Shortwave Cloud Radiative Forcing Research Articles

SUNYA Regional Climate Model Simulations of East Asia Summer Monsoon: Effects of Cloud Vertical Structure on the Surface Energy Balance

We used the State University of New York at Albany (SUNYA) regional climate model to study the effect of cloud vertical distribution in affecting the surface energy balance of the East Asia summer monsoon (EASM). Simulations were conducted for the summers of 1988 and 1989, during which large contrast in the intra-seasonal cloud radiative forcing (CRF) was observed at the top of the atmosphere. The model results indicate that both the high and low clouds are persistent throughout the summer months in both years. Because of large cloud water, low clouds significantly reduce the solar radiation flux reaching the surface, which nevertheless still dominate the surface energy balance, accounting for more than 50% of the surface heating. The low clouds also contribute significantly the downward longwave radiation to the surface with values strongly dependent on the cloud base temperature. The presence of low clouds effectively decreases the temperature and moisture gradients near surface, resulting in a substantial decrease in the sensible and latent heat fluxes from surface, which partially compensate the decrease of the net radiative cooling of the surface. For example, in the two days, May 8 and July 11 of 1988, the total cloud cover of 80% is simulated, but the respective low cloud cover (water) was 63% (114 gm^(-2)) and 22% (21 gm^(-2)). As a result, the downward solar radiation is smaller by 161 Wm-2 in May 8. On the other hand, the cloud temperature was 10℃ lower, yielding 56 Wm^(-2) smaller downward longwave radiation. The near surface temperature and gradient is more than 1℃ smaller (and moisture gradient), leading to 21 and 81 Wm^(-2) smaller sensible heat and latent heat fluxes. It is also demonstrated that the model is capable to reproduce the intra-seasonal variation of shortwave CRF, and catches the relationship between total cloud cover and SW CRF. The model results show the dominance of high cloud on the regional mean longwave CRF and low cloud on the intra-seasonal variation of shortwave CRF, indicating the importance of cloud vertical structure. The strong negative feedbacks from the responses of latent and sensible heat flux tend to limit the effects of low clouds on the surface temperature simulations, as evidently the surface air temperatures bias of only 0.34℃ in the EASM simulations while the variances of the surface radiative fluxes and heat fluxes are, respectively, in the ranges of 100-200 and 60-110 Wm^(-2) when total cloud cover are all near 80%. Therefore, it is also concluded that surface air temperature, precipitation, and total cloud cover, which are three frequently examined variables for climate models, are not sufficient for model evaluation, but instead the cloud vertical structure needs to be examined.

Physical Processes Associated with the Tropical Atlantic SST Meridional Gradient

Abstract The major modes of seasonal sea surface temperature (SST) meridional gradient and their connection with some regional mean SST indices in the Atlantic Ocean are examined using reanalysis data. The focus of the work is on the evolution of the dominant mode of the meridional SST gradient in boreal spring and the associated physical processes. The spatial distribution of the dominant mode in boreal spring is a seesaw pattern, reflecting the opposite variation of the meridional SST gradient between the subtropical and tropical North Atlantic, which resulted from a coherent warming or cooling with maxima along 10°–15°N. It is confirmed that this mode is dominated by the wind–evaporation–SST feedback. The feedback persists a longer time in the western Atlantic than in the eastern. The contribution to the SST variation is mainly from latent heat flux. The surface longwave and shortwave cloud radiative forcings are mainly determined by low cloud cover variations. The authors also found that the thermodynamic mode that peaked in boreal spring becomes weak in the following boreal summer. A similar thermodynamic mode appears in a northward position in boreal autumn, and its life cycle is shorter than the one in boreal spring. In contrast to the leading mode in boreal spring, it is shown that the leading mode in boreal summer is a dynamical air–sea feedback mode, reflecting a coherent warming or cooling pattern extending from the Angolan coast toward the equator in the Gulf of Guinea. The thermodynamic processes act as a negative feedback. The net surface latent heat flux anomalies are the leading damping factor, while the sensible heat flux plays the same role on a smaller scale.

Relationship between Shortwave Cloud Radiative Forcing and Local Meteorological Variables Compared in Observations and Several Global Climate Models

Abstract The relations between local monthly mean shortwave cloud radiative forcing and aspects of the resolved-scale meteorological fields are investigated in hindcast simulations performed with 12 of the global coupled models included in the model intercomparison conducted as part of the preparation for Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In particular, the connection of the cloud forcing over tropical and subtropical ocean areas with resolved midtropospheric vertical velocity and with lower-level relative humidity are investigated and compared among the models. The model results are also compared with observational determinations of the same relationships using satellite data for the cloud forcing and global reanalysis products for the vertical velocity and humidity fields. In the analysis the geographical variability in the long-term mean among all grid points and the interannual variability of the monthly mean at each grid point are considered separately. The shortwave cloud radiative feedback (SWCRF) plays a crucial role in determining the predicted response to large-scale climate forcing (such as from increased greenhouse gas concentrations), and it is thus important to test how the cloud representations in current climate models respond to unforced variability. Overall there is considerable variation among the results for the various models, and all models show some substantial differences from the comparable observed results. The most notable deficiency is a weak representation of the cloud radiative response to variations in vertical velocity in cases of strong ascending or strong descending motions. While the models generally perform better in regimes with only modest upward or downward motions, even in these regimes there is considerable variation among the models in the dependence of SWCRF on vertical velocity. The largest differences between models and observations when SWCRF values are stratified by relative humidity are found in either very moist or very dry regimes. Thus, the largest errors in the model simulations of cloud forcing are prone to be in the western Pacific warm pool area, which is characterized by very moist strong upward currents, and in the rather dry regions where the flow is dominated by descending mean motions.

A comparison of low-latitude cloud properties and their response to climate change in three AGCMs sorted into regimes using mid-tropospheric vertical velocity

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 Climatology of Midlatitude Continental Clouds from the ARM SGP Central Facility. Part II: Cloud Fraction and Surface Radiative Forcing

Abstract Data collected at the Department of Energy Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Central Facility (SCF) are analyzed to determine the monthly and hourly variations of cloud fraction and radiative forcing between January 1997 and December 2002. Cloud fractions are estimated for total cloud cover and for single-layered low (0–3 km), middle (3–6 km), and high clouds (>6 km) using ARM SCF ground-based paired lidar–radar measurements. Shortwave (SW) and longwave (LW) fluxes are derived from up- and down-looking standard precision spectral pyranometers and precision infrared radiometer measurements with uncertainties of ∼10 W m−2. The annual averages of total and single-layered low-, middle-, and high-cloud fractions are 0.49, 0.11, 0.03, and 0.17, respectively. Both total- and low-cloud amounts peak during January and February and reach a minimum during July and August; high clouds occur more frequently than other types of clouds with a peak in summer. The average annual downwelling surface SW fluxes for total and low clouds (151 and 138 W m−2, respectively) are less than those under middle and high clouds (188 and 201 W m−2, respectively), but the downwelling LW fluxes (349 and 356 W m−2) underneath total and low clouds are greater than those from middle and high clouds (337 and 333 W m−2). Low clouds produce the largest LW warming (55 W m−2) and SW cooling (−91 W m−2) effects with maximum and minimum absolute values in spring and summer, respectively. High clouds have the smallest LW warming (17 W m−2) and SW cooling (−37 W m−2) effects at the surface. All-sky SW cloud radiative forcing (CRF) decreases and LW CRF increases with increasing cloud fraction with mean slopes of −0.984 and 0.616 W m−2 %−1, respectively. Over the entire diurnal cycle, clouds deplete the amount of surface insolation more than they add to the downwelling LW flux. The calculated CRFs do not appear to be significantly affected by uncertainties in data sampling and clear-sky screening. Traditionally, cloud radiative forcing includes not only the radiative impact of the hydrometeors, but also the changes in the environment. Taken together over the ARM SCF, changes in humidity and surface albedo between clear and cloudy conditions offset ∼20% of the NET radiative forcing caused by the cloud hydrometeors alone. Variations in water vapor, on average, account for 10% and 83% of the SW and LW CRFs, respectively, in total cloud cover conditions. The error analysis further reveals that the cloud hydrometeors dominate the SW CRF, while water vapor changes are most important for LW flux changes in cloudy skies. Similar studies over other locales are encouraged where water and surface albedo changes from clear to cloudy conditions may be much different than observed over the ARM SCF.

Transmission of Solar Radiation by Clouds over Snow and Ice Surfaces. Part II: Cloud Optical Depth and Shortwave Radiative Forcing from Pyranometer Measurements in the Southern Ocean

Abstract Downward solar irradiance at the sea surface, measured on several voyages of an icebreaker in the Southern Ocean, is used to infer transmittance of solar radiation by clouds. Together with surface albedo estimated from coincident hourly sea ice reports, instantaneous cloud radiative forcing and effective cloud optical depth are obtained. Values of “raw cloud transmittance” (trc), the ratio of downward irradiance under cloud to downward irradiance measured under clear sky, vary from 0.1 to 1.0. Over sea ice, few values of trc were observed between 0.8 and 1.0, possibly due to the threshold nature of the aerosol-to-cloud-droplet transition. This sparsely populated region of transmittances is referred to as the Köhler gap. The instantaneous downward shortwave cloud radiative forcing is computed, as well as the time-averaged net forcing. The net forcing at a solar zenith angle of 60° is typically −250 W m−2 over open ocean, but only half this value over sea ice because of the higher surface albedo and less frequent occurrence of clouds. “Effective” optical depths τ (for a radiatively equivalent horizontally homogeneous cloud) are classified by season and surface type. The frequency distributions of τ are well fitted by decaying exponentials, giving a characteristic optical depth of 15 at 47°S, increasing to 24 in the region of maximum cloud cover at 58°S, and decreasing to 11 at 67°S near the coast of Antarctica.

Large-scale changes in the cloud radiative forcing over the Indian region

Abstract Based on earth radiation budget experiment (ERBE) data, earlier studies have shown that in tropical deep-convective regions there is a near cancellation between shortwave and longwave cloud radiative forcing and El Nino event affects the cloud radiative forcing in the Pacific Ocean. The present study investigates these features over the Indian region (0–30°N, 60–120°E) in the peak monsoon month July, (being a representative month of the southwest monsoon season) using satellite measurements of ERBE and International Satellite Cloud Climatology Project (ISCCP) cloud data during the period 1985–1989. It has been observed from the study that a unique imbalance is seen between shortwave cloud radiative forcing (SWCRF) and longwave cloud radiative forcing (LWCRF). It is found to be prominent when the magnitude of LWCRF is higher than 50 W m−2. Net cloud radiative forcing (NCRF) is highly negative in the Indian region from Arabian sea to Indochina and near zero in the equatorial Indian Ocean. The ratio N, i.e., N = - SWCRF / LWCRF , is showing significantly higher values for all the years, the variation of observed N against SWCRF also shows higher values with SWCRF during average of 1985–1989, 1987 and 1988 because it is more than ( N = ∼ 1 ) that mentioned by Kiehl and Ramanathan (1990, Comparison of cloud forcing derived from the earth radiation budget experiment with that simulated by NCAR community climate model. Journal of Geophysical Research 95, 11679–11698) suggesting more imbalance between SWCRF and LWCRF and the lowering of the cloud top pressure particularly in 1987. Cloud radiative forcing (CRF) components are undergoing year-to-year variability with maximum magnitude in 1988 and minimum in 1987 similar to rainfall variation over the region, indicating an association between monsoon rainfall activity and CRF. Large-scale reduction occurred in the magnitudes of CRF and cloud physical properties in the Arabian Sea and south Bay of Bengal especially in 1987, and NCEP/NCAR-vertical velocity for 700 h Pa is also found considerably weak in 1987 in the Arabian Sea and Bay of Bengal i.e., the reduction in the vertical velocity might have affected the cloud formation and their vertical growth and ultimately changed the CRF. We attribute this reduction to the anomalous subsidence associated with the 1987 El Nino event.

A study of convective systems, water vapor and top of the atmosphere cloud radiative forcing over the Indian Ocean using INSAT-1B and ERBE data

The distribution of cloud radiative forcing (CRF) at the top of the atmosphere over the Indian Ocean is investigated using satellite observations. Two key regions are considered: The eastern Indian Ocean and the Bay of Bengal which experience maximum upper-level cloudiness in winter and summer respectively. It is found that longwave CRF in the Bay of Bengal during summer is similar to that over the eastern Indian Ocean during winter. On the other hand shortwave CRF magnitude is larger in the Bay of Bengal. These differences explain the net CRF difference between the two regions. The stronger shortwave forcing seems to be related to the Upper-Level Cloudiness being larger over the Bay than over the eastern Indian Ocean. The reasons for the longwave CRF similarities are analysed in more details. Using the results from a convective system classification method, it is first shown that the longwave radiative properties of the individual systems do not vary much from one region to another. The distribution of the different kind of systems, a proxy for the vertical cloudiness structure, does not either indicate strong difference between the regions. It is then proposed that the substantial precipitable water vapour amount observed over the Bay of Bengal damps the effects of the upper-level cloudiness on radiation compared to the relatively dryer eastern Indian Ocean area; yielding to similar LW CRF in both region despite more Upper-Level Cloudiness over the Bay of Bengal. These observations are supported by idealised radiative transfer computations. The distribution of cloudiness and radiative forcing is then analysed over the whole tropical Indian Ocean for each season. July is characterized by a low longwave CRF regime (relative to January) over the most convectively active part of the Ocean. The non linear damping effect of water vapor on longwave CRF is also shown to contribute to this regime. Overall, this study reaffirms the need for simultaneous documentation of the cloud systems properties together with their moist environment in order to understand the overall net radiative signature of tropical convection at the top of the atmosphere (TOA).

Cloud Parameterizations in SUNYA Regional Climate Model for the East Asia Summer Monsoon Simulations

East Asia summer monsoon (EASM) simulations are conducted to evaluate three schemes which determine cloud properties used in the radiation calculation of the State University of New York at Albany (SUNYA) regional climate model (RCM). Scheme-Ⅰ uses diagnostic cloud cover and cloud water while Scheme-Ⅱ uses prognostic cloud water along with overcast sky; both schemes are commonly employed in RCMs. In Scheme-Ⅲ, cloud cover is determined by diagnostic formula, but the cloud water is calculated by the weighted means of its diagnosed and prognosed values. Therefore, Scheme-Ⅲ considers the subgrid-scale clouds as Scheme-Ⅰ and maintains consistent cloud properties in radiation calculation and microphysical processes as Scheme-Ⅱ. Cloud radiative forcing (CRF) which provides a quantification of the cloud-radiation-climate interaction is adopted to compare the three schemes in simulating the 1991 EASM, characterized by large amounts of cloud and persistent rainfall over Yangtze-Huai River valley. With these three cloud schemes, the SUNYA RCM is capable of simulating the intra-seasonal variations of observed cloud cover and longwave CRF. The transition of shortwave CRF is not properly simulated due to the constantly presented low-level clouds. Mostly the magnitude of CRF is overestimated by 13-22 W m^(-2) for shortwave CRF and by 12-16 W m^(-2) for longwave CRF. It is also found that the surface temperature biases are highly correlated (with correlation coefficient greater than 0.8) to the shortwave CRF biases. Therefore, Scheme-Ⅲ resulting in less low-level cloud water and the least shortwave CRF biases simulates surface temperatures in better agreement with observations. Analyses of surface energy balance components indicate that the CRF changes dominate the surface temperature responses and the consequent surface latent heat and sensible heat flux feedbacks significantly offset them. Finally, comparisons of the diurnal variations of simulated cloud water among the three schemes show that Scheme-Ⅲ provides consistency between cloud microphysics and radiation.

Clouds and Shortwave Fluxes at Nauru. Part II: Shortwave Flux Closure

Abstract The datasets currently being collected by the Atmospheric Radiation Measurement (ARM) program on the islands of Nauru and Manus represent the longest time series of ground-based cloud measurements in the tropical western Pacific region. In this series of papers, a shortwave flux closure study is presented using observations collected at the Nauru site between June 1999 and May 2000. The first paper presented frequency of occurrence of nonprecipitating clouds detected by the millimeter-wavelength cloud radar (MMCR) at Nauru and statistics of their retrieved microphysical properties. This paper presents estimates of the cloud radiative effect over the study period and results from a closure study in which retrieved cloud properties are input to a radiative transfer model and the modeled surface fluxes are compared to observations. The average surface shortwave cloud radiative forcing is 48.2 W m−2, which is significantly smaller than the cloud radiative forcing estimates found during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) field project. The difference in the estimates during the two periods is due to the variability in cloud amount over Nauru during different phases of the El Niño–Southern Oscillation (ENSO). In the closure study, modeled and observed surface fluxes show large differences at short time scales, due to the temporal and spatial variability of the clouds observed at Nauru. Averaging over 60 min reduces the average root-mean-square difference in total flux to 10% of the observed flux. Modeled total downwelling fluxes are unbiased with respect to the observed fluxes while direct fluxes are underestimated and diffuse fluxes are overestimated. Examination of the differences indicates that cloud amount derived from the ground-based measurements is an overestimate of the radiatively important cloud amount due to the anisotropy of the cloud field at Nauru, interpolation of the radar data, uncertainty in the microwave brightness temperature measurements for thin clouds, and the uncertainty in relating the sixth moment of the droplet size distribution observed by the radar to the more radiatively important moments.

Cloud Radiative Forcing in Pacific, African, and Atlantic Tropical Convective Regions

Abstract The high degree of cancellation between longwave (LW) and shortwave (SW) cloud radiative forcing (CRF) observed in the Pacific warm pool region has generally been assumed to be a property of all convective regions in the Tropics. Analysis of the (Earth Radiation Budget Experiment) ERBE-like data from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Terra satellite reveals that a similar degree of cancellation occurs over the African convective region at monthly and longer time scales, but only in the area average. In the Atlantic intertropical convergence zone (ITCZ), the degree of cancellation is lower, particularly during the summer months, where the area-average SW forcing typically exceeds the LW forcing by more than 20 W m−2. This behavior is similar to that found previously for the eastern Pacific ITCZ region, which is consistent with the similarity in dynamics between these two regions. Over Africa, substantial seasonal and spatial variations in net CRF occur, wit...

A New Parameterization for Shallow Cumulus Convection and Its Application to Marine Subtropical Cloud-Topped Boundary Layers. Part II: Regional Simulations of Marine Boundary Layer Clouds

Abstract The impact of physical parameterizations on simulations of cloud-topped marine boundary layers is investigated using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Three-month MM5 simulations of the northeast and southeast Pacific during June–August 1987 are boundary forced with time- varying ECMWF analyses. Runs with four planetary boundary layer (PBL) parameterizations already implemented in MM5 are compared with runs using new parameterizations of boundary layer turbulence and shallow cumulus convection (ShCu) described in a companion paper. Numerous modifications to the MM5 that allow it to be used as a regional climate model are described. The simulated 3-month mean shortwave cloud radiative forcing (SWCF) and vertical structure of cloud-topped boundary layers in the northeast Pacific are sensitive to the PBL/shallow convection schemes. All four current MM5 PBL schemes [the Blackadar, Medium-Range Forecast (MRF), Burk–Thompson, and Gayno–Seaman schemes] produc...

Evaluation of cloud thermodynamic phase parametrizations in the LMDZ GCM by using POLDER satellite data

Realistic simulations of clouds are of uppermost importance for climate modelling using general circulation models. Satellite data are well suited to evaluate model parametrizations. In this study we use the Laboratoire de Météorologie Dynamique general circulation model (LMDZ). We evaluate the current LMDZ cloud phase parametrization, in which the repartition of condensed cloud water between liquid and ice is a function of the local temperature. Three parameters are used to derive a relation between liquid cloud water content and temperature, two of which are not physically based. We use the POLDER‐1 satellite data to infer more realistic parameters by establishing statistical relationships between cloud top thermodynamical phase and cloud top temperature, consistently in both satellite data and model results. We then perform a multitude of short model integrations and derive a best estimate for the lowest local temperature where liquid water can exist in a cloud (Tice = −32°C in our parametrization). The other parameter which describes the shape of the transition between ice and liquid water is also estimated. A longer simulation has then been performed with the new parameters, resulting in an improvement in the representation of the shortwave cloud radiative forcing.

Angular Distribution Models for Top-of-Atmosphere Radiative Flux Estimation from the Clouds and the Earth’s Radiant Energy System Instrument on the Tropical Rainfall Measuring Mission Satellite. Part I: Methodology

Top-of-atmosphere (TOA) radiative fluxes from the Clouds and the Earth s Radiant Energy System (CERES) are estimated from empirical angular distribution models (ADMs) that convert instantaneous radiance measurements to TOA fluxes. This paper evaluates the accuracy of CERES TOA fluxes obtained from a new set of ADMs developed for the CERES instrument onboard the Tropical Rainfall Measuring Mission (TRMM). The uncertainty in regional monthly mean reflected shortwave (SW) and emitted longwave (LW) TOA fluxes is less than 0.5 W/sq m, based on comparisons with TOA fluxes evaluated by direct integration of the measured radiances. When stratified by viewing geometry, TOA fluxes from different angles are consistent to within 2% in the SW and 0.7% (or 2 W/sq m) in the LW. In contrast, TOA fluxes based on ADMs from the Earth Radiation Budget Experiment (ERBE) applied to the same CERES radiance measurements show a 10% relative increase with viewing zenith angle in the SW and a 3.5% (9 W/sq m) decrease with viewing zenith angle in the LW. Based on multiangle CERES radiance measurements, 18 regional instantaneous TOA flux errors from the new CERES ADMs are estimated to be 10 W/sq m in the SW and, 3.5 W/sq m in the LW. The errors show little or no dependence on cloud phase, cloud optical depth, and cloud infrared emissivity. An analysis of cloud radiative forcing (CRF) sensitivity to differences between ERBE and CERES TRMM ADMs, scene identification, and directional models of albedo as a function of solar zenith angle shows that ADM and clear-sky scene identification differences can lead to an 8 W/sq m root-mean-square (rms) difference in 18 daily mean SW CRF and a 4 W/sq m rms difference in LW CRF. In contrast, monthly mean SW and LW CRF differences reach 3 W/sq m. CRF is found to be relatively insensitive to differences between the ERBE and CERES TRMM directional models.

Comparing cloud radiative properties between the Eastern China and the Indian monsoon region

Based on the data from International Satellite Cloud Climatology Project (ISCCP) and Earth Radiation Budget Experiment (ERBE), the climatic cloud properties and cloud radiative forcing in the eastern China and the Indian monsoon region are compared. Although both of the Indian monsoon region and the eastern China are included in the Asian monsoon region and the seasonal cycles of rainfall are in phase, the properties of clouds and related cloud radiative forcing are significantly different. All of cloud components in the Indian region have similar phase structure of seasonal cycle. The maximum cloud fractions occur in the summer monsoon period and high clouds dominate the total cloud fraction. However, the seasonal features of clouds in the eastern China are complex. It is the mid-low clouds rather than high clouds dominating the total cloud fraction. The maximum total cloud fraction occurs in spring season. The total cloud and mid-low cloud fractions in winter season are larger than that in summer season. A unique global distinction of clouds in the eastern China is the largest cover of nimbostratus clouds. Reflecting to the cloud properties, the maximums of negative short wave, positive long wave and negative net cloud radiative forcing in the Indian monsoon region are in the summer season. In the eastern China, large negative short wave cloud radiative forcing occurs in early summer. The annual mean negative net cloud radiative forcing in the eastern China is obviously larger than that in the Indian region.

Parametrization of solar radiation in inhomogeneous stratocumulus: Albedo bias

Low-level cloud fields with typical albedos between 50% and 70% contribute significantly to short-wave cloud radiative forcing. Consequently, the earth's radiative budget is sensitive to extended stratocumulus which typically occurs in sub-tropical subsidence regions over the oceans. It is therefore vital to represent the cloud albedo of such cloud fields accurately in climate models. Nevertheless, cloud fields are modelled as homogeneous plane-parallel clouds, which leads to an overestimation of the albedo by up to 15% compared with real inhomogeneous clouds, both types of cloud fields having the same mean cloud optical depth. This so-called plane-parallel cloud albedo bias has given rise to a number of studies that present methods for reducing the albedo bias for stratiform clouds in climate models. Using Monte Carlo simulations and the Independent Pixel Approximation (IPA) the inhomogeneous cloud albedo is investigated with the aim of deriving a parametrization of the albedo bias. The new approach used here is that the variance of the cloud optical depth is parametrized as a function of grid size and mean cloud liquid-water content. The variance of the cloud optical depth is derived from turbulence processes. For the determination of the albedo bias an analytical solution is presented which is based on a simplified distribution function of the cloud optical depth and the IPA method. When used in conjunction with the parametrized variance of the cloud optical depth, a box function, which replaces the distribution function, produces comparable results to Monte Carlo simulations within an acceptable accuracy of a few per cent.

Interactions among deep convection, sea surface temperature and radiation in the Asian monsoon region

The climatic interactions among deep convection, sea surface temperature and radiation in the Asian monsoon region have been examined using various satellite-derived data sets of the period 1983-90. Annual average Frequency of Deep Convection (FDC) is maximum over the equatorial east Indian ocean and adjoining west Pacific and Indonesian region. Maximum FDC zone shifts to Bay of Bengal during the monsoon (June-September) season.
 
 There is weak relationship between the variations in FDC and SST in the Indian ocean. Deep convective activity was suppressed over most of the tropical Indian ocean during El Nino of 1987 in spite of warmer SSTs. The pattern of inter-annual variation between FDC and SST behaves differently in the Indian ocean basin as compared to the Pacific ocean basin. Deep convective clouds interact with radiation very effectively in the Asian monsoon region to cause large net negative cloud radiative forcing. Variation in FDC explains more than 70% of the variation in surface short-wave cloud radiative forcing (SWCRF) and long wave cloud radiative forcing (LWCRF) in the atmosphere.
 
 On inter-annual scale, warmer SSTs may not necessarily increase deep convection in the Indian ocean. However, the inter-annual variation of deep convective clouds influences significantly the radiative budget of the surface-atmosphere system in the Asian monsoon region. The satellite observations suggest that warmer SSTs in the Indian ocean might have resulted from an increase in the absorbed solar radiation at the surface due to a reduction in deep convective cloud cover.

Outgoing longwave radiation and cloud radiative forcing of the Tibetan Plateau

In order to study the energy balance and the cloud radiative forcing (CRF) of the Tibetan Plateau in detail, 2 years of GMS5 satellite data are employed to analyze the monthly mean outgoing longwave radiation (OLR) and CRF. It should be noted that the temporal resolution of GMS5 data is 1 hour, so the data can be used to study the diurnal variations of OLR. First, a method is presented to retrieve the OLR from split‐window channels (10.5–11.5 and 11.5–12.5 μm) and the water vapor channel (6.5–7.0 μm) of GMS5. The method applies the discrete ordinates radiative transfer (DISORT) model together with the radiosonde profiles of the Tibetan Plateau to simulate radiances and fluxes of the three channels. A regression relationship is then developed to calculate the OLR from the observations of the three channels. Since the Tibetan Plateau is located nearly out of the effective observational range of the GMS5 satellite, the regression results of GMS5's split‐window channels and water vapor channel are corrected by using simultaneously retrieved results from TIROS Operational Vertical Sounder (TOVS). The correlation coefficient of GMS5 and TOVS results is 0.8510, which is large enough for 1% significant level. The OLR distributions are calculated for the Tibetan Plateau using 2 years of GMS5 data and the regression and correction methods. The average of the OLR images for the same month and same time gives the monthly mean OLR distribution for each hour. The 24‐hour OLR distributions of the same month are then averaged to yield the monthly mean OLR distribution for that month. Then our monthly mean OLR distributions are compared with the Clouds and the Earth's Radiant Energy System (CERES) results, and they are generally in good agreement with differences of <10% for January and 5% for July. Analyzing the monthly mean OLR distributions for different seasons, we find that during the winter season the OLR distribution exhibits low values over the Tibetan Plateau but high values for areas off the Tibetan Plateau. During the summer season the OLR of the southern part is smaller than that of the northern part. Studying the monthly mean diurnal variations of OLR, we find that the diurnal variations of OLR are affected by diurnal cycles of cloud quantity and surface temperature. The relief of the Tibetan Plateau is very high, and the radiative heating is intense after sunrise. The OLR is greatly influenced by the surface and reaches a maximum value soon after sunrise, but the time the minimum OLR emerges varies. After the OLR distributions of the Tibetan Plateau are obtained, the role of clouds in the climate system is also studied. In order to calculate the CRF the International Satellite Cloud Climatology Project (ISCCP) cloud detection algorithm is used to detect the clear pixels for each image. The clear‐sky components of OLR and albedo for different months and hours are then derived and averaged over a month to obtain the monthly mean clear‐sky OLR and albedo for each hour. Finally, data are averaged over 24 hours to give the monthly mean shortwave CRF (SWCRF), longwave CRF (LWCRF), and CRF. The results show that the CRF over the Tibetan Plateau is negative most of the time. This means the CRF is dominated by cooling effects, and the distribution pattern is mainly determined by the SWCRF component. While the CRFs to the south and the north of the Tibetan Plateau are different, there are obvious annual variations with heating effects in the summer‐autumn season and cooling effects in the winter‐spring season.

Examining the sensitivity of Earth's climate to the removal of ozone, landmasses and enhanced ocean heat transport in the GENESIS global climate model

This paper examines the cloud radiative forcing and its impacts on the surface climate for global climate model simulations that use reduced ozone concentrations and land fractions as boundary conditions. In one simulation using present-day land continents, ozone concentrations are reduced to zero and compared to the present-day climate simulation. In the second set of simulations under global ocean conditions, the implied poleward transport of heat by the ocean is varied. The removal of ozone causes an increase in longwave cloud radiative forcing at the top of the atmosphere and the surface. The increase in longwave forcing melts sea-ice and snow at high latitudes leading 10–14°C warmer temperatures and globally a 2°C increase. The global ocean simulations lead to higher cloud fractions than present-day simulation. Without poleward transport of heat by the ocean, surface temperatures cool as a result of higher cloud fractions. Increasing the ocean heat transport by a factor of 3.33 brings about ice-free conditions. An 11°C difference in globally averaged surface air temperatures is found between the enhanced and zero poleward oceanic heat transport simulations. The longwave cloud radiative forcing from high cloud fractions enhance the surface warming in the polar regions during the winter season. Conversely, during the summer season, a high cloud fraction increases the shortwave cloud radiative forcing producing only moderately warm temperatures in the polar regions. High cloud fractions in polar regions during warm periods throughout geologic times may help to explain the reduced equator to pole temperature gradient.

A new parameterization scheme for the optical properties of ice crystals for use in general circulation models of the atmosphere

Abstract We present a new parameterization scheme for the optical properties of ice crystals, for use in general circulation models of the atmosphere (GCMs). The scheme consists of separate parameterizations for five different ice crystals shapes: hexagonal columns, hexagonal plates, bullet rosettes, spheres and planar polycrystals. The optical properties are specified as functions of the mean maximum dimension of the ice crystal size distribution using polynomial fits to a more elaborate scheme developed by the last author. The present scheme, based on 10th order fits, gives excellent accuracy and is efficient enough for use in GCMs. First, the scheme is tested in 1-dimensional models, showing a large sensitivity of albedo and emissivity to both ice crystal sizes and shapes. Also, long- and shortwave cloud radiative forcings at the top of the atmosphere are quite sensitive to those two parameters, in such a manner that the two effects do not cancel. It is shown that this may have a significant impact on climate sensitivity, through variations in net cloud forcing.