中国东部和印度季风区云辐射特性的比较

基于 ＩＳＣＣＰ和 ＥＱＢＥ资料,本文比较了中国东部和印度季风区的云和云辐射强迫的气候特征。虽然它们同属于亚洲季风区,并且有相似的降水季节特征,但它们各自的云和云辐射强迫特征差异很大。在印度区域,所有的云量有着相同的季节变化,最大云量分布都出现在夏季,且总云量中以高云量为主。而中国东部云量的季节变化都比较复杂,在总云量中以中、低云量为主,最大总云量出现在春季。冬季的总云量和中、低云量要大于夏季。在全球云量分布中,中国东部最典型的特征是：该地区为全球最大的雨层云覆盖区。与云的分布和变化相关,印度季风区最大的负短波云辐射强迫,最大的正的长波辐射强迫和最大的负的净云辐射强迫发生在夏季,而在中国东部,大的负的短波云辐射强迫发生在春夏之交。年平均的负的短波云辐射强迫在中国东部地区明显要大于在印度季风区。     

Sensitivity Study of Anthropogenic Aerosol Indirect Forcing through Cirrus Clouds with CAM5 Using Three Ice Nucleation Parameterizations

Quantifying the radiative forcing due to aerosol–cloud interactions especially through cirrus clouds remains challenging because of our limited understanding of aerosol and cloud processes. In this study, we investigate the anthropogenic aerosol indirect forcing (AIF) through cirrus clouds using the Community Atmosphere Model version 5 (CAM5) with a state-of-the-art treatment of ice nucleation. We adopt a new approach to isolate anthropogenic AIF through cirrus clouds in which ice nucleation parameterization is driven by prescribed pre-industrial (PI) and presentday (PD) aerosols, respectively. Sensitivities of anthropogenic ice AIF (i.e., anthropogenic AIF through cirrus clouds) to different ice nucleation parameterizations, homogeneous freezing occurrence, and uncertainties in the cloud microphysics scheme are investigated. Results of sensitivity experiments show that the change (PD minus PI) in global annual mean longwave cloud forcing (i.e., longwave anthropogenic ice AIF) ranges from 0.14 to 0.35 W m^–2, the change in global annual mean shortwave cloud forcing (i.e., shortwave anthropogenic ice AIF) from–0.47 to–0.20 W m^–2, and the change in net cloud forcing from–0.12 to 0.05 W m^–2. Our results suggest that different ice nucleation parameterizations are an important factor for the large uncertainty of anthropogenic ice AIF. Furthermore, improved understanding of the spatial and temporal occurrence characteristics of homogeneous freezing events and the mean states of cirrus cloud properties are also important for constraining anthropogenic ice AIF.     

An investigation of cloud cover change in response to thermal forcing

The role of cloud cover in determining the sensitivity of climate has been a source of great uncertainty. This article reviews the distributions of cloud cover change from several climate sensitivity experiments conducted at the Geophysical Fluid Dynamics Laboratory of NOAA (GFDL) and other institutions. Two of the sensitivity experiments conducted at GFDL used a general circulation model with a limited computational domain and idealized geography, whereas three other experiments were conducted by the use of a global model with realistic geography. A thermal forcing imposed was either a change of solar constant or that of the CO₂-concentration in the atmosphere. It was found that in all five cases, clouds were decreased in the moist, convectively active regions such as the tropical and middle latitude rainbelts, whereas they increased in the stable region near the model surface from middle to higher latitudes. In addition, cloud also increased in the lower model stratosphere and generally decreased in the middle and upper troposphere for practically all latitudes.A comparison of the cloud changes obtained from investigations carried out at other institutions reveals certain qualitative (but not necessarily quantitative) similarities to the GFDL results. These similarities include a general reduction of tropospheric cloud cover especially in the vicinity of the rainbelts, a general increase of lower stratospheric cloud cover for almost all latitudes and an increase of low stratiform cloud in high latitudes.     

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.     

Parametrizations of global thermal emissions for simple climate models

 A new, observationally based, parametrization of the thermal emissions by the Earth-atmosphere system for use in simple climate models is presented and discussed. The parametrization, allowing for the modelling of outgoing longwave fluxes in both clear and cloudy sky conditions, introduces temperature-dependent emissivities in a grey-body formulation, whose expressions are based on data from by the International Satellite Cloud Climatology Program (ISCCP). The proposed parametrization is compared to traditional ones and is used in a simple energy-balance model. It is shown how, unlike the traditional parametrizations, the proposed one correctly reproduces the runaway greenhouse phenomenon, i.e. the impossibility of an equilibrium for values of the solar constant larger than a critical one, which is known to occur in more complex radiative-convective models. It is also seen that the new parametrization induces the existence of a varying number of equilibria, depending on the value of the solar forcing, in agreement with previous studies. Further, the new parametrization is used to explore the effects of clouds on the Earth-atmosphere energy balance. It is found that for certain values of the model parameters the radiative effect of clouds might be either net cooling or net warming, depending on the magnitude of the solar forcing. Received: 12 January 1998 / Accepted: 23 July 1998     

Albedo enhancement of marine clouds to counteract global warming: impacts on the hydrological cycle

Recent studies have shown that changes in solar radiation affect the hydrological cycle more strongly than equivalent CO₂ changes for the same change in global mean surface temperature. Thus, solar radiation management ??geoengineering?? proposals to completely offset global mean temperature increases by reducing the amount of absorbed sunlight might be expected to slow the global water cycle and reduce runoff over land. However, proposed countering of global warming by increasing the albedo of marine clouds would reduce surface solar radiation only over the oceans. Here, for an idealized scenario, we analyze the response of temperature and the hydrological cycle to increased reflection by clouds over the ocean using an atmospheric general circulation model coupled to a mixed layer ocean model. When cloud droplets are reduced in size over all oceans uniformly to offset the temperature increase from a doubling of atmospheric CO₂, the global-mean precipitation and evaporation decreases by about 1.3% but runoff over land increases by 7.5% primarily due to increases over tropical land. In the model, more reflective marine clouds cool the atmospheric column over ocean. The result is a sinking motion over oceans and upward motion over land. We attribute the increased runoff over land to this increased upward motion over land when marine clouds are made more reflective. Our results suggest that, in contrast to other proposals to increase planetary albedo, offsetting mean global warming by reducing marine cloud droplet size does not necessarily lead to a drying, on average, of the continents. However, we note that the changes in precipitation, evaporation and P-E are dominated by small but significant areas, and given the highly idealized nature of this study, a more thorough and broader assessment would be required for proposals of altering marine cloud properties on a large scale.     

南半球中高纬度区域不同类型云的辐射特性

利用CloudSat的2B-CLDCLASS-LIDAR云分类产品和2B-FLXHR-LIDAR辐射产品4 a（2007-2010年）的数据,定量分析了单层云（高云、中云、低云）和3种双层云（如:高云与中云共存、高云与低云共存以及中云与低云共存）在南半球中高纬度（40°-65°S）的云量、云辐射强迫和云辐射加热率。其中云辐射加热率定义为有云时的大气加热率廓线与晴空大气加热率廓线的差值。结果表明:研究区域盛行单层低云和单层中云,其云量分别为44.1%和10.3%。并且,中云重叠低云在双层云中云量也是最大（8.7%）。不同类型云的云量也显著影响着其云辐射强迫。单层低云在大气层顶、地表以及大气中的净云辐射强迫分别是-64.8、-56.5和-8.4 W/m2,其绝对值大于其他类型云。虽然单层的中云在大气层顶和地表的净辐射强迫也为负值,但其在大气中的净云辐射强迫为正值（2.3 W/m2）。最后,讨论了不同类型云对大气中辐射能量垂直分布的影响。所有类型云的短波（或长波）云辐射加热率都随高度升高表现为由负值转为正值（或由正值转为负值）。对于大部分云,其净云辐射加热率主要由长波云辐射加热率决定。这些研究结果旨在为模式中云重叠参数化方案在区域的适用性评估及改进提供观测依据。      

Design and performance of a new cloud microphysics scheme developed for the ECHAM general circulation model

A new cloud microphysics scheme including a prognostic treatment of cloud ice (PCI) is developed to yield a more physically based representation of the components of the atmospheric moisture budget in the general circulation model ECHAM. The new approach considers cloud water and cloud ice as separate prognostic variables. The precipitation formation scheme for warm clouds distinguishes between maritime and continental clouds by considering the cloud droplet number concentration, in addition to the liquid water content. Based on several observational data sets, the cloud droplet number concentration is derived from the sulfate aerosol mass concentration as given from the sulfur cycle simulated by ECHAM. Results obtained with the new scheme are compared to satellite observations and in situ measurements of cloud physical and radiative properties. In general, the standard model ECHAM4 and also PCI capture the overall features, and the simulated results usually lie within the range of observed uncertainty. As compared to ECHAM4, only slight improvements are achieved with the new scheme. For example, the overestimated liquid water path and total cloud cover over convectively active regions are reduced in PCI. On the other hand, some shortcomings of the standard model such as underestimated shortwave cloud forcing over the extratropical oceans of the respective summer hemisphere are more pronounced in PCI.This paper was presented at the Third International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 4–9 Sept. 1995 under the auspices of the Max Planck Institute for Meteorology, Hamburg. Editor for these papers is L. Dümenil.     

一维辐射对流模式对云-辐射强迫的数值模拟研究

利用一维辐射－对流气候模式, 详细研究了云量、云光学厚度以及云高等要素的变化对大气顶和地面太阳短波辐射和红外长波辐射通量以及云的辐射强迫的影响, 给出了计算这些物理量的经验拟合公式。结果表明, 云具有极为重要的辐射－气候效应。云量、云光学厚度以及云高即使只有百分之几的变化, 所带来的辐射强迫也可以与大气二氧化碳浓度加倍所产生的辐射强迫（3.75 W/m2）相比拟。例如, 当分别给它们+3%的扰动时, 即取云量变化0.015, 云光学厚度变化0.27, 以及云高变化0.15 km时（在实际的地球大气中, 这种尺度的变化是完全可能发生的）, 那么,可以得到地气系统的太阳短波辐射强迫-3.10 W/m2以及红外长波辐射强迫-1.77 W/m2, 二者之和为-4.78 W/m2, 已经完全可以抵消大气二氧化碳浓度加倍所产生的辐射强迫。但是, 当云量、云光学厚度以及云高向相反方向产生类似扰动时, 所产生的辐射强迫可能极大地放大二氧化碳浓度增加所产生的增强温室效应。因此, 研究结果揭示出, 不管是为了解释过去的气候变化, 还是预测未来的气候变化, 亟待加强在一个变化了的气候环境（例如地面温度升高）下, 云将发生何种变化的研究。     

Simulations of 20th and 21st century Arctic cloud amount in the global climate models assessed in the IPCC AR4

Simulations of late 20th and 21st century Arctic cloud amount from 20 global climate models (GCMs) in the Coupled Model Intercomparison Project phase 3 (CMIP3) dataset are synthesized and assessed. Under recent climatic conditions, GCMs realistically simulate the spatial distribution of Arctic clouds, the magnitude of cloudiness during the warmest seasons (summer–autumn), and the prevalence of low clouds as the predominant type. The greatest intermodel spread and most pronounced model error of excessive cloudiness coincides with the coldest seasons (winter–spring) and locations (perennial ice pack, Greenland, and the Canadian Archipelago). Under greenhouse forcing (SRES A1B emissions scenario) the Arctic is expected to become cloudier, especially during autumn and over sea ice, in tandem with cloud decreases in middle latitudes. Projected cloud changes for the late 21st century depend strongly on the simulated modern (late 20th century) annual cycle of Arctic cloud amount: GCMs that correctly simulate more clouds during summer than winter at present also tend to simulate more clouds in the future. The simulated Arctic cloud changes display a tripole structure aloft, with largest increases concentrated at low levels (below 700 hPa) and high levels (above 400 hPa) but little change in the middle troposphere. The changes in cloud radiative forcing suggest that the cloud changes are a positive feedback annually but negative during summer. Of potential explanations for the simulated Arctic cloud response, local evaporation is the leading candidate based on its high correlation with the cloud changes. The polar cloud changes are also significantly correlated with model resolution: GCMs with higher spatial resolution tend to produce larger future cloud increases.     

A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing

Snow surface and sea-ice energy budgets were measured near 87.5°N during the Arctic Summer Cloud Ocean Study (ASCOS), from August to early September 2008. Surface temperature indicated four distinct temperature regimes, characterized by varying cloud, thermodynamic and solar properties. An initial warm, melt-season regime was interrupted by a 3-day cold regime where temperatures dropped from near zero to ?7°C. Subsequently mean energy budget residuals remained small and near zero for 1 week until once again temperatures dropped rapidly and the energy budget residuals became negative. Energy budget transitions were dominated by the net radiative fluxes, largely controlled by the cloudiness. Variable heat, moisture and cloud distributions were associated with changing air-masses. Surface cloud radiative forcing, the net radiative effect of clouds on the surface relative to clear skies, is estimated. Shortwave cloud forcing ranged between ?50 W m^?2 and zero and varied significantly with surface albedo, solar zenith angle and cloud liquid water. Longwave cloud forcing was larger and generally ranged between 65 and 85 W m^?2, except when the cloud fraction was tenuous or contained little liquid water; thus the net effect of the clouds was to warm the surface. Both cold periods occurred under tenuous, or altogether absent, low-level clouds containing little liquid water, effectively reducing the cloud greenhouse effect. Freeze-up progression was enhanced by a combination of increasing solar zenith angles and surface albedo, while inhibited by a large, positive surface cloud forcing until a new air-mass with considerably less cloudiness advected over the experiment area.     

A three-dimensional study of mesoscale model response to radiative forcing

An infrared radiation parameterization has been applied to a detailed three-dimensional mesoscale model in order to determine whether radiative forcing significantly affects mesoscale atmospheric processes. By taking into account water vapor, liquid water, and carbon dioxide absorption, the scheme differentiates between cloud and clear air regions. The parametric model is presented, along with an overview of the associated mesoscale model.Comparisons between a control run in which only a uniform cooling rate of l K day^–1 is specified, and runs with the infrared scheme are made for 12-hr simulations. The major feature of the radiative forcing is seen to be strong cloud-top cooling. This leads to enhanced destabilization of the upper cloud layer, which in turn results in faster growth of clouds (and which extend to higher levels) than in the control experiment. The deeper clouds force a more vigorous secondary circulation, in which thermodynamic feedbacks between clouds and their environment are substantially stronger than in the case with only a constant cooling rate. This confirms findings made in previous studies undertaken in small-scale numerical models. The discussion also focuses upon a simulation in which the cloud-top infrared cooling has been smoothed out over neighboring vertical levels, in order to represent a cloud-top height distribution crudely. The results indicate that although the absolute values of cloud-top cooling are reduced with respect to the unfiltered case, the fact that cooling extends even higher than previously predicted leads to the formation of thicker, more vigorous clouds. These clouds interact more intensely with their environment than in the unfiltered situation, thereby considerably modifying the mesoscale atmosphere.     

A regional and global analysis of carbon dioxide physiological forcing and its impact on climate

An increase in atmospheric carbon dioxide concentration has both a radiative (greenhouse) effect and a physiological effect on climate. The physiological effect forces climate as plant stomata do not open as wide under enhanced CO₂ levels and this alters the surface energy balance by reducing the evapotranspiration flux to the atmosphere, a process referred to as ‘carbon dioxide physiological forcing’. Here the climate impact of the carbon dioxide physiological forcing is isolated using an ensemble of twelve 5-year experiments with the Met Office Hadley Centre HadCM3LC fully coupled atmosphere–ocean model where atmospheric carbon dioxide levels are instantaneously quadrupled and thereafter held constant. Fast responses (within a few months) to carbon dioxide physiological forcing are analyzed at a global and regional scale. Results show a strong influence of the physiological forcing on the land surface energy budget, hydrological cycle and near surface climate. For example, global precipitation rate reduces by ~3% with significant decreases over most land-regions, mainly from reductions to convective rainfall. This fast hydrological response is still evident after 5 years of model integration. Decreased evapotranspiration over land also leads to land surface warming and a drying of near surface air, both of which lead to significant reductions in near surface relative humidity (~6%) and cloud fraction (~3%). Patterns of fast responses consistently show that results are largest in the Amazon and central African forest, and to a lesser extent in the boreal and temperate forest. Carbon dioxide physiological forcing could be a source of uncertainty in many model predicted quantities, such as climate sensitivity, transient climate response and the hydrological sensitivity. These results highlight the importance of including biological components of the Earth system in climate change studies.     

Changes in Arctic clouds during intervals of rapid sea ice loss

We investigate the behavior of clouds during rapid sea ice loss events (RILEs) in the Arctic, as simulated by multiple ensemble projections of the 21st century in the Community Climate System Model (CCSM3). Trends in cloud properties and sea ice coverage during RILEs are compared with their secular trends between 2000 and 2049 during summer, autumn, and winter. The results suggest that clouds promote abrupt Arctic climate change during RILEs through increased (decreased) cloudiness in autumn (summer) relative to the changes over the first half of the 21st century. The trends in cloud characteristics (cloud amount, water content, and radiative forcing) during RILEs are most strongly and consistently an amplifying effect during autumn, the season in which RILEs account for the majority of the secular trends. The total cloud trends in every season are primarily due to low clouds, which show a more robust response than middle and high clouds across RILEs. Lead-lag correlations of monthly sea ice concentration and cloud cover during autumn reveal that the relationship between less ice and more clouds is enhanced during RILEs, but there is no evidence that either variable is leading the other. Given that Arctic cloud projections in CCSM3 are similar to those from other state-of-the-art GCMs and that observations show increased autumn cloudiness associated with the extreme 2007 and 2008 sea ice minima, this study suggests that the rapidly declining Arctic sea ice will be accentuated by changes in polar clouds.     

FGOALS2两个模式版本对青藏高原东侧冬季层云的模拟特征

冬季青藏高原东部（22°N～32°N，102°E～118°E）层云区是唯一存在于副热带陆地的层云密集区，环流特征较为复杂，大多数耦合气候系统模式对该地区层云的模拟存在较大的偏差。对该地区层云模拟能力的系统分析评估是改进模式性能的重要基础。本文基于国际卫星云计划（ISCCP）卫星资料，评估了中国科学院大气物理研究所两个版本的气候系统模式FGOALS-s2和FGOALS-g2的大气环流模式试验（AMIP）对青藏高原东侧层云的模拟能力。通过分析云辐射强迫等相关特征、大气环流、稳定度、以及地表气温和云的关系，探讨了模式偏差的可能原因。结果表明，两个模式都不同程度地低估了青藏高原东侧的低层云量和云水含量。在垂直结构模拟方面，FGOALS-s2模式能较好地模拟出高原东侧低云主导的特征，其模拟的云顶高度与卫星资料更为接近；而FGOALS-g2模式则高估了该地区的平均云顶高度。分析表明，两个模式均低估了高原东侧的低层稳定度，同时不同程度地低估了该地区中低层水平水汽输送，导致层云云量的模拟偏少。此外，FGOALS-g2高估了高原东侧的上升运动和垂直水汽输送，使得模拟的低云偏少而云顶高度偏高。     

利用FY-4A卫星反演产品对飞机增雨作业的分析

本文利用FY-4A卫星反演的云降水微物理特征参数,分析了2018年10月8日的一次飞机增雨前层状云的微物理结构和特征参数变化,得到四川盆地秋季层状云增雨潜力区的分布,结果表明:①此次作业过程,四川盆地大部分地区覆盖中低云,局部有高云,中低云含有丰富的过冷水,云底粒子较小10～15μm,主要通过凝结过程增长;②此次作业过程,云系随时间的演变特征为从中午到傍晚,云层加厚,云顶升高,粒子有效半径增长,有大片过冷水区,但是缺乏大滴和冰晶,降水不充分;③在无高云配置下,中低云区产生的地面降水较小,而有高低云共同配置和粒子有效半径较大的地区,降水更为充沛;④利用统计检验中的区域对比分析、双比分析和区域历史回归分析方法对本次飞机增雨进行分析后表明,对具有丰富过冷水区的中低云进行人工引晶后,绝对增雨量分别为1.81mm (对比分析)、1.26mm (双比分析)、1.69mm (区域历史回归分析)。利用FY-4A卫星反演方法能够提供云和降水高时空分辨率的物理特征参数演变,丰富了云和降水宏微观物理信息,为人工增雨判别增雨潜力区和准确把握增雨时机提供了一种新的方法。      

Climate sensitivity to cloud optical properties

A radiative–convective model was developed to investigate the sensitivity of climate to cloud optical properties and the related feedback processes. This model demonstrates that the Earth's surface temperature increases with cloud optical depth when the clouds are very thin but decreases with cloud optical depth when the cloud shortwave (solar) radiative forcing is larger than the cloud longwave (terrestrial) radiative forcing. When clouds are included in the model, the magnitude of the greenhouse effect due to a doubling of the CO₂ concentration varies with the cloudoptical depth: the thicker the clouds, the weaker the greenhouse warming. In addition, a small variation in the cloud droplet size has a larger impact on the equilibrium state temperature in the lower atmosphere than the warming caused by a doubling of the CO₂ concentration: a 2% increase in the average cloud droplet size per degree increase in temperature doubles the warming caused by the doubling of the CO₂ concentration. These findings suggest that physically reliable correlations between the cloud droplet size and macrophysical meteorological variables such as temperature, wind and water vapor fields are needed on a global climate scale to assess the climate impact of increases in greenhouse gases.     

Combining ERBE and ISCCP data to assess clouds in the Hadley Centre, ECMWF and LMD atmospheric climate models

 This study compares radiative fluxes and cloudiness fields from three general circulation models (the HadAM4 version of the Hadley Centre Unified model, cycle 16r2 of the ECMWF model and version LMDZ 2.0 of the LMD GCM), using a combination of satellite observations from the Earth Radiation Budget Experiment (ERBE) and the International Satellite Cloud Climatology Project (ISCCP). To facilitate a meaningful comparison with the ISCCP C1 data, values of column cloud optical thickness and cloud top pressure are diagnosed from the models in a manner consistent with the satellite view from space. Decomposing the cloud radiative effect into contributions from low-medium- and high-level clouds reveals a tendency for the models' low-level clouds to compensate for underestimates in the shortwave cloud radiative effect caused by a lack of high-level or mid-level clouds. The low clouds fail to compensate for the associated errors in the longwave. Consequently, disproportionate errors in the longwave and shortwave cloud radiative effect in models may be taken as an indication that compensating errors are likely to be present. Mid-level cloud errors in the mid-latitudes appear to depend as much on the choice of the convection scheme as on the cloud scheme. Convective and boundary layer mixing schemes require as much consideration as cloud and precipitation schemes when it comes to assessing the simulation of clouds by models. Two distinct types of cloud feedback are discussed. While there is reason to doubt that current models are able to simulate potential `cloud regime' type feedbacks with skill, there is hope that a model capable of simulating potential `cloud amount' type feedbacks will be achievable once the reasons for the remaining differences between the models are understood. Received: 23 January 2000 / Accepted: 24 January 2001     

Climate feedback efficiency and synergy

Earth’s climate sensitivity to radiative forcing induced by a doubling of the atmospheric CO₂ is determined by feedback mechanisms, including changes in atmospheric water vapor, clouds and surface albedo, that act to either amplify or dampen the response. The climate system is frequently interpreted in terms of a simple energy balance model, in which it is assumed that individual feedback mechanisms are additive and act independently. Here we test these assumptions by systematically controlling, or locking, the radiative feedbacks in a state-of-the-art climate model. The method is shown to yield a near-perfect decomposition of change into partial temperature contributions pertaining to forcing and each of the feedbacks. In the studied model water vapor feedback stands for about half the temperature change, CO₂-forcing about one third, while cloud and surface albedo feedback contributions are relatively small. We find a close correspondence between forcing, feedback and partial surface temperature response for the water vapor and surface albedo feedbacks, while the cloud feedback is inefficient in inducing surface temperature change. Analysis suggests that cloud-induced warming in the upper tropical troposphere, consistent with rising convective cloud anvils in a warming climate enhances the negative lapse-rate feedback, thereby offsetting some of the warming that would otherwise be attributable to this positive cloud feedback. By subsequently combining feedback mechanisms we find a positive synergy acting between the water vapor feedback and the cloud feedback; that is, the combined cloud and water vapor feedback is greater than the sum of its parts. Negative synergies surround the surface albedo feedback, as associated cloud and water vapor changes dampen the anticipated climate change induced by retreating snow and ice. Our results highlight the importance of treating the coupling between clouds, water vapor and temperature in a deepening troposphere.     

海盐气溶胶和硫酸盐气溶胶在云微物理过程中的作用

利用大气气溶胶和云分档模式研究海盐气溶胶和硫酸盐气溶胶在云微物理过程中的作用, 计算结果表明:云中液态水含量随高度的分布并不随海盐、硫酸盐的数目以及云团上升速度的变化而变化; 随着云滴数目的增加, 云滴的有效半径会减小; 硫酸盐对云滴数目影响起主导作用, 海盐在水汽相对充足情况下增加了云滴数目, 在水汽相对不足的情况下减少了云滴数目; 硫酸盐粒子浓度特别强的情况下 (人类活动污染比较严重时), 如果水汽相对不足, 云滴数目会明显小于硫酸盐粒子浓度; 而海盐粒子的存在, 加剧了水汽的供应不足, 从而可以在很大程度上进一步降低云滴数目。也就是说, 在有些情况下, 如果不考虑海盐气溶胶的作用, 硫酸盐气溶胶对云特性的影响会被过高估计。