Longwave Cloud Forcing Research Articles

Recent Changes in Downward Longwave Radiation at King Sejong Station, Antarctica

AbstractEffects of cloud, air temperature, and specific humidity on downward longwave irradiance and their long-term variabilities are examined by analyzing the measurements made at the King Sejong Station in the Antarctic Peninsula during the period of 1996–2006.It has been shown that the downward longwave irradiance (DLR) is significantly correlated with three variables: air temperature, specific humidity, and cloudiness. Based on the relationship of the three variables with DLR, a multiple linear regression model has been developed in order to evaluate the relative contribution of each of the variables to the variation of DLR. The three variables together explained 75% of all the variance in daily mean DLR. The respective contribution from specific humidity and cloudiness to the variation of DLR was 46% and 23%; thus most of the DLR variability can be explained by the variations in the two variables. The annual mean of longwave cloud forcing shows 52 W m−2 with no remarkable seasonal cycle. It is also noted that the overcast cloud effect gives an increase by 65 W m−2 with respect to clear-sky flux throughout the year. It is suggested that the multiple regression model can be used to estimate the radiative forcings of variables influencing the DLR variability.A highly significant decrease in DLR with an average of −1.52 W m−2 yr−1 (−0.54% yr−1) is found in an analysis from the time series of the deseasonalized monthly mean values. Accordingly, the atmospheric flux emissivity, air temperature, and specific humidity have also decreased in their time series, while the cloudiness has increased insignificantly. Consequently, it may be concluded that the recent decrease in DLR is mainly attributed to the net cooling effect due to the decrease in air temperature and specific humidity, which overwhelm the slight warming effect in cloudiness. Analysis of mean monthly trends for individual months shows that, as for the annual data, the variations in DLR are mostly associated with those of air temperature, specific humidity, and cloudiness.

Dynamic Effects on the Tropical Cloud Radiative Forcing and Radiation Budget

Abstract Vertical velocity is used to isolate the effect of large-scale dynamics on the observed radiation budget and cloud properties in the tropics, using the methodology suggested by Bony et al. Cloud and radiation budget quantities in the tropics show well-defined responses to the large-scale vertical motion at 500 hPa. For the tropics as a whole, the ratio of shortwave to longwave cloud forcing (hereafter N) is about 1.2 in regions of upward motion, and increases to about 1.9 in regions of strong subsidence. If the analysis is restricted to oceanic regions with SST > 28°C, N does not increase as much for subsiding motions, because the stratocumulus regions are eliminated, and the net cloud forcing decreases linearly from about near zero for zero vertical velocity to about −15 W m−2 for strongly subsiding motion. Increasingly negative cloud forcing with increasing upward motion is mostly related to an increasing abundance of high, thick clouds. Although a consistent dynamical effect on the annual cycle of about 1 W m−2 can be identified, the effect of the probability density function (PDF) of the large-scale vertical velocity on long-term trends in the tropical mean radiation budget is very small compared to the observed variations. Observed tropical mean changes can be as large as ±3 W m−2, while the dynamical components are generally smaller than ±0.5 W m−2. For relatively small regions in the east and west Pacific, changes in the relative magnitude of longwave and shortwave cloud forcing can be related to the PDF of vertical velocity. The east Pacific in 1987 and 1998 showed large reductions of N in association with an increase in the fraction of the area in the domain with upward motion, and concomitant increases in high cloud. For the west Pacific in 1998, a large increase in N was caused not so much by a change in the mean vertical motion, but rather by a shift from top- to bottom-heavy upward motion.

Towards constraining climate sensitivity by linear analysis of feedback patterns in thousands of perturbed-physics GCM simulations

A linear analysis is applied to a multi-thousand member “perturbed physics" GCM ensemble to identify the dominant physical processes responsible for variation in climate sensitivity across the ensemble. Model simulations are provided by the distributed computing project, climate prediction.net . A principal component analysis of model radiative response reveals two dominant independent feedback processes, each largely controlled by a single parameter change. The leading EOF was well correlated with the value of the entrainment coefficient—a parameter in the model’s atmospheric convection scheme. Reducing this parameter increases high vertical level moisture causing an enhanced clear sky greenhouse effect both in the control simulation and in the response to greenhouse gas forcing. This effect is compensated by an increase in reflected solar radiation from low level cloud upon warming. A set of ‘secondary’ cloud formation parameters partly modulate the degree of shortwave compensation from low cloud formation. The second EOF was correlated with the scaling of ice fall speed in clouds which affects the extent of cloud cover in the control simulation. The most prominent feature in the EOF was an increase in longwave cloud forcing. The two leading EOFs account for 70% of the ensemble variance in λ—the global feedback parameter. Linear predictors of feedback strength from model climatology are applied to observational datasets to estimate real world values of the overall climate feedback parameter. The predictors are found using correlations across the ensemble. Differences between predictions are largely due to the differences in observational estimates for top of atmosphere shortwave fluxes. Our validation does not rule out all the strong tropical convective feedbacks leading to a large climate sensitivity.

Observed Relationships between Arctic Longwave Cloud Forcing and Cloud Parameters Using a Neural Network

Abstract A neural network technique is used to quantify relationships involved in cloud–radiation feedbacks based on observations from the Surface Heat Budget of the Arctic (SHEBA) project. Sensitivities of longwave cloud forcing (CFL) to cloud parameters indicate that a bimodal distribution pattern dominates the histogram of each sensitivity. Although the mean states of the relationships agree well with those derived in a previous study, they do not often exist in reality. The sensitivity of CFL to cloud cover increases as the cloudiness increases with a range of 0.1–0.9 W m−2 %−1. There is a saturation effect of liquid water path (LWP) on CFL. The highest sensitivity of CFL to LWP corresponds to clouds with low LWP, and sensitivity decreases as LWP increases. The sensitivity of CFL to cloud-base height (CBH) depends on whether the clouds are below or above an inversion layer. The relationship is negative for clouds higher than 0.8 km at the SHEBA site. The strongest positive relationship corresponds to clouds with low CBH. The dominant mode of the sensitivity of CFL to cloud-base temperature (CBT) is near zero and corresponds to warm clouds with base temperatures higher than −9°C. The low and high sensitivity regimes correspond to the summer and winter seasons, respectively, especially for LWP and CBT. Overall, the neural network technique is able to separate two distinct regimes of clouds that correspond to different sensitivities; that is, it captures the nonlinear behavior in the relationships. This study demonstrates a new method for evaluating nonlinear relationships between climate variables. It could also be used as an effective tool for evaluating feedback processes in climate models.

Surface Cloud Forcing in the East Pacific Stratus Deck/Cold Tongue/ITCZ Complex*

Abstract Data from the Eastern Pacific Investigation of Climate Studies (EPIC) mooring array are used to evaluate the annual cycle of surface cloud forcing in the far eastern Pacific stratus cloud deck/cold tongue/intertropical convergence zone complex. Data include downwelling surface solar and longwave radiation from 10 EPIC-enhanced Tropical Atmosphere Ocean (TAO) moorings from 8°S, 95°W to 12°N, 95°W, and the Woods Hole Improved Meteorology (IMET) mooring in the stratus cloud deck region at 20°S, 85°W. Surface cloud forcing is defined as the observed downwelling radiation at the surface minus the clear-sky value. Solar cloud forcing and longwave cloud forcing are anticorrelated at all latitudes from 12°N to 20°S: clouds tended to reduce the downward solar radiation and to a lesser extent increase the downward longwave radiation at the surface. The relative amount of solar radiation reduction and longwave increase depends upon cloud type and varies with latitude. A statistical relationship between solar and longwave surface cloud forcing is developed for rainy and dry periods and for the full record length in six latitudinal regions: northeast tropical warm pool, ITCZ, frontal zone, cold tongue, southern, and stratus deck regions. The buoy cloud forcing observations and empirical relations are compared with the International Satellite Cloud Climatology Project (ISCCP) radiative flux data (FD) dataset and are used as benchmarks to evaluate surface cloud forcing in the NCEP Reanalysis 2 (NCEP2) and 40-yr ECMWF Re-Analysis (ERA-40). ERA-40 and NCEP2 cloud forcing (both solar and longwave) showed large discrepancies with observations, being too large in the ITCZ and equatorial regions and too weak under the stratus deck at 20°S and north to the equator during the cool season from July to December. In particular the NCEP2 cloud forcing at the equator was nearly identical to the ITCZ region and thus had significantly larger solar cloud forcing and smaller longwave cloud forcing than observed. The net result of the solar and longwave cloud forcing deviations is that there is too little radiative warming in the ITCZ and southward to 8°S during the warm season and too much radiative warming under the stratus deck at 20°S and northward to the equator during the cold season.

Cloud Radiative Forcing of the Arctic Surface: The Influence of Cloud Properties, Surface Albedo, and Solar Zenith Angle

Abstract An annual cycle of cloud and radiation measurements made as part of the Surface Heat Budget of the Arctic (SHEBA) program are utilized to determine which properties of Arctic clouds control the surface radiation balance. Surface cloud radiative forcing (CF), defined as the difference between the all-sky and clear-sky net surface radiative fluxes, was calculated from ground-based measurements of broadband fluxes and results from a clear-sky model. Longwave cloud forcing (CFLW) is shown to be a function of cloud temperature, height, and emissivity (i.e., microphysics). Shortwave cloud forcing (CFSW) is a function of cloud transmittance, surface albedo, and the solar zenith angle. The annual cycle of Arctic CF reveals cloud-induced surface warming through most of the year and a short period of surface cooling in the middle of summer, when cloud shading effects overwhelm cloud greenhouse effects. The sensitivity of CFLW to cloud fraction is about 0.65 W m−2 per percent cloudiness. The sensitivity of ...

Some considerations on the infrared cloud forcing

Long‐wave spectral cloud forcing is computed for realistic clouds of various optical thickness embedded in the tropical and subarctic winter standard atmospheres. The strict connections between the integrated long‐wave cloud forcing (LCF) and cloud's altitude and transmissivity are shown. LCF, especially in presence of ice clouds, grows almost linearly with cloud transmissivity, and largest values are found in presence of high and opaque cirri. We have also considered the effect of incomplete spectral coverage in broadband long‐wave radiometry when estimating LCF. The error has been computed as a function of edge cutoff (two different filters are used) and cloud's transmissivity and top altitude. Uncorrected measurements from sensors with incomplete spectral coverage generally underestimate the real LCF, especially for the highest cirrus clouds in the tropics. A correction is possible since the percentage error is practically constant with cloud's transmissivity.

Inconsistencies between satellite estimates of longwave cloud forcing and dynamical fields from reanalyses

The greenhouse effect of cloud may be quantified as the difference between outgoing longwave radiation (OLR) and its clear‐sky component (OLRc). Clear‐sky measurements from satellite preferentially sample drier, more stable conditions relative to the monthly‐mean state. The resulting observational bias is evident when OLRc is stratified by vertical motion; differences to climate model OLRc of 15 Wm−2 occur over warm regions of strong ascent. Using data from the ECMWF 40‐year reanalysis, an estimate of cloud longwave radiative effect is made which is directly comparable with standard climate model diagnostics. The impact of this methodology on the cancellation of cloud longwave and shortwave radiative forcing in the tropics is estimated.

Influence of Dynamics on the Changes in Tropical Cloud Radiative Forcing during the 1998 El Niño

Abstract Satellite measurements of the radiation budget and data from the U.S. National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis are used to investigate the links between anomalous cloud radiative forcing over the tropical west Pacific warm pool and the tropical dynamics and sea surface temperature (SST) distribution during 1998. The ratio, N, of the shortwave cloud forcing (SWCF) to longwave cloud forcing (LWCF) (N = −SWCF/LWCF) is used to infer information on cloud altitude. A higher than average N during 1998 appears to be related to two separate phenomena. First, dynamic regime-dependent changes explain high values of N (associated with low cloud altitude) for small magnitudes of SWCF and LWCF (low cloud fraction), which reflect the unusual occurrence of mean subsiding motion over the tropical west Pacific during 1998, associated with the anomalous SST distribution. Second, Tropics-wide long-term changes in the spatial-mean cloud forcing, independent of ...

Cloud radiative forcing at the top of the atmosphere during FIRE ACE derived from AVHRR data

Cloud radiative forcing at the top of the atmosphere is derived from narrowband visible and infrared radiances from NOAA‐12 and NOAA‐14 advanced very high resolution radiometer (AVHRR) data taken over the Arctic Ocean during the First ISCCP Regional Experiment Arctic Cloud Experiment (FIRE ACE) during spring and summer 1998. Shortwave and longwave fluxes at the top of the atmosphere (TOA) were computed using narrowband‐to‐broadband conversion formulae based on coincident Earth Radiation Budget Experiment (ERBE) broadband and AVHRR narrowband radiances. The NOAA‐12/NOAA‐14 broadband data were validated using model calculations and coincident broadband flux radiometer data from the Surface Heat Budget of the Arctic Ocean experiment and from aircraft data. The AVHRR TOA albedos agreed with the surface‐ and aircraft‐based albedos to within one standard deviation of ±0.029 on an instantaneous basis. Mean differences ranged from −0.012 to 0.023 depending on the radiometer and platform. AVHRR‐derived longwave fluxes differed from the model calculations using aircraft‐ and surface‐based fluxes by −0.2 to −0.3 W m−2, on average, when the atmospheric profiles were adjusted to force agreement between the observed and the calculated downwelling fluxes. The standard deviations of the differences were less than 2%. Mean total TOA albedo for the domain between 72°N and 80°N and between 150°W and 180°W changed from 0.695 in May to 0.510 during July, while the longwave flux increased from 217 to 228 W m−2. Net radiation increased from −89 to −2 W m−2 for the same period. Net cloud forcing varied from −15 W m−2 in May to −31 W m−2 during July, while longwave cloud forcing was nearly constant at ∼8 W m−2. Shortwave cloud forcing dominated the cloud effect, ranging from −22 W m−2 during May to −40 W m−2 in July. The mean albedos and fluxes are consistent with previous measurements from the ERBE, except during May when the albedo and longwave flux were greater than the maximum ERBE values. The cloud‐forcing results, while similar to some earlier estimates, are the most accurate values hitherto obtained for regions in the Arctic. When no significant melting was present, the clear‐sky longwave flux showed a diurnal variation similar to that over land under clear skies. These data should be valuable for understanding the Arctic energy budget and for constraining models of atmosphere and ocean processes in the Arctic.

Dynamic and Thermodynamic Regulation of Ocean Warming*

The relative roles of clouds, surface evaporation, and ocean heat transport in limiting maximum sea surface temperatures (SSTs) in the western Pacific warm pool are investigated by means of simple and intermediate coupled ocean‐atmosphere models. The authors first take an analytical approach by constructing a conceptual two-box model that contains dynamic coupling among the Walker circulation, SST, and ocean thermocline and thermodynamic coupling, which includes shortwave and longwave cloud forcing and latent and sensible heat fluxes at the ocean surface. In a realistic parameter regime, the three mechanisms mentioned above are all essential in limiting the SSTs within the observed range. The lack of any one mechanism would lead to an equilibrium SST that is too high, although unstable warming due to the super greenhouse effect would not occur. The analysis of the surface heat balance from the simple box model indicates that in the western Pacific warm pool, cloud reflection has a dominant effect, followed by evaporation and ocean dynamics. The simple model results are further evaluated numerically by using an intermediate coupled ocean‐atmosphere model. With the forcing of the annual-mean solar radiation, this model is capable of simulating a realistic annual mean climate in the tropical Pacific. The authors then introduce an initial SST perturbation and examine how the perturbation evolves with time in the presence of clouds, surface evaporation, and ocean dynamic processes. Four experiments have been designed. In the first three experiments, each of the three processes is studied separately; in the last experiment, they are combined. The intermediate model results indicate that in the western Pacific warm pool, the largest negative feedback comes from the cloud shortwave radiation forcing, followed by the surface evaporation and ocean heat transport. The sensitivity of the model to various initial SST perturbation patterns is also investigated.

Implications of Microphysics for Cloud-Radiation Parameterizations: Lessons from TOGA COARE

Abstract A single-column model (SCM) and observational data collected during TOGA COARE were used to investigate the sensitivity of model-produced cloud properties and radiative fluxes to the representation of cloud microphysics in the cloud-radiation parameterizations. Four 78-day SCM numerical experiments were conducted for the atmospheric column overlying the COARE Intensive Flux Array. Each SCM experiment used a different cloud-radiation parameterization with a different representation of cloud microphysics. All the SCM experiments successfully reproduced most of the observed temporal variability in precipitation, cloud fraction, shortwave and longwave cloud forcing, and downwelling surface shortwave flux. The magnitude and temporal variability of the downward surface longwave flux was overestimated by all the SCM experiments. This bias is probably due to clouds forming too low in the model atmosphere. Time-averaged model results were used to examine the sensitivity of model performance to the differe...

A possible change in cloud radiative forcing due to aircraft exhaust

Aircraft exhaust may reduce the crystal size in natural cirrus. This work investigates the change in cloud radiative forcing from such a size reduction by assuming a constant ice water content. A 1‐dim model with radiative properties that depend on the mean crystal size is used to compute the radiative transfer for an atmospheric column. The results show that the negative shortwave cloud forcing is enhanced with smaller crystals as they mainly increase the reflectivity of clouds. The change in the longwave cloud forcing is always positive although its magnitude depends strongly on the ice water path. The weighted sum of SW and LW cloud forcings depends on the mean crystal size, surface albedo and ice water content. It appears that there is a range of diameters between 15 and 25 µm where the response to a reduction in crystal size is fairly insensitive. Below and above this range the change is negative or positive, respectively. In regions of dense airtraffic the magnitude of the change in cloud forcing could be on the order of 0.3 W m−2 under the assumption of a 20% decrease of the mean crystal size from about 30 µm to 24 µm. Aircraft exhaust thus has the potential to affect the climate but the results should be taken with caution as they are based on parameterized optical properties for cirrus clouds.

Radiative effects of convection in the tropical Pacific

The radiative effects of tropical clouds at the tropopause and the ocean surface have been estimated by using in situ measurements from the Central Equatorial Pacific Experiment (CEPEX). The effect of clouds is distinguished from the radiative effects of the surrounding atmosphere by calculating the shortwave and longwave cloud forcing. These terms give the reduction in insolation and the increase in absorption of terrestrial thermal emission associated with clouds. At the tropopause the shortwave and longwave cloud forcing are nearly equal and opposite, even on daily timescales. Therefore the net effect of an ensemble of convective clouds is small compared to other radiative terms in the surface‐tropospheric heat budget. This confirms the statistical cancellation of cloud forcing observed in Earth radiation budget measurements from satellites. At the surface the net effect of clouds is to reduce the radiant energy absorbed by the ocean. Under deep convective clouds the diurnally averaged reduction exceeds 150 W m−2. The divergence of flux in the cloudy atmosphere can be estimated from the difference in cloud forcing at the surface and tropopause. The CEPEX observations show that the atmospheric cloud forcing is nearly equal and opposite to the surface forcing. Based upon the frequency of convection, the atmospheric forcing approaches 100 W m−2 when the surface temperature is 303 K. The cloud forcing is closely related to the frequency of convective cloud systems. This relation is used in conjunction with cloud population statistics derived from satellite to calculate the change in surface cloud forcing with sea surface temperature. The net radiative cooling of the surface by clouds increases at a rate of 20 W m−2K−1 during the CEPEX observing period.

Tropical cirrus cloud radiative forcing: Sensitivity studies

We have performed one dimensional radiative transfer calculations to evaluate the impact of cirrus clouds on the tropical radiation budget. We investigate the sensitivity of solar and infrared fluxes to cloud optical depth, particle size distributions, and cloud height. If the observed solar cloud forcing in excess of 100 W‐m−2 is to be attributed to cirrus anvils alone, then the optical depth of these anvils must be at least 5 (assuming 50% cloud cover and an ice crystal effective radius of 15 µm). The net radiative forcing of cirrus near the tropical tropopause is positive (heating) for cloud optical depths less than about 16 and negative (cooling) for larger optical depths. If cirrus clouds alone are responsible for the equal and opposite shortwave and longwave cloud forcing in excess of 100 W‐m−2 observed by ERBE, then the cirrus must typically take the form of deep, optically thick clouds with relatively small particles (radii of 10‐20 µm) and cloud‐tops well below the tropopause. The maintenance of this balance on monthly time scales can be attributed to a variety of correlations: The cloud cover of optically thick cirrus or thin cirrus overlying low‐level stratus clouds could vary; or cirrus anvil height could increase along with a decrease in the ice crystal effective radius and an increase in optical depth. It would be of great interest to determine observationally which of these correlations is responsible for the observed lack of variation in cloud forcing.

Seasonal variation of surface and atmospheric cloud radiative forcing over the globe derived from satellite data

Global distributions of surface and atmospheric cloud radiative forcing parameters have been derived using parameterized radiation models with satellite meteorological data from the International Satellite Cloud Climatology Project, and directly measured top‐of‐atmosphere radiative fluxes from the Earth Radiation Budget Experiment. Specifically, shortwave, longwave, and total cloud forcing at the surface, and column‐averaged values of longwave cloud forcing of the atmosphere were derived for the midseasonal months of April, July, and October 1985 and January 1986, covering a complete annual cycle. Seasonal variability is illustrated by comparing the results for July 1985 and January 1986, which represent the seasonal extremes. Surface shortwave cloud forcing is always negative, representing a cooling of the surface, with strongest cooling (−120 to −180 W m−2 ) occurring over midlatitude storm tracks of the summer hemisphere. Surface longwave cloud forcing is always positive, representing a warming of the surface, with strongest warming (60 to 75 W m−2) occurring over storm tracks of the winter hemisphere. Zonal averages show the entire summer hemisphere dominated by shortwave cooling, the middle and high latitudes of the winter hemisphere dominated by longwave warming, and a broad zone of transition in between. The globally averaged total cloud forcing amounts to a cooling throughout the year, ranging from a low of about −12 W m−2 for July 1985 to a high of about −25 W m−2 for January 1986. The longwave cloud forcing of the atmosphere shows a strong warming over deep convective regions in the tropics and a moderate cooling outside the tropics, amounting to a weak cooling (−2 to −5 W m−2) in the global average. Comparisons of the results with general circulation model simulations show broad qualitative agreement regarding the locations of prominent warming and cooling regions. Quantitative comparisons, on the other hand, show significant differences between the magnitudes of warming and cooling in these regions. Most of the larger differences can be attributed to known deficiences of the general circulation model simulations. Comparisons with satellite‐derived results of other investigators show much better agreement.

Satellite validation of GCM‐simulated annual cycle of the earth radiation budget and cloud forcing

Earth Radiation Budget Experiment (ERBE) data are used to validate radiative fluxes and cloud radiative forcing (CRF) simulated by the Laboratoire de Météorologie Dynamique (LMD) general circulation model (GCM). The emphasis of the work is on the development of new tests to obtain more significant elements of comparison between model simulations and satellite observations. These tests are applied to the clear‐sky fluxes and the cloud radiative forcing. The validation of the CRF described by a model requires to test the consistency between the solar or shortwave (SW: 0.2 to 5 μm) and longwave (LW: 5 to 50 μm) cloud forcing. For this purpose, we compute the mean cloud perturbation of the planetary albedo as a function of the LW cloud forcing for both model results and ERBE observations. In the SW spectral domain, the consideration of total fluxes does not provide very constraining elements of validation because most of the observed variations are prescribed (incoming solar radiation, solar zenith angle). We therefore distinguish the part of the SW seasonal variations related only to the variation of external parameters (mainly the insolation) from the part which arises from the combined variation of internal climate parameters (mainly cloud albedo and snow/ice cover) with the insolation. Fourier analysis is used to study the seasonal amplitude and phase of the CRF. The seasonal variation of the cloudiness is, respectively, out of phase (in phase) with the insolation in mid‐latitudes (in low and high latitudes). We show that this acts to enhance (to reduce) the seasonal amplitude of the absorbed SW flux in mid‐latitudes (in low and high latitudes). Finally, we show that the impact of the seasonal variation of the cloudiness on the variation of the net CRF is less than 10 W m−2.

The Effect of Cloud Type on Earth's Energy Balance: Global Analysis

Abstract The role of fractional area coverage by cloud types in the energy balance of the earth is investigated through joint use of International Satellite Cloud Climatology Project (ISCCP) C1 cloud data and Earth Radiation Budget Experiment (ERBE) broadband energy flux data for the one-year period March 1985 through February 1986. Multiple linear regression is used to relate the radiation budget data to the cloud data. Comparing cloud forcing estimates obtained from the ISCCP-ERBE regression with those derived from the ERBE scene identification shows generally good agreement except over snow, in tropical convective regions, and in regions that are either nearly cloudless or always overcast. It is suggested that a substantial fraction of the disagreement in longwave cloud forcing in tropical convective regions is associated with the fact that the ERBE scene identification does not take into account variations in upper-tropospheric water vapor. On a global average basis, low clouds make the largest contri...

The response of a general circulation model to cloud longwave radiative forcing. II: Further studies

AbstractFollowing on from the first part of this study, the impact of cloud longwave forcing on the general circulation has been studied further using a series of 510‐day, constant January integrations with the NCAR Community Climate Model. The sensitivity of the global response to the vertical profile of the forcing has been assessed by replacing the cloud prediction scheme by that used in the ECMWF model. The results confirm those reported in Part I and emphasize the influence of tropical cirrus clouds on the local thermal structure and on the strength of the subtropical jets. The impact on the model's hydrological cycle of the cloud longwave forcing associated with boundary‐layer clouds is also shown to be important.Several integrations are described which assess the relative importance of the three major tropical forcing maxima over Indonesia, South Africa and South America in determining both the local and remote responses. For each region, the perturbation to the upper tropospheric diabatic heating by the radiative effects of the cirrus clouds excites an anticyclonic vorticity pair, located near the longitude of the forcing and almost symmetric about the equator. The influence of these anticyclones on the upper tropospheric circulation is substantial. The results also indicate that a perturbation to the diabatic heating over South America, provided in this case by the cloud longwave forcing, may have an important effect on the Walker circulation and on the extra‐tropical flow. The implications of these results for the problem of deforestation are discussed.

Seasonal variation of cloud radiative forcing derived from the Earth Radiation Budget Experiment

The NASA Earth Radiation Budget Experiment (ERBE), flying aboard multiple satellites, is providing new insights into the climate system. Monthly averaged clear‐sky and cloudy sky flux data derived from the ERBE are used to assess the impact of clouds on the Earth's radiation balance. This impact is examined in terms of three quantities: longwave, shortwave, and net cloud forcing. Overall, clouds appear to cool the Earth‐atmosphere system. The global mean cooling varied from 14 to 21 W m−2 between April 1985 and January 1986. Hemispherically, the longwave and shortwave cloud forcing nearly cancel each other in the winter hemisphere, while in the summer the negative shortwave cloud forcing is significantly lower than the longwave cloud forcing, producing a strong cooling. Thus clouds significantly reduce the seasonal changes in the net radiative heating of the planet. This reduction is particularly strong over the mid‐ and high‐latitude oceans, where they reduce the summer and spring solar heating by as much as 100–150 W m−2. In the low latitudes, the longwave and shortwave cloud forcing reach peak values over the convectively disturbed regions and tend to offset each other to a large extent. This feature, when combined with the large cooling effect over mid‐ and high‐latitude oceans, leads to the conclusion that clouds significantly reduce the equator‐to‐pole radiative heating gradient of the planet during spring and summer. In the tropical convective regions the large magnitudes of the shortwave and longwave forcing and the near cancellation of the two suggest that clouds have a significant influence on the vertical distribution of heating between the atmosphere and the surface. Thus the ERBE data reveal that globally, hemispherically, and zonally, clouds have a significant effect on the radiative heating gradients. Comparisons of the ERBE results with general circulation models (GCMs) show that global net cloud forcing can be determined reasonably well from some current versions of the GCMs. Modeled regional and zonal values of radiative cloud forcing, however, indicate a need for considerable improvement.