Microphysical and radiative effects of ice clouds on diurnal variations of tropical convective and stratiform rainfall

Microphysical and radiative effects of ice clouds on diurnal variations of tropical convective and stratiform rainfall are examined with the equilibrium simulation data from three experiments conducted with a two-dimensional cloud resolving model with imposed temporally and zonally invariant winds and sea surface temperature and zero mean vertical velocity. The experiment without ice radiative effects is compared with the control experiment with ice microphysics (both the ice radiative and microphysical effects) to study effects of ice radiative effects on diurnal rainfall variations whereas it is compared with the experiment without ice microphysics to examine ice microphysical effects on the diurnal rainfall variations. The ice radiative processes mainly affect diurnal cycle of convective rainfall whereas the ice microphysical processes have important impacts on the diurnal cycles of both convective and stratiform rainfall. Turning off the ice radiative effects generally enhances convective rainfall during the morning and evening and suppresses convective rainfall in the afternoon whereas turning off the ice microphysical effects generally suppresses convective and stratiform rainfall during the morning and enhances convective and stratiform rainfall in the afternoon and evening. The ice radiative and microphysical effects on the diurnal cycle of surface rainfall are mainly associated with that of vapor condensation and deposition, which is controlled by air temperature through saturation specific humidity. The ice effects on the diurnal cycle of local temperature tendency are largely explained by that of latent heating since the diurnal cycle of radiation is insensitive to the ice effects.     

冷水面对积云的影响——数值试验

采用二维平面对称暖积云模式研究了夏季午后水面对移行过境积云的影响.讨论了过冷水面积云发展演变过程、流场结构和雷达回波结构.试验中考虑整层均匀环境风,并对水陆温差进行了适当处理.计算表明,冷水面对过境积云的影响表现在两个方面:水陆环流辐合上升运动使移近水面的积云受到一定程度增强;水陆环流的下沉运动和冷变性气层的共同作用使积云移入水面后发展受抑,强度减弱.受冷水面作用后的积云流场和液水场均发生很大变化,有些积云移过水面后可以重行增强.     

Clear-sky aerosol radiative forcing effects based on multi-site AERONET observations over Europe

Summary One of the great unknowns in climate research is the contribution of aerosols to climate forcing and climate perturbation. In this study, retrievals from AERONET are used to estimate the direct clear-sky aerosol top-of-atmosphere and surface radiative forcing effects for 12 multi-site observing stations in Europe. The radiative transfer code sdisort in the libRadtran environment is applied to accomplish these estimations. Most of the calculations in this study rely on observations which have been made for the years 1999, 2000, and 2001. Some stations do have observations dating back to the year of 1995. The calculations rely on a pre-compiled aerosol optical properties database for Europe. Aerosol radiative forcing effects are calculated with monthly mean aerosol optical properties retrievals and calculations are presented for three different surface albedo scenarios. Two of the surface albedo scenarios are generic by nature bare soil and green vegetation and the third relies on the ISCCP (International Satellite Cloud Climatology Project) data product. The ISCCP database has also been used to obtain clear-sky weighting fractions over AERONET stations. The AERONET stations cover the area 0° to 30° E and 42° to 52° N. AERONET retrievals are column integrated and this study does not make any seperation between the contribution of natural and anthropogenic components. For the 12 AERONET stations, median clear-sky top-of-atmosphere aerosol radiative forcing effect values for different surface albedo scenarios are calculated to be in the range of −4 to −2 W/m². High median radiative forcing effect values of about −6 W/m² were found to occur mainly in the summer months while lower values of about −1 W/m² occur in the winter months. The aerosol surface forcing also increases in summer months and can reach values of −8 W/m². Individual stations often have much higher values by a factor of 2. The median top-of-atmosphere aerosol radiative forcing effect efficiency is estimated to be about −25 W/m² and their respective surface efficiency is around −35 W/m². The fractional absorption coefficient is estimated to be 1.7, but deviates significantly from station to station. In addition, it is found that the well known peak of the aerosol radiative forcing effect at a solar zenith angle of about 75° is in fact the average of the peaks occurring at shorter and longer wavelengths. According to estimations for Central Europe, based on mean aerosol optical properties retrievals from 12 stations, the critical threshold of the aerosol single scattering albedo, between cooling and heating in the presence of an aerosol layer, is close between 0.6 and 0.76.     

气候模式分辨率对气溶胶气候效应模拟结果的影响

气候模式分辨率作为影响模式模拟结果的重要因素,其对气溶胶与云相互作用的影响尚未全面认识。利用公共大气模型CAM5.3在3种分辨率（2°、1°、0.5°）下,分别采用2000年和1850年气溶胶排放情景进行试验,检验提高分辨率是否能改进气候模式的模拟能力,分析不同分辨率下气溶胶气候效应的异同,探索模式分辨率对气溶胶气候效应数值模拟结果的影响。通过观测资料与模式结果对比发现,提高分辨率可以明显改进模式对总云量、云短波辐射强迫的模拟能力,0.5°分辨率下模拟结果与观测更接近,其他变量并无明显改善。在不同分辨率下,全球平均的气溶胶气候效应较为一致,总云量、云水路径均增加,云短波和长波辐射强迫均加强,而云顶的云滴有效半径和降水均减小,地面气温降低。不同分辨率下,气溶胶增加引起的气溶胶光学厚度、云水路径、地面温度、云短波和长波辐射强迫变化的纬向平均分布相似但大小存在差异;而降水和云量变化的纬向分布与大小均存在较大差异,在区域尺度上还存在较大的不确定性。全球平均而言, 0.5°分辨率下气溶胶的间接辐射强迫相比1°分辨率下的结果降低了2.5%,相比2°分辨率下的结果降低了6.4%。提高模式分辨率可以部分改进模式模拟能力,同时,气溶胶的间接效应随着模式分辨率的提高而减弱。但气溶胶引起的云量、降水的变化在不同分辨率下差异较大,存在较大的不确定性。      

The sensitivity of climate simulations to the specification of mixed phase clouds

In the prognostic stratiform cloud scheme used in the UK Meteorological Office Unified Model, mixed phase clouds are assumed to exist between 0 and –15 °C. Recent observations of such clouds, carried out using the C-130 aircraft of the Meteorological Research Flight, suggest that a smaller range of 0 to –9°C may be more appropriate. The sensitivity of cloud and radiation fields as simulated by a 5 × 7.5° latitude-longitude version of the Unified Model to such a reduction in the temperature range of mixed phase clouds is considered. Using a smaller temperature range systematic errors in the radiation budget of the model are reduced in mid-latitudes, bringing the model into closer agreement with ERBE data. The sensitivity of model albedo to an increase in the temperature range over which mixed phase clouds are assumed to exist, suggested by previous observational studies, is also considered together with the impact of removing the mixed phase part of the precipitation parametrization altogether.     

Model studies of the impact of chemical inhomogeneity on SO2 oxidation in warm stratiform clouds

A one-dimensional, time-dependent model of the physics and chemistry of a warm stratiform cloud is used to study the possible impact of chemical inhomogeneity among cloud and raindrops on the oxidation of SO₂ in clouds. The effects of chemical inhomogeneity are examined using two contrasting models: In Model 1 a bulk-solution parameterization is adopted which effectively treats all cloud and raindrops as if they are chemically homogeneous; in Model 2 we allow the cloud and raindrops to have a dichotomous distribution. The dichotomous distribution in Model 2 is simulated by assuming that the two groups of cloud droplets nucleate from two chemically distinct populations of condensation nuclei; one being acidic and the other being alkaline. While the two models yield essentially identical results when the ambient levels of H₂O₂ are greater than the ambient levels of SO₂, the rate of conversion of SO₂ to sulfuric acid and the amount of sulfate removed in the precipitation can be significantly enhanced in Model 2 over that of Model 1 under conditions of oxidant limitation (i.e., H₂O₂ < SO₂). This enhancement is critically dependent upon the fraction of alkaline nuclei assumed to be present in Model 2 and arises from the rapid increase in the aqueous-phase reaction between O₃+S_IV at high pH. Our results suggest that cloud models which adopt a bulk-solution parameterization for cloud droplet chemistry, may underestimate the amount of in-cloud SO₂ oxidation under oxidant-limited conditions.     

A comparison of solar radiative flux above clouds from MODIS with BALTIMOS regional climate model simulations

A comparison study for the solar radiative flux above clouds is presented between the regional climate model system BALTEX integrated model system (BALTIMOS) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations. For MODIS, an algorithm has been developed to retrieve reflected shortwave fluxes over clouds. The study area is the Baltic Sea catchment area during an 11-month period from February to December 2002. The intercomparison focuses on the variations of the daily and seasonal cycle and the spatial distributions. We found good agreement between the observed and the simulated data with a bias of the temporal mean of 13.6 W/m² and a bias of the spatial mean of 35.5 W/m². For summer months, BALTIMOS overestimates the solar flux with up to 90 W/m² (20%). This might be explained by the insufficient representation of cirrus clouds in the regional climate model.     

Greenhouse sensitivity experiments with penetrative cumulus convection and tropical cirrus albedo effects

Results are presented from two versions of a global R15 atmospheric general circulation model (GCM) coupled to a nondynamic, 50-m deep, slab ocean. Both versions include a penetrative convection scheme that has the effect of pumping more moisture higher into the troposphere. One also includes a simple prescribed functional dependence of cloud albedo in areas of high sea-surface temperature (SST) and deep convection. Previous analysis of observations has shown that in regions of high SST and deep convection, the upper-level cloud albedos increase as a result of the greater optical depth associated with increased moisture content. Based on these observations, we prescribe increased middle- and upper-level cloud albedos in regions of SST greater than 303 K where deep convection occurs. This crudely accounts for a type of cloud optical property feedback, but is well short of a computed cloud-optical property scheme. Since great uncertainty accompanies the formulation and tuning of such schemes, the prescribed albedo feedback is an intermediate step to examine basic feedbacks and sensitivities. We compare the two model versions (with earlier results from the same model with convective adjustment) to a model from the Canadian Climate Centre (CCC) having convective adjustment and a computed cloud optical properties feedback scheme and to several other GCMs. The addition of penetrative convection increases tropospheric moisture, cloud amount, and planetary albedo and decreases net solar input at the surface. However, the competing effect of increased downward infrared flux (from increased tropospheric moisture) causes a warmer surface and increased latent heat flux. Adding the prescribed cirrus albedo feedback decreases net solar input at the surface in the tropics, since the cloud albedos increase in regions of high SST and deep convection. Downward infrared radiation (from increased moisture) also increases, but this effect is overpowered by the reduced solar input in the tropics. Therefore, the surface is somewhat cooler in the tropics, latent heat flux decreases, and global average sensitivity to a doubling of CO₂ with regard to temperature and precipitation/evaporation feedback is reduced. Similar processes, evident in the CCC model with convective adjustment and a computed cloud optical properties feedback scheme, occur over a somewhat expanded latitudinal range. The addition of penetrative convection produces global effects, as does the prescribed cirrus albedo feedback, although the strongest local effects of the latter occur in the tropics.Portions of this study are supported by the Office of Health and Environmental Research of the U.S. Department of Energy as part of its Carbon Dioxide Research Program, and by the Electric Power Research Institute as part of its Model Evaluation Consortium for Climate Assessment ProjectThe National Center for Atmospheric Research is sponsored by the National Science Foundation     

Microwave Simulations of Precipitation Distribution with Two Radiative Transfer Models

Two microwave radiative transfer models of precipitating cloud are used to simulate the microwave upwelling radiances emerging from precipitating clouds. Comparison of the simulation results shows that significant difference of microwave upwelling radiances exists between these two radiative transfer models. Analysis of these differences in different cloud and precipitation conditions shows that it is complicated but has certain trend for different microwave frequencies. The results may be useful to quantitative rainfall rate retrieval of real precipitating clouds.     

层状云微物理参数反演及其辐射效应的个例研究

利用毫米波云雷达、微波辐射计联合反演方法,对2015年11月11日安徽寿县的一次层状云过程的云参数进行了反演,将所得云参数加入到SBDART辐射传输模式中,进行辐射通量计算,并将计算的地面辐射通量与观测的地面辐射通量进行了对比分析。研究表明:1)利用毫米波雷达和微波辐射计数据联合反演的云参数比较可靠;2)利用SBDART模式并结合反演的云参数,可以准确实时地计算地面及其他高度层的长短波辐射通量;3)在反演的云参数中,光学厚度对地面各种辐射通量的影响是最大的,云层的光学厚度越大,到达地面的太阳短波辐射越小,地面反射短波辐射也越小。另外云底温度越高,云体向下发射的红外长波辐射越大。地面向上的长波辐射是地面温度的普朗克函数,随地面温度而变;4)云对地面的短波辐射强迫为负值,对地面有降温的作用。云对地面的长波辐射强迫是一个正值,对地面有一个增温的作用;5)云对地面的净辐射强迫随时间变化很大,它的正负与太阳高度角和云参数有关。     

风切变对积云发展影响的数值模拟研究

本文采用二维暖积云模式研究了风切变对积云发展和降水大小的影响。计算结果表阴:一般有风切变情况下积云的发展和降水量都明显减弱。这是由于水平气流的作用促进了云内外水汽和热量交换,减弱了积云发展的条件。计算还表明:在多种不同风速廓线和大气层结下风切变对云和降水发展均不利。 我们发现当云移行前方有高湿区时,在中等的风切变下可能有利于积云发展和降水形成。这是由于在水汽场和风场的配置下,云中得到充沛的水汽补充,促进了云和降水的发展,总降水量可增加2—3倍。     

Indirect radiative forcing and climatic effect of the anthropogenic nitrate aerosol on regional climate of China

The regional climate model (RegCM3) and a tropospheric atmosphere chemistry model (TACM) were coupled, thus a regional climate chemistry modeling system (RegCCMS) was constructed, which was applied to investigate the spatial distribution of anthropogenic nitrate aerosols, indirect radiative forcing, as well as its climatic effect over China. TACM includes the thermodynamic equilibrium model ISORROPIA and a condensed gas-phase chemistry model. Investigations show that the concentration of nitrate aerosols is relatively high over North and East China with a maximum of 29 μg m^-3 in January and 8 μg m^-3 in July. Due to the influence of air temperature on thermodynamic equilibrium, wet scavenging of precipitation and the monsoon climate, there are obvious seasonal differences in nitrate concentrations. The average indirect radiative forcing at the tropopause due to nitrate aerosols is -1.63 W m^-2 in January and -2.65 W m^-2 in July, respectively. In some areas, indirect radiative forcing reaches $-$10 W m^-2. Sensitivity tests show that nitrate aerosols make the surface air temperature drop and the precipitation reduce on the national level. The mean changes in surface air temperature and precipitation are -0.13 K and -0.01 mm d^-1 in January and -0.09 K and -0.11 mm d^-1 in July, respectively, showing significant differences in different regions.     

Seasonal and spatial variation of radiative effects of anthropogenic sulfate aerosol

On the basis of the emission data of the industrial sulphur dioxide (SO2) and observed climate fields over East Asia, the distribution of anthropogenic sulfate aerosol (SO) with seasonal variation in the troposphere is simulated and analyzed by a regional sulfur transport model, and the direct radiative effects of SO under different weather conditions are also calculated using the discrete ordinate method. The results show that the concentration of SO has significant seasonal and spatial variations resulting from the effects of SO2 emission source and precipita-tion and wind fields. Both the concentration of SO2 and its radiative forcing have the largest values in October and the lowest in July. SO causes the decrease of the radiation flux absorbed by earth-atmosphere and the cooling of air temperature by scattering more solar radiation back into space. Besides, the radiative and climatic effects of SO are related to the types and height and optical thickness, etc., of the clouds.     

Numerical simulations of stratiform boundary-layer clouds on the meso-γ-scale. Part I: The influence of terrain height differences

A higher order closure mesoscale model is used to study the influence of terrain height differences on the meso--scale on stratiform boundary-layer clouds. The model is hydrostatic, has a terrain-following coordinate system and a sub-grid scale condensation scheme. It also has a radiation parameterisation for shortwave and longwave radiation in order to calculate radiative cooling/heating. The simulations show that the cloud base height variations induced by the terrain can be much larger than motivated by terrain height variations alone. It is also shown how this behavior is dependent on upstream boundary-layer conditions and/or changes in the turbulence field. Other features studied include the wave in the lee of a ridge/hill and the associated lifting of the cloud base. The results are compared with some simpler physical models, and limitations in those models are demonstrated.     

The assessment of aerosol optical properties over Mohal in the northwestern Indian Himalayas using satellite and ground-based measurements and an influence of aerosol transport on aerosol radiative forcing

The present study deals with the aerosol optical properties which are assessed during the period 2007 to 2009 over Mohal (31.9oN, 77.12oE) in the northwestern Indian Himalaya, using ground-based measurements and multi-satellite data. The daily average value of aerosol optical depth (AOD) at 500?nm, ?ngstr?m exponent and turbidity coefficient are 0.24?±?0.08, 1.02?±?0.34 and 0.13?±?0.05, respectively. The comparative study of satellite and ground-based measurements reveals that the percentage retrieval for daily AOD at 550?nm over Mohal within the expected accuracy (???? _p?? ?=?±0.05?±?0.15?? _p?? ) is around 87%, with a significant correlation coefficient of 0.76. The present study suggests that the retrieval of AOD through satellite data is able to characterise the distribution of AOD over Mohal. However, further efforts are needed in order to eliminate systematic errors in the existing Moderate Resolution Imaging Spectroradiometer (MODIS) algorithm. The transport of desert dust and anthropogenic aerosol during high aerosol loading days caused a significant reduction in surface-reaching solar radiation by 149 and 117%, respectively. This large reduction in surface-reaching solar radiation increased the atmospheric heating rate by 0.93 and 0.72?K?day^?1, respectively. This study indicates significant climatic implications due to the transport of aerosols in the northwestern Indian Himalaya.