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1.
Richard T. Wetherald 《Climatic change》2011,109(3-4):569-582
The effects that low clouds in sub-tropical to tropical latitudes have in determining a given model’s climate sensitivity is investigated by analyzing the cloud data produced by 16 “slab” or mixed-layer models submitted to the PCMDI and CFMIP archives and their respective response to a doubling of CO2. It is found that, within the context of the 16 models analyzed, changes of these low clouds appear to play a major role in determining model sensitivity but with changes of middle cloud also contributing especially from middle to higher latitudes. It is noted that the models with the smallest overall cloud change produce the smallest climate sensitivities and vice versa although the overall signs of the respective cloud feedbacks are positive. It is also found that the amounts of low cloud as simulated by the respective control runs have very little correlation with their respective climate sensitivities. In general, the overall latitude-height patterns of cloud change as derived from these more recent experiments agree quite well with those obtained from much earlier studies which include increases of the highest cloud, decreases of cloud lower down in the middle and lower tropospheric and small increases of low clouds. Finally, other mitigating factors are mentioned which could also affect the spread of the resulting climate sensitivities. 相似文献
2.
Matthew C. Wyant Christopher S. Bretherton Julio T. Bacmeister Jeffrey T. Kiehl Isaac M. Held Ming Zhao Stephen A. Klein Brian J. Soden 《Climate Dynamics》2006,27(2-3):261-279
Low-latitude cloud distributions and cloud responses to climate perturbations are compared in near-current versions of three leading U.S. AGCMs, the NCAR CAM 3.0, the GFDL AM2.12b, and the NASA GMAO NSIPP-2 model. The analysis technique of Bony et al. (Clim Dyn 22:71–86, 2004) is used to sort cloud variables by dynamical regime using the monthly mean pressure velocity ω at 500 hPa from 30S to 30N. All models simulate the climatological monthly mean top-of-atmosphere longwave and shortwave cloud radiative forcing (CRF) adequately in all ω-regimes. However, they disagree with each other and with ISCCP satellite observations in regime-sorted cloud fraction, condensate amount, and cloud-top height. All models have too little cloud with tops in the middle troposphere and too much thin cirrus in ascent regimes. In subsidence regimes one model simulates cloud condensate to be too near the surface, while another generates condensate over an excessively deep layer of the lower troposphere. Standardized climate perturbation experiments of the three models are also compared, including uniform SST increase, patterned SST increase, and doubled CO2 over a mixed layer ocean. The regime-sorted cloud and CRF perturbations are very different between models, and show lesser, but still significant, differences between the same model simulating different types of imposed climate perturbation. There is a negative correlation across all general circulation models (GCMs) and climate perturbations between changes in tropical low cloud cover and changes in net CRF, suggesting a dominant role for boundary layer cloud in these changes. For some of the cases presented, upper-level clouds in deep convection regimes are also important, and changes in such regimes can either reinforce or partially cancel the net CRF response from the boundary layer cloud in subsidence regimes. This study highlights the continuing uncertainty in both low and high cloud feedbacks simulated by GCMs. 相似文献
3.
To investigate the hydrologic changes of climate in response to an increase of CO2-concentration in the atmosphere, the results from numerical experiments with three climate models are analyzed and compared
with each other. All three models consist of an atmospheric general circulation model and a simple mixed layer ocean with
a horizontally uniform heat capacity. The first model has a limited computational domain and simple geography with a flat
land surface. The second model has a global computational domain with realistic geography. The third model is identical to
the second model except that it has a higher computational resolution. In each numerical experiment, the CO2-induced change of climate is evaluated based upon a comparison between the two climates of a model with normal and four times
the normal concentration of carbon dioxide in air.
It is noted that the zonal mean value of soil moisture in summer reduces significantly in two separate zones of middle and
high latitudes in response to the increase of the CO2-concentration in air. This CO2-induced summer dryness results not only from the earlier ending of the snowmelt season, but also from the earlier occurrence
of the spring to summer reduction in rainfall rate. The former effect is particularly important in high latitudes, whereas
the latter effect becomes important in middle latitudes. Other statistically significant changes include large increases in
both soil moisture and runoff rate in high latitudes of a model during most of the annual cycle with the exception of the
summer season. The penetration of moisture-rich, warm air into high latitudes is responsible for these increases. 相似文献
4.
Yoko Tsushima S. Emori T. Ogura M. Kimoto M. J. Webb K. D. Williams M. A. Ringer B. J. Soden B. Li N. Andronova 《Climate Dynamics》2006,27(2-3):113-126
We have conducted a multi-model intercomparison of cloud-water in five state-of-the-art AGCMs run for control and doubled carbon dioxide climates. The most notable feature of the differences between the control and doubled carbon dioxide climates is in the distribution of cloud-water in the mixed-phase temperature band. The difference is greatest at mid and high latitudes. We found that the amount of cloud ice in the mixed phase layer in the control climate largely determines how much the cloud-water distribution changes for the doubled carbon dioxide climate. Therefore evaluation of the cloud ice distribution by comparison with data is important for future climate sensitivity studies. Cloud ice and cloud liquid both decrease in the layer below the melting layer, but only cloud liquid increases in the mixed-phase layer. Although the decrease in cloud-water below the melting layer occurs at all latitudes, the increase in cloud liquid in the mixed-phase layer is restricted to those latitudes where there is a large amount of cloud ice in the mixed-phase layer. If the cloud ice in the mixed-phase layer is concentrated at high latitudes, doubling of carbon dioxide might shift the center of cloud water distribution poleward which could decrease solar reflection because solar insolation is less at higher latitude. The magnitude of this poleward shift of cloud water appears to be larger for the higher climate sensitivity models, and it is consistent with the associated changes in cloud albedo forcing. For the control climate there is a clear relationship between the differences in cloud-water and relative humidity between the different models, for both magnitude and distribution. On the other hand the ratio of cloud ice to cloud-water follows the threshold temperature which is determined in each model. Improved measurements of relative humidity could be used to constrain the modeled representation of cloud water. At the same time, comparative analysis in global cloud resolving model simulations is necessary for further understanding of the relationships suggested in this paper. 相似文献
5.
A full global atmosphere-ocean-land vegetation model is used to examine the coupled climate/vegetation changes in the extratropics
between modern and mid-Holocene (6,000 year BP) times and to assess the feedback of vegetation cover changes on the climate
response. The model produces a relatively realistic natural vegetation cover and a climate sensitivity comparable to that
realized in previous studies. The simulated mid-Holocene climate led to an expansion of boreal forest cover into polar tundra
areas (mainly due to increased summer/fall warmth) and an expansion of middle latitude grass cover (due to a combination of
enhanced temperature seasonality with cold winters and interior drying of the continents). The simulated poleward expansion
of boreal forest and middle latitude expansion of grass cover are consistent with previous modeling studies. The feedback
effect of expanding boreal forest in polar latitudes induced a significant spring warming and reduced snow cover that partially
countered the response produced by the orbitally induced changes in radiative forcing. The expansion of grass cover in middle
latitudes worked to reinforce the orbital forcing by contributing a spring cooling, enhanced snow cover, and a delayed soil
water input by snow melt. Locally, summer rains tended to increase (decrease) in areas with greatest tree cover increases
(decreases); however, for the broad-scale polar and middle latitude domains the climate responses produced by the changes
in vegetation are relatively much smaller in summer/fall than found in previous studies. This study highlights the need to
develop a more comprehensive strategy for investigating vegetation feedbacks. 相似文献
6.
The general circulation model (GCM) used in this study includes a prognostic cloud scheme and a rather detailed radiation scheme. In a preceding paper, we showed that this model was more sensitive to a global perturbation of the sea surface temperatures than most other models with similar physical parametrization. The experiments presented here show how this feature might depend on some of the cloud modelling assumptions. We have changed the temperature at which the water clouds are allowed to become ice clouds and analyzed separately the feedbacks associated with the variations of cloud cover and cloud radiative properties. We show that the feedback effect associated with cloud radiative properties is positive in one case and negative in the other. This can be explained by the elementary cloud radiative forcing and has implications concerning the use of the GCMs for climate sensitivity studies. 相似文献
7.
Prof. K. N. Liou J. L. Lee S. C. Ou Q. Fu Y. Takano 《Meteorology and Atmospheric Physics》1991,46(1-2):41-50
Summary Parameterization programs for cloud microphyscs and radiative transfer involving ice clouds have been developed in terms of the mean effective size and ice water path. The mean effective size appears to be adequate in representing the ice crystal size distribution for radiative parameterizations. For a given ice water path, smaller mean effective sizes reflect more solar radiation, emit more IR radiation and enhance net radiative heating/cooling at the cloud top and bottom than larger sizes. The presence of small ice crystals may generate steeper lapse rates in clouds. A 3-D global cloud model that prescribes the horizontal wind fields in a 24 hour period is used to investigate the sensitivity of the mean effective size of ice crystals on the simulation of radiative heating, temperature, cloud cover and ice water content. A variation in the mean effective size from 75 to 50 m in a 24 hour prediction on simulation generates more cooling above the high cloud top and a decrease of temperature. These results lead to an increase of high cloud cover in some latitudes by as much as 4% and, at the same time, a decrease of middle cloud cover by 3–4% in latitudes between 60°S and 60°N.With 7 Figures 相似文献
8.
NUMERICAL EXPERIMENTS FOR THE IMPACT OF ANTARCTIC ICE COVER AND SEA SURFACE TEMPERATURE ON CLIMATE VARIABILITY* 
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下载免费PDF全文 By using a three-level atmospheric general circulation model(AGCM),we have completed several numerical experiments to study the impacts of sea surface temperature anomaly(SSTA) and antarctic ice cover anomaly(AICA) during 1981-1983 on climate variability.The results show that during the El Nino period of 1982-1983 the impact of SSTA overrides that of AICA.SSTA mainly affects equatorial zonal circulation and produces PNA wave train,and SE-NW wave train in East Asia to influence the weather of China.AICA produces west-east anomalous vortex streets in the middle latitudes of both hemispheres and affects the intensity of the polar vortex of Southern Hemisphere. 相似文献
9.
In recent years, a substantial reduction of the sea ice in the Arctic has been observed. At the same time, the near-surface
air in this region is warming at a rate almost twice as large as the global average—this phenomenon is known as the Arctic
amplification. The role of the ice-albedo feedback for the Arctic amplification is still a matter of debate. Here the effect
of the surface-albedo feedback (SAF) was studied using a coupled climate model CCSM3 from the National Center for Atmospheric
Research. Experiments, where the SAF was suppressed by locking the surface albedo in the entire coupled model system, were
conducted. The results reveal polar temperature amplification when this model, with suppressed albedo, is forced by a doubling
of the atmospheric CO2 content. Comparisons with variable albedo experiments show that SAF amplifies the surface-temperature response in the Arctic
area by about 33%, whereas the corresponding value for the global-mean surface temperature is about 15%. Even though SAF is
an important process underlying excessive warming at high latitudes, the Arctic amplification is only 15% larger in the variable
than in the locked-albedo experiments. It is found that an increase of water vapour and total cloud cover lead to a greenhouse
effect, which is larger in the Arctic than at lower latitudes. This is expected to explain a part of the Arctic surface–air-temperature
amplification. 相似文献
10.
Abstract We use eddy life‐cycle simulations to evaluate the response of atmospheric transient eddies to a global warming caused by CO2 doubling in the CCC general circulation model. In simulations using Northern Hemisphere winter conditions, transient waves attain larger kinetic energy and encompass a wider range of latitudes in the warmer climate. This behaviour contrasts with a previous investigation that used output from the NCAR and GFDL models. Our analysis indicates two primary factors for the difference between model responses: (1) a smaller change in the mid‐latitude temperature gradient in the CCC model, which allows (2) increased atmospheric water vapour in mid‐latitudes to catalyze a more rapidly evolving life‐cycle. 相似文献
11.
12.
K. D. Williams M. A. Ringer C. A. Senior M. J. Webb B. J. McAvaney N. Andronova S. Bony J. -L. Dufresne S. Emori R. Gudgel T. Knutson B. Li K. Lo I. Musat J. Wegner A. Slingo J. F. B. Mitchell 《Climate Dynamics》2006,26(2-3):145-165
Most of the uncertainty in the climate sensitivity of contemporary general circulation models (GCMs) is believed to be connected with differences in the simulated radiative feedback from clouds. Traditional methods of evaluating clouds in GCMs compare time–mean geographical cloud fields or aspects of present-day cloud variability, with observational data. In both cases a hypothetical assumption is made that the quantity evaluated is relevant for the mean climate change response. Nine GCMs (atmosphere models coupled to mixed-layer ocean models) from the CFMIP and CMIP model comparison projects are used in this study to demonstrate a common relationship between the mean cloud response to climate change and present-day variability. Although atmosphere–mixed-layer ocean models are used here, the results are found to be equally applicable to transient coupled model simulations. When changes in cloud radiative forcing (CRF) are composited by changes in vertical velocity and saturated lower tropospheric stability, a component of the local mean climate change response can be related to present-day variability in all of the GCMs. This suggests that the relationship is not model specific and might be relevant in the real world. In this case, evaluation within the proposed compositing framework is a direct evaluation of a component of the cloud response to climate change. None of the models studied are found to be clearly superior or deficient when evaluated, but a couple appear to perform well on several relevant metrics. Whilst some broad similarities can be identified between the 60°N–60°S mean change in CRF to increased CO2 and that predicted from present-day variability, the two cannot be quantitatively constrained based on changes in vertical velocity and stability alone. Hence other processes also contribute to the global mean cloud response to climate change. 相似文献
13.
R. Bintanja 《Theoretical and Applied Climatology》1997,56(1-2):1-24
Summary In this paper a simple climate model is presented which is used to perform some sensitivity experiments. The atmospheric part is represented by a vertically and zonally averaged layer in which the surface air temperature, radiative fluxes at the surface and at the top of the atmosphere, the turbulent fluxes between atmosphere and surface and the snow cover are calculated. This atmospheric layer is coupled to a two-dimensional advection-diffusion ocean model in which the zonal overturning pattern is prescribed. The ocean model evaluates the temperature distribution, the amount of sea-ice and the meridional and vertical heat fluxes. The present-day climate simulated by the model compares reasonably well with observations of the seasonal and latitudinal distribution of temperature, radiation, surface alebdo, sea-ice and snow cover and meridional energy fluxes. Then, the sensitivity of the model-simulated present-day climate to perturbations in the incident solar radiation at the top of the atmosphere is investigated. The temperature response displays large latitudinal and seasonal variations, which is in qualitative agreement with results obtained with other climate models. It is found that the seasonal variation of sea-ice cover (and hence, the effective oceanic heat capacity) is one of the most important elements determining seasonal variations in climate sensitivity. Differences in sensitivity between the seasonal and annual mean version of the model are discussed. Finally, the equilibrium response to perturbations in some selected model variables is presented; these variables include meridional diffusion coefficients, drag coefficient, sea-ice thickness, atmospheric CO2-concentration and cloud optical thickness.With 13 Figures 相似文献
14.
A method is described for evaluating the ‘partial derivatives’ of globally averaged top-of-atmosphere (TOA) radiation changes
with respect to basic climate model physical parameters. This method is used to analyse feedbacks in the Australian Bureau
of Meteorology Research Centre general circulation model. The parameters considered are surface temperature, water vapour,
lapse rate and cloud cover. The climate forcing which produces the changes is a globally uniform sea surface temperature (SST)
perturbation. The first and second order differentials of model parameters with respect to the forcing (i.e. SST changes)
are estimated from quadratic least square fitting. Except for total cloud cover, variables are found to be strong functions
of global SST. Strongly non-linear variations of lapse rate and high cloud amount and height appear to relate to the non-linear
response in penetrative convection. Globally averaged TOA radiation differentials with respect to model parameters are also
evaluated. With the exception of total cloud contributions, a high correlation is generally found to exist, on the global
mean level, between TOA radiation and the respective parameter perturbations. The largest non-linear terms contributing to
radiative changes are those due to lapse rate and high cloud. The contributions of linear and non-linear terms to the overall
radiative response from a 4 K SST perturbation are assessed. Significant non-linear responses are found to be associated with
lapse rate, water vapour and cloud changes. Although the exact magnitude of these responses is likely to be a function of
the particular model as well as the imposed SST perturbation pattern, the present experiments flag these as processes which
cannot properly be understood from linear theory in the evaluation of climate change sensitivity.
Received: 16 January 1997/Accepted: 9 May 1997 相似文献
15.
区域气候模式对中国东部季风雨带演变的模拟 总被引：28，自引：6，他引：22
本文给出了应用区域气候模式对中国东部夏季雨带演变过程模拟的主要结果。模拟试验分别对正常季风年（1979）和湿季风年（1991）（均由观测场驱动）以及连续3年（全球大气环流模式驱动）的夏季降水场进行,并同观测场进行了比较。结果表明,模式基本上能抓住夏季雨带的主要位置和它的演变特征,与实况相比明显优于全球模式的结果。但模拟的雨带具体位置并不总是与观测值十分吻合,有些旬（或候）差别比较大,模拟的降水量与观测值的相关系数最大仅0.40左右。以上结果表明,为正确模拟东亚季风雨带的演变还需要对决定区域气候的主要物理过程在参数化方面作进一步的改进。 相似文献
16.
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。 相似文献
17.
欧亚大陆积雪对我国春季气候可预报性的影响 总被引：1，自引：0，他引：1
利用大气环流模式IAP9L_CoLM,通过两组集合后报试验,考察了欧亚大陆积雪对我国春季气候可预报性的影响。一组试验为常规后报试验,积雪是由模式陆面过程预报得到的,另一组试验为积雪试验,模式积分过程中欧亚大陆雪水当量由微波遥感积雪资料替代,一天替换一次。通过分析两组试验后报结果的差异,来考察欧亚大陆积雪对我国春季（3~5月）气候可预报性的影响。分析表明:欧亚大陆积雪模拟水平的改善能提高春季欧亚大陆中高纬环流场（海平面气压场和中、高层位势高度场）的可预报性,模式对我国春季气温异常的年际变化和空间分布的可预报能力也有显著增强。对我国春季降水,虽然预报技巧较低,但引入较真实的欧亚积雪作用后,由于中高纬环流场预报技巧的改进导致降水的预测能力也有所改进。个例分析也表明,欧亚中高纬春季积雪异常模拟水平的改善导致了欧亚中高纬贝加尔湖及以南区域环流场可预报性的提高,最终使中国东部区域春季气候异常模拟技巧得以改善。以上结果也证实,欧亚大陆积雪是影响东亚区域春季气候的一个重要因子,要提高模式对中国春季气候的预报技巧,积雪模拟水平的改进是非常必要的。 相似文献
18.
Effect of tropospheric temperature change on the zonal mean circulation and SH winter extratropical cyclones 总被引：1，自引：1，他引：0
This study aims to understand the mechanisms which cause an overall reduction of SH extratropical cyclone activity with a
slight increase in the high latitudes in a warmer climate simulated in general circulation models (GCMs) with increasing CO2. For this purpose, we conducted idealized model experiments by forcing warm temperature anomalies to the areas where climate
change models exhibit local maximum warming—the tropics in the upper troposphere and the polar regions in the lower troposphere—simultaneously
and separately. The Melbourne University atmospheric GCM (R21) coupled with prescribed SST was utilized for the experiments.
Our results demonstrate that the reduction of SH extratropical cyclone frequency and depth in the midlatitudes but the slight
increase in the high latitudes suggested in climate change models result essentially from the tropical upper tropospheric
warming. With this tropical warming, the enhanced static stability which decreases baroclinicity in the low and midlatitudes
turns out to be a major contributor to the decrease of cyclone activity equatorward of 45°S whereas the increased meridional
temperature gradient in the high latitudes seems an important mechanism for the increase of cyclone activity over 50°–60°S. 相似文献
19.
赵勇 《沙漠与绿洲气象(新疆气象)》2013,7(6):1-7
基于1961-2008年天山区域24站云量的逐日资料,使用相关和M原K检验等统计方法,分析中国天山区域云量的时空变化特征及其与降水的关系。结果表明：（1）春季、秋季和冬季,伊犁河谷以北总云量最多,夏季则在中天山和东天山的部分区域最多,低云量在夏季占总云量的比重最大;（2）区域平均总云量在春季和秋季呈减少趋势;低云量在各季节均呈增加趋势,尤其在冬季和夏季;（3）总云量的年代际变化不明显,而低云量自20世纪90年代至今,都处在高值期。（4）低云量在春季、夏季和秋季,均在20世纪90年代,而冬季在2000年左右发生了由少到多的气候突变;总云量未发生明显的气候突变。（5）总云量和低云量均和同期降水有较好的相关性。春季低云量和夏季降水,相关系数可达0.52。 相似文献
20.
We investigate the role of clouds and radiation in the general circulation of the atmosphere using a model designed for 30-day predictions.Comprehensive verifications of 30-day predictions for the 500 hPa geopotential height field have been carried out,using the data from ECMWF objective analyses that cover the period from May 5 to June 3,1982.We perform three model simulations,including experiments with interactive cloud formation,without clouds,and without radiative heating.The latter two experiments allow us to study the effects of cloud/radiation interactions and feedbacks on the predicted vertical velocity,and the meridional and zonal wind profiles,averaged over a 30-day period.We demonstrate that the Hadley circulation is maintained by the presence of clouds.The radiative cooling in the atmosphere intensifies the vertical motion in low latitudes and,to some extent,also strengthens the overall meridional circulation.The meridional winds are correctly reproduced in the model if clouds are incorporated.The zonal winds are significantly affected by clouds and radiative cooling.Without an appropriate incorporation of these physical elements,the model results would deviate significantly from observations.The presence of clouds strengthens the westerlies in middle and high levels.In May,the northerly movemen of the jet stream over eastern Asia is,in part,associated with the presence of clouds. 相似文献