x 2° of latitude and longitude which are repeatable from successive images 30 minutes apart to within approximately 1K. To facilitate the interpretation of the clear sky brightness temperatures, a simplified model of the radiative transfer at 6.7-µm is presented. This model, based upon a set of irregularly spaced, strongly absorbing, pressure-broadened lines, demonstrates that accurate to within approximately 1 K or 10% of the actual relative humidity, the brightness temperature at 6.7-µm is proportional to the natural logarithm of the appropriate vertical average of the relative humidity divided by the cosine of the viewing zenith angle. Estimates of upper tropospheric water vapor inferred in this way from GOES E observations are presented for July 1987. The geographic distribution reflects many well-known features of the large-scale atmospheric circulation. A clear dependence of the greenhouse effect of upper tropospheric water vapor upon the large-scale dynamics is also demonstrated. Finally, the observed relationship between the upper tropospheric relative humidity and the occurrence of upper tropospheric cloud cover is presented and its implications for the parameterization of clouds in general circulation models are discussed.
