UV attenuation in the cloudy atmosphere

Ultraviolet (UV) energy absorption plays a very important role in the Earth–atmosphere system. Based on observational data for Beijing, we suggest that some atmospheric constituents utilize or transfer UV energy in chemical and photochemical (C&P) reactions, in addition to those which absorb UV energy directly. These constituents are primarily volatile organic compounds (VOCs) emitted from both vegetative and anthropogenic sources. The total UV energy loss in the cloudy atmosphere for Beijing in 1990 was 78.9 Wm⁻². This attenuation was caused by ozone (48.3 Wm⁻²), other compounds in the atmosphere (26.6 Wm⁻²) and a scattering factor (4.0 Wm⁻²). Our results for a cloudy atmosphere in the Beijing area show that the absorption due to these other compounds occurs largely through the mediation of water vapor. This fraction of energy loss has not been fully accounted for in previous models. Observations and previous models results suggest that 1) a cloudy atmosphere absorbs 25∼30 Wm⁻² more solar shortwave radiation than models predict; and 2) aerosols can significantly decrease the downward mean UV-visible radiation and the absorbed solar radiation at the surface by up to 28 and 23 Wm⁻², respectively. Thus, quantitative study of UV and visible absorption by atmospheric constituents involved in homogeneous and heterogeneous C&P reactions is important for atmospheric models.     

On the vertical profile of surface radiative fluxes in southwest Germany

Summary Vertical profile of surface radiative fluxes in an area of heterogeneous terrain in south-west Germany is presented. Main data sets utilized for the study were recorded during the REgio KLIma Projekt (REKLIP). Supporting observational data were provided by the German weather service and German geophysical consultant service. Elevation of the study sites ranged from 212 m a.s.l. to 1489 m a.s.l. From May to September, monthly mean albedo was generally low at the study sites, ranging from 19% to 24%. For the other months, monthly mean albedo lie between 22% and 25% at the lowland site but extended between 27% and 71% at the highly elevated mountain site. Following the altitudinal increase in surface albedo, net radiative flux and radiation efficiency declined with elevation at an annual mean of 1.15 Wm⁻²/100 m and 0.008/100 m respectively. Absorbed shortwave radiation and effective terrestrial radiation showed mean decline of 1.54 Wm⁻²/100 m and 0.34 Wm⁻²/100 m, respectively, with the mean sky-to-earth radiation deficit amounting to about 52 Wm⁻² for the lowland site and 73 Wm⁻² for the highest elevated site. Some empirical models which express shortwave and longwave radiative fluxes in terms of meteorological variables have been validated for the lowland and mountain sites. Monthly mean daily total estimates of solar radiation obtained from ?ngst?m-Prescott relation were quite consistent with observed values. Parameterisation of downward atmospheric radiation under all sky condition was achieved by extending Brutsaert clear sky atmospheric model. Relationship between outgoing longwave radiation and screen temperature at the study sites was best described by an exponential function unlike the linear relationship proposed by Monteith and Unsworth. Net radiative flux for the lowland and mountain sites has been expressed in terms of absorbed shortwave radiation, cloud amount and screen temperature. Received March 5, 2001 Revised October 29, 2001     

Solar absorption at the surface and in the atmosphere as determined by different observational methods

Summary The concept of effective cloud cover, elaborated on the basis of an assumption that changes in the net radiation at the top of the atmosphere are mainly caused by changing cloudiness, has been used to deduce solar surface radiation from satellite data. It has been shown that the method permits a calculation of solar surface absorption distributions that agree well with the results obtained by other authors and that the existing disagreement can be to a great extent ascribed to the differences in the data sets and analysis periods. The method allows use of early satellite measurements to get longer time series of the surface radiation budget. In this study, it has been applied to the Nimbus-7 ERB WFOV data for 1979–1986.The net solar flux at the TOA (top of the atmosphere) can be partitioned into absorption at the surface and within the atmosphere. The geographical distributions of all the three quantities as well as the zonal averages of the surface absorption for January and July have been described. Special objectives of the present study are to estimate the interannual standard deviation for the 8-year period and to analyse the shortwave cloud-radiative forcing distributions at the surface and especially within the atmosphere.The standard deviation of the TOA and the surface solar absorption shows a temporal asymmetry, being much larger in January than in July. Noticeable is the disappearance of the wintertime strong variability over the central Pacific in July. As can be expected, the strong variability areas coincide with the strong variability areas of the cloud amount, showing the values up to 27 Wm^–2 at the surface.According to our estimate, the shortwave cloud forcing at the surface is everywhere stronger than that at the TOA, so that the cloud forcing of the atmosphere is negative. This means that in the belt of 58.5° N–58.5° S a cloudy atmosphere absorbs more solar energy than a cloud-free atmosphere. Our mean annual value of the atmospheric cloud forcing for this belt is –11 Wm^–2 which is somewhat stronger than that obtained by other investigators. It must be stressed that this value is within the uncertainty limits.Shortwave cloud forcing of the atmosphere is the strongest in the lower latitude areas of heavy cloudiness above the continents and negligible in the midlatitudes in winter. This gives evidence that the value of the shortwave cloud forcing of the atmosphere is modified by a combination of cloud absorption and cloud albedo.With 4 Figures     

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.     

Regional climate simulation with a high resolution GCM: surface radiative fluxes

The ability of a high resolution (T106) version of the ECHAM3 general circulation model to simulate regional scale surface radiative fluxes has been assessed using observations from a new compilation of worldwide instrumentally-measured surface fluxes (Global Energy Balance Archive, GEBA). The focus is on the European region where the highest density of observations is found, and their use for the validation of global and regional climate models is demonstrated. The available data allow a separate assessment of the simulated fluxes of surface shortwave, longwave, and net radiation for this region. In summer, the incoming shortwave radiation calculated by the ECHAM3/T106 model is overestimated by 45 W m^–2 over most of Europe, which implies a largely unrealistic forcing on the model surface scheme and excessive surface temperatures. In winter, too little incoming shortwave radiation reaches the model surface. Similar tendencies are found over large areas of the mid-latitudes. These biases are consistent with deficiencies in the simulation of cloud amount, relative humidity and clear sky radiative transfer. The incoming longwave radiation is underestimated at the European GEBA stations predominantly in summer. This largely compensates for the excessive shortwave flux, leading to annual mean net radiation values over Europe close to observations due to error cancellation, a feature already noted in the simulated global mean values in an earlier study. Furthermore, the annual cycle of the simulated surface net radiation is strongly affected by the deficiencies in the simulated incoming shortwave radiation. The high horizontal resolution of the GCM allows an assessment of orographically induced flux gradients based on observations from the European Alps. Although the model-calculated and observed flux fields substantially differ in their absolute values, several aspects of their gradients are realistically captured. The deficiencies identified in the model fields are generally consistent at most stations, indicating a high degree of representativeness of the measurements for their larger scale setting.     

A hybrid technique for computing the monthly mean net longwave surface radiation over oceanic areas

Summary Global maps of the monthly mean net upward longwave radiation flux at the ocean surface have been obtained for April, July, October 1985 and January 1986. These maps were produced by blending information obtained from a combination of general circulation model cloud radiative forcing fields, the top-of-the-atmosphere cloud radiative forcing from ERBE and TOVS profiles and sea surface temperature on ISCCP C1 tapes. The fields are compatible with known meteorological regimes of atmospheric water vapor content and cloudiness. There is a vast area of high net upward longwave radiation flux (> 80 W m^–2) in the eastern Pacific Ocean throughout most of the year. Areas of low net upward longwave radiation flux (< 40 Wm^–2) are the tropical convective regions and extra tropical regions that tend to have persistent low cloud cover. The technique used in this study relies on GCM simulations and so is subject to some of the uncertainties associated with the model. However, all input information regarding temperature, moisture and cloud cover is from satellite data having near global coverage. This feature of the procedure alone warrants its consideration for further use in compiling global maps of the net longwave radiation at the surface over the oceans.With 9 Figures     

The Arctic surface energy budget as simulated with the IPCC AR4 AOGCMs

Ensembles of simulations of the twentieth- and twentyfirst-century climate, performed with 20 coupled models for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment, provide the basis for an evaluation of the Arctic (70°–90°N) surface energy budget. While the various observational sources used for validation contain differences among themselves, some model biases and across-model differences emerge. For all energy budget components in the twentieth-century simulations (the 20C3M simulation), the across-model variance and the differences from observational estimates are largest in the marginal ice zone (Barents, Kara, Chukchi Seas). Both downward and upward longwave radiation at the surface are underestimated in winter by many models, and the ensenmble mean annual net surface energy loss by longwave radiation is 35 W/m², which is less than for the NCEP and ERA40 reanalyses but in line with some of the satellite estimates. Incoming solar radiation is overestimated by the models in spring and underestimated in summer and autumn. The ensemble mean annual net surface energy gain by shortwave radiation is 39 W/m², which is slightly less than for the observational based estimates, In the twentyfirst-century simulations driven by the SRES A2 scenario, increased concentrations of greenhouse gasses increase (average for 2080–2100 minus average for 1980–2000 averages) the annual average ensemble mean downward longwave radiation by 30.1 W/m². This was partly counteracted by a 10.7 W/m² reduction in downward shortwave radiation. Enhanced sea ice melt and increased surface temperatures increase the annual surface upward longwave radiation by 27.1 W/m² and reduce the upward shortwave radiation by 13.2 W/m², giving an annual net (shortwave plus longwave) surface radiation increase of 5.8 W/m² , with the maximum changes in summer. The increase in net surface radiation is largely offset by an increased energy loss of 4.4 W/m² by the turbulent fluxes.     

Lidar remote sounding of cirrus clouds and comparison of simulated fluxes with surface and METEOSAT observations

Three cirrus cloud cases have been remotely sounded near Paris by a ground-based backscatter lidar and broadband radiometers. Some cirrus properties (optical depth, emissivity, height) are derived from these measurements and used to compare radiative transfer calculations to surface and METEOSAT observations of broadband irradiances.For a useful comparison, the three cirrus cases were selected to have different morphologies and optical properties: June 29, 1993—thin cirrus cloud (thickness 1.5 km, optical depth 0.22); September 6, 1993—thick cirrus cloud (thickness 5 km, optical depth 2.7); and November 16, 1993—inhomogeneous and geometrically thick cirrus cloud (thickness 3.5–6.5 km) but optically thin (optical depth 0.82).At surface, the differences between measurements and model range from 1.5 to 4 Wm⁻² for longwave fluxes, and from 20 to 70 Wm⁻² for shortwave fluxes.At the top of the atmosphere, the differences between METEOSAT measurements and model are in fair agreement for longwave fluxes (up to 50 Wm⁻²). However, unexpected high differences are found for shortwave fluxes (up to 144 Wm⁻²) due to cirrus clouds heterogeneities and uncertainties in their microphysical properties and especially the occurrence of high reflectivity due to horizontally oriented ice crystals at the cloud top, which are not taken into account by the Model presently.     

Climate sensitivity and climate change under strong forcing

A version of the National Centre for Atmospheric Research (NCAR) coupled climate model is integrated under current climate conditions and in a series of experiments with climate forcings ranging from modest to very strong. The purpose of the experiments is to investigate the nature and behaviour of the climate feedback/sensitivity of the model, its evolution with time and climate state, the robustness of model parameterizations as forcing levels increase, and the possibility of a “runaway” warming under strong forcing. The model is integrated for 50 years, or to failure, after increasing the solar constant by 2.5, 10, 15, 25, 35, and 45% of its control value. The model successfully completes 50 years of integration for the 2.5, 10, 15, and 25% solar constant increases but fails for increases of 35% and 45%. The effective global climate sensitivity evolves with time and analysis indicates that a new equilibrium will be obtained for the 2.5, 10, and 15% cases but that runaway warming is underway for the 25% increase in solar constant. Feedback processes are analysed both locally and globally in terms of longwave and shortwave, clear-sky/surface, and cloud forcing components. Feedbacks in the system must be negative overall and of sufficient strength to balance the positive forcing if the system is to attain a new equilibrium. Longwave negative feedback processes strengthen in a reasonably linear fashion as temperature increases but shortwave feedback processes do not. In particular, solar cloud feedback becomes less negative and, for the 25% forcing case, eventually becomes positive, resulting in temperatures that “run away”. The conditions under which a runaway climate warming might occur have previously been investigated using simpler models. For sufficiently strong forcing, the greenhouse effect of increasing water vapour in a warmer atmosphere is expected to overwhelm the negative feedback of the longwave cooling to space as temperature increases. This is not, however, the reason for the climate instability experienced in the GCM. Instead, the model experiences a “cloud feedback” warming whereby the decrease in cloudiness that occurs when temperature increases beyond a critical value results in an increased absorption of solar radiation by the system, leading to the runaway warming.     

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。     

RegCM4对中国东部区域气候模拟的辐射收支分析

利用卫星和再分析数据,评估了区域气候模式Reg CM4对中国东部地区辐射收支的基本模拟能力,重点关注地表净短波(SNS)、地表净长波(SNL)、大气顶净短波(TNS)、大气顶净长波(TNL)4个辐射分量。结果表明:1)短波辐射的误差值在夏季较大,而长波辐射的误差值在冬季较大。但各辐射分量模拟误差的空间分布在冬、夏季都有较好的一致性。2)对于地表辐射通量,SNS表现为正偏差(向下净短波偏多),在各分量中误差最大,区域平均误差值近50 W/m2;SNL表现为负偏差(向上净长波偏多);对于大气顶辐射通量,TNS和TNL分别表现为"北负南正"的误差分布和整体正偏差。3)利用空间相关和散点线性回归方法对4个辐射分量的模拟误差进行归因分析,发现在云量、地表反照率、地表温度三个直接影响因子中,云量模拟误差的贡献最大,中国东部地区云量模拟显著偏少。     

The impact of geophysical parameters on longwave radiation budget at the top and base of the atmosphere

Summary The effect of clouds on longwave radiation budget at the top and base of the atmosphere is studied by using the HIRS2/MSU-retrieved temperature and humidity fields, and cloud fields and the International Satellite Cloud Climatology Project-produced fields. Detailed studies are carried out at four selected sites: one at Equatorial Eastern Pacific (ITCZ) area, one at Libyan Desert (Libya), one at Ottawa, Montreal (Ottawa), and one at central Europe (Europe). The monthly mean differences in outgoing longwave radiation (OLR) (the ISCCP-based OLR minus the HIRS2-based OLR), ranging from –2.8 Wm^–2 at ITCZ to –15.4 Wm^–2 at Ottawa, are less than the monthly mean differences in surface downward flux, ranging from –2.7 Wm^–2 at Libya to 40.6 Wm^–2 at the ITCZ. The large differences in surface downward flux are mainly due to large differences in cloud amount and moisture in the low levels of the atmosphere.Monthly mean OLR and surface downward flux can be derived either (1) from instantaneous temperature, humidity, and cloud fields over a month period or (2) from monthly mean temperature, humidity, and cloud fields. The monthly mean OLR and surface downward flux derived from the first approach is compared with the second. The differences in OLR are small, ranging from –0.05 Wm^–2 to 6.2 Wm^–2, and the differences in surface downward flux is also small, ranging from 0.4 Wm^–2 to 6.4 Wm^–2.List of Acronyms AVHRR Advanced Very High Resolution radiometer - ERB Earth Radiation Budget - ERBE Earth Radiation Budget Experiment - FGGE First Global GARP Experiment - GARP Global Atmospheric Research Program - GCM General Circulation Model - GISS Goddard Institute for Space Studies - GLA Goddard Laboratory for Atmospheres - GMS Geostationary Meteorological Satellite - GOES Geostationary Operational Environmental Satellite - HIRS2 High Resolution Infrared Radiation Sounder/2 - ISCCP International Satellite Cloud Climatology Project - IR Infrared - MSU Microwave Sounding Unit - NFOV Narrow Field of View - NOAA National Oceanic and Atmospheric Administration - NESDIS National Environmental Satellite Data Information Service - TOVS TIROS Operational Vertical Sounder With 4 Figures     

利用MODIS和CERES遥感数据研究青藏高原的云辐射强迫效应

准确估算青藏高原的云辐射效应,对分析该地区的近地面感热通量十分重要。本文首先利用加权平均方法,分别将中分辨率成像光谱仪(MODIS)、测云雷达(CPR)和云与地球辐射能量系统(CERES)的像元数据进行融合。利用这些数据,分析了青藏高原上多云个例(2017年5月5日)与少云个例(2017年8月2日)情况下的可见光通道和热红外通道的信号、云参数和大气长短波辐射强迫等的差异。研究表明,少云时高原地区的大气顶大气长波辐射强迫为108.3 W·m-2,多云时为104.5 W·m-2。同时少云个例中塔里木盆地的大气顶大气长波辐射强迫为200.7 W·m-2,表明该辐射强迫受到地表热力状况影响较大。深厚与浅薄云区的云顶高度相差不大,但多云个例中深厚云区的短波辐射强迫是浅薄云区的2倍多,这一比例远大于长波。这表明短波辐射强迫对云厚度较敏感。最后,本文分析了CERES观测的大气顶长短波辐射分别与MODIS热红外和可见光通道之间的关系,结果表明它们存在很好的相关性(相关系数超过0.95),MODIS的可见光通道可以用于估算大气顶的短波辐射量,而MODIS的热红外通道只可用来估算云区的大气顶长波辐射量。     

Surface‐absorbed and top‐of‐atmosphere radiation fluxes for the Mackenzie River basin from satellite observations and a regional climate model and an evaluation of the model

Abstract

Both the earth‐reflected shortwave and outgoing longwave radiation (OLR) fluxes at the top of the atmosphere (TOA) as well as surface‐absorbed solar fluxes from Canadian Regional Climate Model (CRCM) simulations of the Mackenzie River Basin for the period March 2000 to September 2003 are compared with the radiation fluxes deduced from satellite observations. The differences between the model and satellite solar fluxes at the TOA and at the surface, which are used in this paper to evaluate the CRCM performance, have opposite biases under clear skies and overcast conditions, suggesting that the surface albedo is underestimated while cloud albedo is overestimated. The slightly larger differences between the model and satellite fluxes at the surface compared to those at the TOA indicate the existence of a small positive atmospheric absorption bias in the model. The persistent overestimation of TOA reflected solar fluxes and underestimation of the surface‐absorbed solar fluxes by the CRCM under all sky conditions are consistent with the overestimation of cloud fraction by the CRCM. This results in a larger shortwave cloud radiative forcing (CRF) both at the TOA and at the surface in the CRCM simulation. The OLR from the CRCM agrees well with the satellite observations except for persistent negative biases during the winter months under all sky conditions. Under clear skies, the OLR is slightly underestimated by the CRCM during the winter months and overestimated in the other months. Under overcast conditions the OLR is underestimated by the CRCM, suggesting an underestimation of cloud‐top temperature by the CRCM. There is an improvement in differences between model and satellite fluxes compared to previously reported results largely because of changes to the treatment of the surface in the model.     

Climate forcing by carbonaceous and sulfate aerosols

 An atmospheric general circulation model is coupled to an atmospheric chemistry model to calculate the radiative forcing by anthropogenic sulfate and carbonaceous aerosols. The latter aerosols result from biomass burning as well as fossil fuel burning. The black carbon associated with carbonaceous aerosols is absorbant and can decrease the amount of reflected radiation at the top-of-the-atmosphere. In contrast, sulfate aerosols are reflectant and the amount of reflected radiation depends nonlinearly on the relative humidity. We examine the importance of treating the range of optical properties associated with sulfate aerosol at high relative humidities and find that the direct forcing by anthropogenic sulfate aerosols can decrease from −0.81 W m^-2 to −0.55 Wm^-2 if grid box average relative humidity is not allowed to increase above 90%. The climate forcing associated with fossil fuel emissions of carbonaceous aerosols is calculated to range from +0.16 to +0.20 Wm^-2, depending on how much organic carbon is associated with the black carbon from fossil fuel burning. The direct forcing of carbonaceous aerosols associated with biomass burning is calculated to range from −0.23 to −0.16 Wm^-2. The pattern of forcing by carbonaceous aerosols depends on both the surface albedo and the presence of clouds. Multiple scattering associated with clouds and high surface albedos can change the forcing from negative to positive. Received: 29 September 1997 / Accepted: 10 June 1998     

Influences on surface energy fluxes in simulated present and doubled CO2 climates

The surface energy fluxes simulated by the CSIRO9 Mark 1 GCM for present and doubled CO₂ conditions are analyzed. On the global scale the climatological flux fields are similar to those from four GCMs studied previously. A diagnostic calculation is used to provide estimates of the radiative forcing by the GCM atmosphere. For 1 × CO₂, in the global and annual mean, cloud produces a net cooling at the surface of 31 W m^–2. The clear-sky longwave surface greenhouse effect is 311 W m^–2, while the corresponding shortwave term is –79 W m^–2. As for the other GCM results, the CSIRO9 CO₂ surface warming (global mean 4.8°C) is closely related to the increased downward longwave radiation (LW ). Global mean net cloud forcing changes little. The contrast in warming between land and ocean, largely due to the increase in evaporative cooling (E) over ocean, is highlighted. In order to further the understanding of influences on the fluxes, simple physically based linear models are developed using multiple regression. Applied to both 1 × CO₂ and CO₂ December–February mean tropical fields from CSIRO9, the linear models quite accurately (3–5 W m^–2 for 1 × CO₂ and 2–3 W m^–2 for CO₂) relate LW and net shortwave radiation to temperature, surface albedo, the water vapor column, and cloud. The linear models provide alternative estimates of radiative forcing terms to those from the diagnostic calculation. Tropical mean cloud forcings are compared. Over land, E is well correlated with soil moisture, and sensible heat with air-surface temperature difference. However an attempt to relate the spatial variation of LWt within the tropics to that of the nonflux fields had little success. Regional changes in surface temperature are not linearly related to, for instance, changes in cloud or soil moisture.     

Impacts of greenhouse gases and aerosol direct and indirect effects on clouds and radiation in atmospheric GCM simulations of the 1930–1989 period

Among anthropogenic perturbations of the Earths atmosphere, greenhouse gases and aerosols are considered to have a major impact on the energy budget through their impact on radiative fluxes. We use three ensembles of simulations with the LMDZ general circulation model to investigate the radiative impacts of five species of greenhouse gases (CO₂, CH₄, N₂O, CFC-11 and CFC-12) and sulfate aerosols for the period 1930–1989. Since our focus is on the atmospheric changes in clouds and radiation from greenhouse gases and aerosols, we prescribed sea-surface temperatures in these simulations. Besides the direct impact on radiation through the greenhouse effect and scattering of sunlight by aerosols, strong radiative impacts of both perturbations through changes in cloudiness are analysed. The increase in greenhouse gas concentration leads to a reduction of clouds at all atmospheric levels, thus decreasing the total greenhouse effect in the longwave spectrum and increasing absorption of solar radiation by reduction of cloud albedo. Increasing anthropogenic aerosol burden results in a decrease in high-level cloud cover through a cooling of the atmosphere, and an increase in the low-level cloud cover through the second aerosol indirect effect. The trend in low-level cloud lifetime due to aerosols is quantified to 0.5 min day^–1 decade^–1 for the simulation period. The different changes in high (decrease) and low-level (increase) cloudiness due to the response of cloud processes to aerosols impact shortwave radiation in a contrariwise manner, and the net effect is slightly positive. The total aerosol effect including the aerosol direct and first indirect effects remains strongly negative.     

2011年2月—2012年1月东南极高原辐射平衡观测研究

利用东南极高原熊猫-1自动气象站2011年2月—2012年1月观测的辐射资料和相关资料,对辐射分量和辐射平衡的季节变化进行了研究。结果表明,夏季是东南极高原获得太阳能的主要时段,总辐射通量夏季平均为365.0 W/m²,总量达到2752.1 MJ/m²,占全年总辐射量的58%。各个季节均能出现总辐射瞬时值大于大气顶水平总辐射,春季发生频率最高,冬季最小,总辐射平均日变化呈单峰型。大气长波辐射除夏季外,日变化不明显。冰雪面长波辐射除冬季外,各季节平均日变化呈明显的单峰单谷型。净辐射12月和1月为很小的正值,其他月份为负值。年平均净辐射为 -8.7 W/m²,表明地表相对于大气为冷源。该站的辐射平衡特征与其他南极内陆高原站相似,雪面具有强烈的辐射冷却效应,导致净辐射绝对值都小于下降风区。     

Regional climate simulation with a high resolution GCM: surface hydrology

Aspects of the surface hydrology of high resolution (T106) versions of the ECHAM3 and ECHAM4 general circulation models are analysed over the European region and compared with available observations. The focus is on evaporation, and surface measurements are shown to be useful for the identification of systematic deficiencies in the regional-scale performance of climate models on an annual and seasonal basis, such as the excessive summer dryness over continents. The annual mean evaporation at the available European observation sites is overestimated by 4 mm/month by the ECHAM3 T106, quantitatively consistent with an overestimated surface net radiation of 4 Wm^–2 over Europe. In winter, ECHAM3 shows an overestimated evaporation which compensates for an overestimated downward sensible heat flux. This is primarily related to a too strong zonalisation of the large-scale flow and associated overestimated warm air advection and windspeed. Inaccurate local land surface parameters (e.g. leaf area index, roughness length) are minor contributors to the overestimation. In early summer, the excessive solar radiation at the surface calculated with the ECHAM3 radiation scheme generates a too large evaporation and an excessive depletion of the soil moisture reservoirs. This favours the subsequent excessive summer dryness over Europe with too low values of evaporation, convective precipitation and soil moisture content, leading to a too high surface temperature. In the ECHAM4 T106 simulation, the problem of the European summer dryness is largely reduced, and the simulated evaporation as well as convective precipitation, cloud amount and soil moisture content during summer are substantially improved. The new ECHAM4 radiation scheme appears to be an important factor for this improvement, since it calculates smaller insolation values in better agreement with observations and subsequently may avoid an excessive drying of the soil. Received: 20 September 1995 / Accepted: 10 May 1996