The simulation of Arctic clouds and their influence on the winter surface temperature in present-day climate in the CMIP3 multi-model dataset

We investigate the influence of clouds on the surface energy budget and surface temperature in the sea-ice covered parts of the ocean north of the Arctic circle in present-day climate in nine global climate models participating in the Coupled Model Intercomparison Project phase 3, CMIP3. Monthly mean simulated surface skin temperature, radiative fluxes and cloud parameters are evaluated using retrievals from the extended AVHHR Polar Pathfinder (APP-x) product. We analyzed the annual cycle but the main focus is on the winter, in which large parts of the region experience polar night. We find a smaller across-model spread as well as better agreement with observations during summer than during winter in the simulated climatological annual cycles of total cloudiness and surface skin temperature. The across-model spread in liquid and ice water paths is substantial during the whole year. These results qualitatively agree with earlier studies on the present-day Arctic climate in GCMs. The climatological ensemble model mean annual cycle of surface cloud forcing shows good agreement with observations in summer. However, during winter the insulating effect of clouds tends to be underestimated in models. During winter, most of the models as well as the observations show higher monthly mean total cloud fractions, associated with larger positive surface cloud forcing. Most models also show good correlation between the surface cloud forcing and the vertically integrated ice and liquid cloud condensate. The wintertime ensemble model mean total cloud fraction (69%) shows excellent agreement with observations. The across-model spread in the winter mean cloudiness is substantial (36?C94%) however and several models significantly underestimate the cloud liquid water content. If the two models not showing any relationship between cloudiness and surface cloud forcing are disregarded, a tentative across-model relation exists, in such a way that models that simulate large winter mean cloudiness also show larger surface cloud forcing. Even though the across-model spread in wintertime surface cloud forcing is large, no clear relation to the surface temperature is found. This indicates that other processes, not explicitly cloud related, are important for the simulated across-model spread in surface temperature.     

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     

Climate change and the regulation of the surface moisture and energy budgets

The increase in the vigor of the hydrological cycle simulated in a 2 × CO₂ experiment with the Canadian Climate Centre general circulation model is smaller than that obtained by other models which have similar increases in mean surface temperature. The surface energy budget, which encompasses also the moisture budget for the oceans, is analyzed. Changes in the net radiative input to and sensible heat flux from the surface act to warm it. This is balanced, at the new equilibrium, by a change in the latent heat flux which acts to cool it. Although this same general behavior is seen in other models, the increase in radiative input to the surface in the CCC GCM is smaller than in other models while the change in the sensible heat flux is of similar size. As a consequence, the latent heat flux required for balance is smaller. The comparatively small increase in the net radiative input at the surface occurs because of a decrease in the solar component. On average the decrease in solar input in the tropical region outweighs the higher latitude increase associated with the snow/ice albedo feedback. The notable tropical decrease in solar input occurs because the albedo of the clouds increase enough in this region to outweigh a small decrease in cloud amount. The increase in cloud albedo in the warmer and moister tropical atmosphere is a consequence of the parameterized cloud optical properties in the model which play an important role in the regulation of the surface energy and moisture budgets. The results demonstrate some of the consequences of the negative feedback mechanism associated with increasing cloud albedo in the model. They also suggest that the simulated change in the vigor of the hydrological cycle is not a simple function of the average increase in surface temperature but is a consequence of all of the processes in the model which control the available energy at the surface as a function of latitude.     

东亚地区云和地表反照率对硫酸盐直接辐射强迫的影响

文中利用区域气候模式深入探讨了东亚地区云及地表反照率对硫酸盐直接辐射强迫的影响 ,同时定量估算了东亚地区云区气溶胶的直接辐射强迫 ,讨论了硫酸盐对地表和大气短波辐射平衡产生的不同影响。研究表明 :云对气溶胶的直接辐射强迫具有很强的减弱作用 ,这种减弱作用不仅取决于云覆盖份数 ,而且取决于云的光学厚度。就区域平均而言 ,文中模拟的东亚地区气溶胶直接辐射强迫为 - 0 .0 97W /m2 ,占总直接辐射强迫的 10 .4%左右。表明云对硫酸盐直接辐射具有很强的减弱作用 ,在估算其总的直接辐射强迫时 ,云区的贡献不可忽视。较高的地表反照率会减弱硫酸盐的直接辐射强迫 ,而较低地表反照率则会增加硫酸盐的辐射强迫。硫酸盐气溶胶对大气辐射平衡影响非常小 ,但对地表辐射平衡产生重要影响 ,影响程度与大气几乎一致。     

The vertical distribution of climate forcings and feedbacks from the surface to top of atmosphere

The radiative forcings and feedbacks that determine Earth’s climate sensitivity are typically defined at the top-of-atmosphere (TOA) or tropopause, yet climate sensitivity itself refers to a change in temperature at the surface. In this paper, we describe how TOA radiative perturbations translate into surface temperature changes. It is shown using first principles that radiation changes at the TOA can be equated with the change in energy stored by the oceans and land surface. This ocean and land heat uptake in turn involves an adjustment of the surface radiative and non-radiative energy fluxes, with the latter being comprised of the turbulent exchange of latent and sensible heat between the surface and atmosphere. We employ the radiative kernel technique to decompose TOA radiative feedbacks in the IPCC Fourth Assessment Report climate models into components associated with changes in radiative heating of the atmosphere and of the surface. (We consider the equilibrium response of atmosphere-mixed layer ocean models subjected to an instantaneous doubling of atmospheric CO₂). It is shown that most feedbacks, i.e., the temperature, water vapor and cloud feedbacks, (as well as CO₂ forcing) affect primarily the turbulent energy exchange at the surface rather than the radiative energy exchange. Specifically, the temperature feedback increases the surface turbulent (radiative) energy loss by 2.87 W m^?2 K^?1 (0.60 W m^?2 K^?1) in the multimodel mean; the water vapor feedback decreases the surface turbulent energy loss by 1.07 W m^?2 K^?1 and increases the surface radiative heating by 0.89 W m^?2 K^?1; and the cloud feedback decreases both the turbulent energy loss and the radiative heating at the surface by 0.43 and 0.24 W m^?2 K^?1, respectively. Since changes to the surface turbulent energy exchange are dominated in the global mean sense by changes in surface evaporation, these results serve to highlight the fundamental importance of the global water cycle to Earth’s climate sensitivity.     

Spectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust

In May–June 2006, airborne and ground-based solar (0.3–2.2 μm) and thermal infrared (4–42 μm) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m⁻² per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m⁻² were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between −19 and +24 W m⁻² for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect.     

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.     

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     

A local climatic model of the temperature-cloudiness relationship

Summary Filtered secular series of temperature at Zagreb (Croatia) are negatively correlated with simultaneous series of solar radiation and positively correlated with cloudiness.The local balance of radiative fluxes, with cloudiness interference included, provides a basis for the evaluation of a local deterministic model, which can describe both the positive and negative impacts of cloud radiative forcing upon surface temperature. A theoretical temperature series, generated by the model simulation, proves a prevailing greenhouse effect of clouds upon recent climatic variations of temperature at Zagreb.With 5 Figures     

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。     

Cloud effects on air-sea interactions during the 1979 Indian summer monsoon as studied from satellite observations

Summary The Indian summer monsoon, one of the earth's most vigorous and energetic seasonally occurring weather events, influences the global atmospheric circulation. Its onset, duration, and intensity are governed by large- and meso-scale geophysical processes, such as surface solar heating and air-sea interactions. In this paper, using innovative combinations of satellite sensor data, we investigate some of these fundamental processes which are closely tied to clouds and control the monsoon system's evolution. The study, which focuses on the monsoon period of June, 1979, examines the low-frequency variability of clouds and their effects on air-sea processes through an analysis of the complex influence clouds play on the surface heat and water budgets. First, the effects of clouds on both the solar and longwave components of the surface radiation budget are assessed using a cloud radiative forcing parameter. While the effects of clouds on the long-wave irradiance act in a manner opposite to their effects on the shortwave irradiance, only a partial compensation is found to take place and the net effect results in a maximum cloud forcing of 60 Wm^–2 in the southwestern Arabian Sea. Second, employing satellite-derived precipitation and evaporation estimates, the paper analyzes the net surface fresh water budget variability around the monsoon onset. This budget is important in that fresh water affects the upper ocean density distribution and, consequently, the thermohaline circulation. Two regions are found to dominate the analysis: the western Arabian Sea, where evaporation is dominant by more than 10 mm day^–1, and the eastern Arabian Sea, where precipitation is dominant by more than 10 mm day^–1. Thus, a strong zonal gradient of fresh water at the surface is established during the monsoon. The last topic investigated is the intraseasonal variability of convection as analyzed using a cloud parameter indicative of deep convection. Cloud oscillations of 30–50 days, associated with the different phases of the monsoon, are found to propagate northward in the eastern Indian Ocean and eastward in the Bay of Bengal. Our analysis not only supports the hypothesis that the 30–50-day oscillation is driven by deep convection but also, and more importantly, suggests that the ocean thermal forcing is modulated by 30–50-day oscillations through cloud-induced surface radiative forcing. Although the results presented are limited in scope and preliminary because of the diffculty in quantifying the accuracy of the parameters examined, they do demonstrate: 1) the role of clouds in modulating the surface heat and water budgets, 2) the advantage of using combinations of multi-sensor and multi-platform satellite observations to quantify interrelated surface heat/water budget processes, and 3) the potential to examine the intraseasonal variability of air-sea interaction processes associated with the monsoon, even though these processes are not directly measurable from space.With 6 FiguresB. DiJulio passed away in September 1990.     

基于RegCM4.6的中国地区云对地表太阳辐射的影响研究

使用RegCM4.6区域气候模式，选取Emanuel和Mix（Grell+Emanuel）两种积云对流参数化方案，以2016年为例，分别对中国地区云短波辐射强迫及其涉及的物理量进行数值模拟，揭示其时空分布特征，并探究两种积云对流参数方案模拟效果的差异及其原因。结果表明：从季节平均来看，全国地表云短波辐射强迫均为负值，云对地表为冷却效应，冬季最小，春夏季较大。塔里木盆地四季均为辐射强迫低值区，夏季冷却效应最弱，辐射强迫绝对值低于40 W·m-2；全天空地表净短波辐射分布也呈显著季节差异，除夏季外均呈由南向北逐渐递减的分布趋势；晴空地表净短波辐射在横断山脉处和塔里木盆地处均比较低，其中春季最为明显；两个方案所得的季节空间分布特征大致相同，但在数值上存在差异。春季时全国大部分地区全天空地表净短波辐射通量差异最大，在55 W·m-2左右，晴空地表净短波辐射通量在青藏高原处差异在60W·m-2左右。     

Influence of Clouds on UV Irradiance at Ground Level and Backscattered Exittance

The influence of various cloud parameters and the interactions with the ground albedo and the solar zenith angle have been studied by means of model simulations. The radiative transfer model suitable for a cloudy atmosphere as well as for a clear atmosphere has been developed on the basis of the Discrete Ordinate Method. This study leads to a general understanding for cloudy atmospheres: in the presence of a uniform cloud, the cloud scattering is dominant to molecular and aerosol scattering, and it is also wavelength-independent; the ratio of transmitted irradiance in a cloudy atmosphere to that in the background clear atmosphere is independent of cloud height and solar zenith angle. That’s to say, the radiation downwelling out of a cloud is quite isotropic; it decreases approximately exponentially with the cloud optical depth at a rate related to the ground albedo; the reflected irradiance at the top of the atmosphere is dependent on cloud optical depth as well as on solar zenith angle, but not on ground albedo for clouds of not very thin optical depth.     

Effect of surface albedo, water vapour, and atmospheric aerosols on the cloud-free shortwave radiative budget in the Arctic

This study is based on ground-based measurements of downward surface shortwave irradiance (SW), columnar water vapour (wv), and aerosol optical depth (τ) obtained at Thule Air Base (Greenland) in 2007–2010, together with MODIS observations of the surface shortwave albedo (A). Radiative transfer model calculations are used in combination with measurements to separate the radiative effect of A (ΔSW_A), wv (ΔSW_wv), and aerosols (ΔSW_τ) in modulating SW in cloud-free conditions. The shortwave radiation at the surface is mainly affected by water vapour absorption, which produces a reduction of SW as low as ?100 Wm^?2 (?18%). The seasonal change of A produces an increase of SW by up to +25 Wm^?2 (+4.5%). The annual mean radiative effect is estimated to be ?(21–22) Wm^?2 for wv, and +(2–3) Wm^?2 for A. An increase by +0.065 cm in the annual mean wv, to which corresponds an absolute increase in ΔSW_wv by 0.93 Wm^?2 (4.3%), has been observed to occur between 2007 and 2010. In the same period, the annual mean A has decreased by ?0.027, with a corresponding decrease in ΔSW_A by 0.41 Wm^?2 (?14.9%). Atmospheric aerosols produce a reduction of SW as low as ?32 Wm^?2 (?6.7%). The instantaneous aerosol radiative forcing (RF_τ) reaches values of ?28 Wm^?2 and shows a strong dependency on surface albedo. The derived radiative forcing efficiency (FE_τ) for solar zenith angles between 55° and 70° is estimated to be (?120.6 ± 4.3) for 0.1 < A < 0.2, and (?41.2 ± 1.6) Wm^?2 for 0.5 < A < 0.6.     

The impact of boundary forcing on RegCM4.2 surface energy budget

The surface energy budget components from two simulations of the regional climate model RegCM4.2 over the European/North African domain during the period 1989–2005 are analysed. The simulations differ in specified boundary forcings which were obtained from ERA-Interim reanalysis and the HadGEM2-ES Earth system model. Surface radiative and turbulent fluxes are compared against ERA-Interim. Errors in surface radiative fluxes are derived with respect to the Global Energy and Water Cycle Experiment/Surface Radiation Budget satellite-based products. In both space and time, we find a high degree of realism in the RegCM surface energy budget components, but some substantial errors and differences between the two simulations are also present. The most prominent error is an overestimation of the net surface shortwave radiation flux of more than 50 W/m² over central and southeastern Europe during summer months. This error strongly correlates with errors in the representation of total cloud cover, and less strongly with errors in surface albedo. During other seasons, the amplitude of the surface energy budget components is more in line with reference datasets. The errors may limit the usefulness of RegCM simulations in applications (e.g. high-quality simulation-driven impact studies). However, by using a simple diagnostic model for error interpretation, we suggest potential sensitivity studies aiming to reduce the underestimation of cloud cover and overestimation of shortwave radiation flux.     

Impact of clouds on the surface radiation balance of the Arctic Ocean

Summary The relationship between clouds and the surface radiative fluxes over the Arctic Ocean are explored by conducting a series of modelling experiments using a one-dimensional thermodynamic sea ice model. The sensitivity of radiative flux to perturbations in cloud fraction and cloud optical depth are determined. These experiments illustrate the substantial effect that clouds have on the state of the sea ice and on the surface radiative fluxes. The effect of clouds on the net flux of radiation at the surface is very complex over the Arctic Ocean particularly due to the presence of the underlying sea ice. Owing to changes in surface albedo and temperature associated with changing cloud properties, there is a strong non-linearity between cloud properties and surface radiative fluxes. The model results are evaluated in three different contexts: 1) the sensitivity of the arctic surface radiation balance to uncertainties in cloud properties; 2) the impact of interannual variability in cloud characteristics on surface radiation fluxes and sea ice surface characteristics; and 3) the impact of climate change and the resulting changes in cloud properties on the surface radiation fluxes and sea ice characteristics.With 11 Figures     

Spectral dependence on the correction factor of erythemal UV for cloud,aerosol, total ozone,and surface properties: A modeling study

Radiative transfer model simulations were used to investigate the erythemal ultraviolet(EUV) correction factors by separating the UV-A and UV-B spectral ranges. The correction factor was defined as the ratio of EUV caused by changing the amounts and characteristics of the extinction and scattering materials. The EUV correction factors(CFEUV) for UV-A[CFEUV(A)] and UV-B [CFEUV(B)] were affected by changes in the total ozone, optical depths of aerosol and cloud, and the solar zenith angle. The differences between CFEUV(A) and CFEUV(B) were also estimated as a function of solar zenith angle, the optical depths of aerosol and cloud, and total ozone. The differences between CFEUV(A) and CFEUV(B) ranged from-5.0% to 25.0% for aerosols, and from-9.5% to 2.0% for clouds in all simulations for different solar zenith angles and optical depths of aerosol and cloud. The rate of decline of CFEUV per unit optical depth between UV-A and UV-B differed by up to 20% for the same aerosol and cloud conditions. For total ozone, the variation in CFEUV(A) was negligible compared with that in CFEUV(B) because of the effective spectral range of the ozone absorption band. In addition, the sensitivity of the CFEUVs due to changes in surface conditions(i.e., surface albedo and surface altitude) was also estimated by using the model in this study. For changes in surface albedo, the sensitivity of the CFEUVs was 2.9%–4.1% per 0.1 albedo change,depending on the amount of aerosols or clouds. For changes in surface altitude, the sensitivity of CFEUV(B) was twice that of CFEUV(A), because the Rayleigh optical depth increased significantly at shorter wavelengths.     

黑碳气溶胶辐射强迫全球分布的模拟研究

利用一个改进的辐射传输模式,结合全球气溶胶数据集（GADS）,计算晴空条件下冬夏两季黑碳气溶胶的直接辐射强迫在对流层顶和地面的全球分布。计算结果表明,与温室气体引起的整层大气都是正的辐射强迫不同,黑碳气溶胶的辐射强迫在对流层顶为正值,而在地面的辐射强迫却是负值。作者从理论上解释了造成这种结果的原因。对北半球冬季和夏季而言,在对流层顶黑碳气溶胶的全球辐射强迫的平均值分别为0.085W/m2和0.155 W/m2,在地面则分别为-0.37 W/m2和-0.63 W/m2。虽然气溶胶的辐射强迫主要依赖于其本身的光学性质和在大气中的浓度,太阳高度角和地表反照率对黑碳气溶胶的辐射强迫会产生很大的影响。研究指出:黑碳气溶胶在对流层顶正的辐射强迫和在地面负的辐射强迫的绝对值都随太阳天顶角的余弦和地表反照率的增加线性增大;地表反照率对黑碳气溶胶辐射强迫的强度和分布都有重要影响。黑碳气溶胶的辐射强迫分布具有明显的纬度变化特征,冬夏两季的大值区都位于30°N~90°N之间,表明人类活动是造成黑碳气溶胶辐射强迫的主要原因。     

Cirrus cloud horizontal and vertical inhomogeneity effects in a GCM

Summary A set of the inhomogeneity factor for high-level clouds derived from the ISCCP D1 dataset averaged over a five-year period has been incorporated in the UCLA atmospheric GCM to investigate the effect of cirrus cloud inhomogeneity on climate simulation. The inclusion of this inhomogeneous factor improves the global mean planetary albedo by about 4% simulated from the model. It also produces changes in solar fluxes and OLRs associated with changes in cloud fields, revealing that the cloud inhomogeneity not only affects cloud albedo directly, but also modifies cloud and radiation fields. The corresponding difference in the geographic distribution of precipitation is as large as 7 mm day⁻¹. Using the climatology cloud inhomogeneity factor also produces a warmer troposphere related to changes in the cloudiness and the corresponding radiative heating, which, to some extent, corrects the cold bias in the UCLA AGCM. The region around 14 km, however, is cooler associated with increase in the reflected solar flux that leads to a warmer region above. An interactive parameterization for mean effective ice crystal size based on ice water content and temperature has also been developed and incorporated in the UCLA AGCM. The inclusion of the new parameterization produces substantial differences in the zonal mean temperature and the geographic distribution of precipitation, radiative fluxes, and cloud cover with respect to the control run. The vertical distribution of ice crystal size appears to be an important factor controlling the radiative heating rate and the consequence of circulation patterns, and hence must be included in the cloud-radiation parameterization in climate models to account for realistic cloud processes in the atmosphere.