NUMERICAL SIMULATIONS OF EFFECTS OF LAND SURFACE PROCESSES ON CLIMATE——IMPLEMENTING OF SSiB IN IAP/LASG AGCM AND ITS PERFORMANCE

This is an investigation of exchanges of energy and water between the atmosphere and thevegetated continents,and the impact of and mechanisms for land surface-atmosphere interactionson hydrological cycle and general circulation by implementing the Simplified Simple Biosphere(SSiB)model in a modified version of IAP/LASG global spectral general model(L9R15 AGCM).This study reveals that the SSiB model produces a better partitioning of the land surface heat andmoisture fluxes and its diurnal variations,and also gives the transport of energy and water amongatmosphere,vegetation and soil explicitly and realistically.Thus the coupled SSiB-AGCM runslead to the more conspicuous improvement in the simulated circulation,precipitation,mean watervapor content and its transport.particularly in the Asian monsoon region in the real world thanCTL-AGCM runs.It is also pointed out that both the implementation of land surfaceparameterizations and the variations in land surface into the GOALS model have greatly improvedhydrological balance over continents and have a significant impact on the simulated climate.particularly over the massive continents.Improved precipitation recycling model was employed to verify the mechanisms for landsurface hydrology parameterizations on hydrological cycle and precipitation climatology in AGCM.It can be argued that the recycling precipitation rate is significantly reduced,particularly in the aridand semi-arid region of the boreal summer hemisphere,coincident with remarkable reduction inevapotranspiration over the continental area.Therefore the coupled SSiB-AGCM runs reduce thebias of too much precipitation over land surface in most AGCMs,thereby bringing the simulatedprecipitation closer to observations in many continental regions of the world than CTL-AGCMruns.     

陆面过程对气候影响的数值模拟:SSiB与IAP/LASG L9R15 AGCM耦合及其模式性能

利用陆面过程模式 SSi B与 IAP/LASG发展的 L9R1 5AGCM的耦合 1 0 a积分试验 ,研究了全球尺度大气与地表的水分和能量交换以及陆地与大气环流和气候的相互作用。模拟表明 :SSi B模式可模拟出陆地上较为真实的表面通量及其日变化 ,较好地定量描述土壤 -植被 -大气连续体系 ( SPAC)中能量和水分的传输过程。因此 ,将其引入气候模式中能够模拟出比 CTL- AGCM更合理的气候平均状态、水汽分布以及水汽输送的气候特征 ,特别是亚洲夏季风水汽输送独特的地域性 ,再现了大气环流 ,尤其是陆面气候的基本特征。并指出 ,陆面过程参数化的引进及其陆面状况的变化显著地改善了全球陆地上的水分平衡状况。利用改进的再循环降水模式 ,进一步研究了陆面过程参数化明显改进降水模拟的物理机制。指出全球陆地 ,特别是盛夏北半球干旱、半干旱地区的再循环降水率明显减小 ,与陆面上表面潜热通量的显著减小区一致 ,从而克服了许多未耦合陆面过程的 AGCMs因对地表水过程非常简单地参数化导致的普遍存在着整个陆地降水偏高 ,改善了全球陆地上的水分平衡状况。因此 ,在充分耦合的陆气环流模式中模拟的降水分布与实况接近。     

陆面过程模型CoLM与区域气候模式RegCM3的耦合及初步评估

陆面过程通过影响陆面和大气之间物质（如,水分）和能量的交换影响气候, 其参数化方案对数值天气预报、全球及区域气候模拟有重要影响。本研究利用对生物物理、生物化学过程考虑更全面的陆面模式Common Land Model(CoLM) 替代区域气候模式RegCM3原有的陆面模式BATS, 发展了耦合区域气候模式C－RegCM3; 将其应用于东亚地区典型洪涝年份夏季气候模拟以进行评估, 结果表明新耦合的模式C－RegCM3能合理模拟大尺度环流场、近地表气温和降水的分布特征, 对西北半干旱地区降水模拟比RegCM3有所改进。通过利用区域气候模式C－RegCM3及RegCM3对地表能量和水文过程模拟结果的比较, 发现在半干旱、半湿润过渡区C－RegCM3模拟的潜热增大、感热减小; 模拟的地表吸收太阳辐射差异较明显的地区位于模式模拟的主要雨区; C-RegCM3在上述过渡区模拟的夏季地表土壤湿度比RegCM3偏干, 这与它在过渡区降水模拟偏少、蒸散发模拟偏大相对应, 体现了该模式在半干旱、半湿润过渡带模拟出比RegCM3更明显的局地土壤湿度－降水－蒸散发之间的正反馈作用。     

Impact of land surface processes on the South American warm season climate

The present study demonstrates that (1) the simulation of the South American warm season (December?CFebruary) climate by an atmospheric general circulation model (AGCM) is sensitive to the representation of land surface processes, (2) the sensitivity is not confined to the ??hot spot?? in Amazonia, and (3) upgrading the representation of those processes can produce a significant improvement in AGCM performance. The reasons for sensitivity and higher success are investigated based on comparisons between observational datasets and simulations by the AGCM coupled to either a simple land scheme that specifies soil moisture availability or to the Simplified Simple Biosphere Model (SSiB) that allows for consideration of soil and vegetation biophysical process. The context for the study is the UCLA AGCM. The most notable simulation improvements are along the lee of the Andes in the lower troposphere, where poleward flow transports abundant moisture from the Amazon basin to high latitudes, and in the monsoon region where the intensity and pattern of precipitation and upper level ice water content are more realistic. It is argued that a better depiction of the Chaco Low, which is controlled by local effects of land surface processes, decisively contributes to the superior model performance with low-level flows in central South America. The better representation of the atmospheric column static stability and large-scale moisture convergence in tropical South America contribute to more realistic precipitation over the monsoon region. The overall simulation improvement is, therefore, due to a combination of different regional processes. This finding is supported by idealized AGCM experiments.     

Impacts of land surface on climate of july and onset of summer monsoon: a study with an agcm plus ssib

To get more insight into the impacts of land surface processes on climate, a simplified biosphere model (SSiB) developed by Sellers and Xue et al. is implemented into the LASG / IAP spectral climate AGCM (R15L9). The new model has been integrated for more than twenty years. The diagnoses of the integration show that the implementing of the land surface processes has greatly improved the simulation of July climate. It is also shown that the seasonal variations of land surface characteristics have great impacts on the onset of summer monsoon, especially the seasonal march of wind at 850 hPa and precipitation over the regions of summer monsoon.     

一个耦合了简化生物圈方案的全球大气环流谱模式及其性能测试

在美国ＮＭＣ ＡＧＣＭ模式的基础上，参照ＣＯＬＡ低分辨率ＡＧＣＭ模式，移植和改进了一个包括植被层陆面过程的陆－气耦合全球大气环流谱模式。其水平分辨率为菱形截断１５波，垂直方向分为９层，陆面过程采用简化的简单生物圈模式。     

长江中下游1998年夏季梅雨期降水再循环研究

改进了Eltahir的降水再循环率计算模式,引入水汽变化量,使其可用于小于月际时间尺度的降水再循环评估。并利用1998年6~8月间长江中下游162个测站的旬蒸发和降水资料,结合NCEP/NCAR的高空逐日再分析资料,对长江中下游1998年梅雨期的降水再循环率做了计算。分析发现:1998年夏季暴雨时期长江中下游的降水平均约有三成来自当地的蒸发。区域蒸发的水汽在安徽南部和湖北东部对降水的贡献最大。区域平均再循环率的旬变化强烈,最高可达67.8%,最小只有0.8%。表明地表水文敏感,地-气相互作用不稳定。     

AN AGCM +SSiB MODEL SIMULATION ON CHANGES IN PALAEOMONSOON CLIMATE AT 21 KA BP IN CHINA*

The numerical simulation experiment of climate at Last Glacial Maximum (LGM.21 ka BP) in China is made by using an atmospheric general circulation model (AGCM) coupled with land surface processes (AGCM+SSiB) and earth orbital parameters and boundary forcing conditions at21 ka.The modeled climate features are compared with reconstructed conditions at 21 ka from paleo-lake data and pollen data.The results show that the simulated climate conditions at 21 ka in China are fairly comparable with paleo-climatological data.The climate features at 21 ka in China from the experiment are characterized by a drier in the east and a wetter in the west and in the Tibetan Plateau as well.According to the analysis of distribution of pressure and precipitation,as well as the intensity of atmospheric circulation at 21 ka,monsoon circulation in eastern Asia was significantly weak comparing with the present.In the Tibetan Plateau,the intensity of summer monsoon circulation was strengthened,and winter monsoon was a little stronger than the present.The simulation with given forcing boundary conditions,especially the different vegetation coverage,can reproduce the climate condition at the LGM in China,and therefore provides dynamical mechanisms on the climate changes at 21 ka.     

Coupled climate modelling of ocean circulation changes during ice age inception

Freshening of high latitude surface waters can change the large-scale oceanic transport of heat and salt. Consequently, atmospheric and sea ice perturbations over the deep water production sites excite a large-scale response establishing an oceanic "teleconnection" with time scales of years to centuries. To study these feedbacks, a coupled atmosphere-ocean-sea ice model consisting of a two dimensional atmospheric energy and moisture balance model (EMBM) coupled to a thermodynamic sea ice model and an ocean general circulation model is utilised. The coupled model reproduces many aspects of the present oceanic circulation. We also investigate the climate impact of changes in fresh water balance during an ice age initiation. In this experiment part of the precipitation over continents is stored within continental ice sheets. During the buildup of ice sheets the oceanic stratification in the North Atlantic is weakened by a reduced continental run-off leading to an enhanced thermohaline circulation. Under these conditions salinity is redistributed such that deep water is more saline than under present conditions. Once the ice sheets built up, we simulate an ice age climate without net fresh water storage on the continents. In this case the coupled model reproduces the shallow and weak overturning cell, an ice edge advance insulating the upper ocean, and many other aspects of the glacial circulation.     

IAP第四代大气环流模式的耦合气候系统模式模拟性能评估

本文首先扼要介绍了基于中国科学院大气物理研究所(简称IAP)第四代大气环流模式的新气候系统模式-CAS-ESM-C(中国科学院地球系统模式气候系统模式分量)的发展和结构,之后主要对该模式在模拟大气、海洋、陆面和海冰的气候平均态、季节循环以及主要的年际变率等方面的能力做一个初步的评估.结果表明:模式没有明显的气候漂移,各...     

AN AGCM SSiB MODEL SIMULATION ON CHANGES IN PALAEOMONSOON CLIMATE AT 21 KA BP IN CHINA

The numerical simulation experiment of climate at Last Glacial Maximum (LGM.21 ka BP)in China is made by using an atmospheric general circulation model (AGCM) coupled with landsurface processes (AGCM SSiB) and earth orbital parameters and boundary forcing conditions at21 ka.The modeled climate features are compared with reconstructed conditions at 21 ka frompaleo-lake data and pollen data.The results show that the simulated climate conditions at 21 ka inChina are fairly comparable with paleo-climatological data.The climate features at 21 ka in Chinafrom the experiment are characterized by a drier in the east and a wetter in the west and in theTibetan Plateau as well.According to the analysis of distribution of pressure and precipitation,aswell as the intensity of atmospheric circulation at 21 ka,monsoon circulation in eastern Asia wassignificantly weak comparing with the present.In the Tibetan Plateau,the intensity of summermonsoon circulation was strengthened,and winter monsoon was a little stronger than the present.The simulation with given forcing boundary conditions,especially the different vegetationcoverage,can reproduce the climate condition at the LGM in China,and therefore providesdynamical mechanisms on the climate changes at 21 ka.     

The effect of horizontal resolution on simulation of very extreme US precipitation events in a global atmosphere model

We investigate the ability of a global atmospheric general circulation model (AGCM) to reproduce observed 20 year return values of the annual maximum daily precipitation totals over the continental United States as a function of horizontal resolution. We find that at the high resolutions enabled by contemporary supercomputers, the AGCM can produce values of comparable magnitude to high quality observations. However, at the resolutions typical of the coupled general circulation models used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, the precipitation return values are severely underestimated.     

EXTRA-SEASONAL PREDICTIONS OF SUMMER RAINFALL IN CHINA AND ENSO IN 2001 BY CLIMATE MODELS*

China is a monsoon country.The most rainfalls in China concentrate on the summer seasons.More frequent floods or droughts occur in some parts of China.Therefore,the prediction of summer rainfall in China is a significant issue.As we know,the obvious impacts of the sea surface temperature anomalies(SSTA)on the summer rainfall over China have been noticed.The predictions of the SSTA have been involved in the research.The key project on short-term climate modeling prediction system has been finished in 2000.The system included an atmospheric general circulation model named AGCM95,a coupled atmospheric-oceanic general circulation model named AOGCM95,a regional climate model over China named RegCM95,a high-resolution Indian-Pacific OGCM named IPOGCM95,and a simplified atmosphere-ocean dynamic model system named SAOMS95.They became the operational prediction models of National Climate Center(NCC).Extra-seasonal predictions in 2001 have been conducted by several climate models,which were the AGCM95,AOGCM95,RegCM95,IPOGCM95,AIPOGCM95,OSU/NCC,SAOMS95,IAP APOGCM and CAMS/ZS.All of those models predicted the summer precipitation over China and/or the annual SSTA over the tropical Pacific Ocean in the Modeling Prediction Workshop held in March 2001.The assessments have shown that the most models predicted the distributions of main rain belt over Huanan and parts of Jiangnan and droughts over Huabei-Hetao and Huaihe River Valley reasonably.The most models predicted successfully that a weaker cold phase of the SSTA over the central and eastern tropical Pacific Ocean would continue in 2001.The evaluations of extra-seasonal predictions have also indicated that the models had a certain capability of predicting the SSTA over the tropical Pacific Ocean and the summer rainfall over China.The assessment also showed that multi-model ensemble(super ensembles)predictions provided the better forecasts for both SSTA and summer rainfall in 2001,compared with the single model.It is a preliminary assessment for the extra-seasonal predictions by the climate models.The further investigations will be carried out.The model system should be developed and improved.     

Global seasonal climate predictability in a two tiered forecast system. Part II: boreal winter and spring seasons

We examine the Florida Climate Institute–Florida State University Seasonal Hindcast (FISH50) skill at a relatively high (50 km grid) resolution two tiered Atmospheric General Circulation Model (AGCM) for boreal winter and spring seasons at zero and one season lead respectively. The AGCM in FISH50 is forced with bias corrected forecast sea surface temperature averaged from two dynamical coupled ocean–atmosphere models. The comparison of the hindcast skills of precipitation and surface temperature from FISH50 with the coupled ocean–atmosphere models reveals that the probabilistic skill is nearly comparable in the two types of forecast systems (with some improvements in FISH50 outside of the global tropics). Furthermore the drop in skill in going from zero lead (boreal winter) to one season lead (boreal spring) is also similar in FISH50 and the coupled ocean–atmosphere models. Both the forecast systems also show that surface temperature hindcasts have more skill than the precipitation hindcasts and that land based precipitation hindcasts have slightly lower skill than the corresponding hindcasts over the ocean.     

A NUMERICAL STUDY ON THE MEDIUM-RANGE RESPONSES OF THE ATMOSPHERE TO THE ABNORMAL SOIL MOISTURE*

By employing the improved T42L9 spectral model introduced by NMC (Beijing) from ECMWF and utilizing the FGGE-IIIb data covering the period of 14-19 June 1979, the atmospheric responses to the abnormal soil moisture during the medium-range period have been studied numerically. According to the initial field at 12 GMT 14 June, a five-day numerical experiment under different conditions of the soil moisture has been carried out respectively. The monthly mean climatological soil moisture for June has been used in the control experiment in the initial time and it changes with time according to the moisture budget equation at the land surface. Comparing with the experiments with dry or wet soil. one can conclude that: 1) Source of precipitation over continents in summer consists of the land-surface evaporation and the moisture transfer from oceans. Their intensities are comparable during the medium-range time scale when the soil evaporates its moisture sufficiently. Therefore, the soil moisture can influence the global precipitation and the general circulation significantly; 2) By influencing the thermodynamic difference between land and sea,the soil moisture can change the intensity of monsoon and precipitation distribution; 3) The response of the atmosphere to the abnormal soil moisture has the characteristics of geographical distribution and nonlinear interactions; 4) Human activities on the world can influence the environment greatly.     

Evaluation of water and energy budgets in regional climate models applied over Europe

This study presents a model intercomparison of four regional climate models (RCMs) and one variable resolution atmospheric general circulation model (AGCM) applied over Europe with special focus on the hydrological cycle and the surface energy budget. The models simulated the 15 years from 1979 to 1993 by using quasi-observed boundary conditions derived from ECMWF re-analyses (ERA). The model intercomparison focuses on two large atchments representing two different climate conditions covering two areas of major research interest within Europe. The first is the Danube catchment which represents a continental climate dominated by advection from the surrounding land areas. It is used to analyse the common model error of a too dry and too warm simulation of the summertime climate of southeastern Europe. This summer warming and drying problem is seen in many RCMs, and to a less extent in GCMs. The second area is the Baltic Sea catchment which represents maritime climate dominated by advection from the ocean and from the Baltic Sea. This catchment is a research area of many studies within Europe and also covered by the BALTEX program. The observed data used are monthly mean surface air temperature, precipitation and river discharge. For all models, these are used to estimate mean monthly biases of all components of the hydrological cycle over land. In addition, the mean monthly deviations of the surface energy fluxes from ERA data are computed. Atmospheric moisture fluxes from ERA are compared with those of one model to provide an independent estimate of the convergence bias derived from the observed data. These help to add weight to some of the inferred estimates and explain some of the discrepancies between them. An evaluation of these biases and deviations suggests possible sources of error in each of the models. For the Danube catchment, systematic errors in the dynamics cause the prominent summer drying problem for three of the RCMs, while for the fourth RCM this is related to deficiencies in the land surface parametrization. The AGCM does not show this drying problem. For the Baltic Sea catchment, all models similarily overestimate the precipitation throughout the year except during the summer. This model deficit is probably caused by the internal model parametrizations, such as the large-scale condensation and the convection schemes.     

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.     

用一个耦合的全球格点大气环流模式-植被模式模拟中全新世气候变化

用一个耦合的全球格点大气环流模式－植被模式模拟中全新世的气候变化，模拟试验中考虑了地球轨道参数的变化，而其他强迫条件均取成现今值。结果表明，耦合的模式能够模拟出较今强的大尺度夏季风，特别是亚洲－非洲季风，而其他季节和区域的变化值一般都比较小。季风环流和季风降水都大幅度地增大了。结果还显示，耦合模式模拟的大尺度季风系统的变化同单纯大气环流模式模拟的结果非常相似，但是，在非洲北部季风区耦合模式模拟的降水和温度变化较单纯大气模式模拟的值要大，而且，耦合模式模拟的冬季降温值要比单纯大气模式模拟的结果小。     

NUMERICAL SIMULATION ON THE RESPONSE OF LAND SURFACE TO SEVERE WEATHER*

A coupled model of RAMS3b(Regional Atmospheric Modeling System,Version 3b) and LSPM(a land surface process model),in which some basic hydrological processes such as precipitation,evapotranspiration.surface runoff,infiltration and bottom drainage are included,has been established.With the coupled model,we have simulated the response of soil to the sever eweather process which caused the disastrous flood in north italy during 4-7.November,1994,simultaneously compared with the observation and the original RAMS3b,which has a soil and vegetation parameterization scheme(hereafter,SVP) emphasizing on the surface energy fluxes,while some hydrological processes in the soil are not described clearly.The results show that the differences between coupling LSPM and SVP exist mainly in the response of soil to the precipitation.The soil in the SVP never saturates under the strong input of precipitation,while the newly coupled model seems better,the soil has been saturated for one day or more and causes strong surface runoff,which constitutes the flood.Further sensitivity experiments show that the surface hydrological processes are very sensitive to the initial soil moisture and soil type when we compared the results with a relatively dry case and sandy soil.The coupled model has potentiality for simulation on the interaction between regional climate and land surface hydrological processes,and the regional water resources research concerning desertification,drought and flood.     

Numerical simulation of the impact of vegetation index on the interannual variation of summer precipitation in the Yellow River Basin

Two sets of numerical experiments using the coupled National Center for Environmental Prediction General Circulation Model （NCEP/GCM T42L18） and the Simplified Simple Biosphere land surface scheme （SSiB） were carried out to investigate the climate impacts of fractional vegetation cover （FVC） and leaf area index （LAI） on East Asia summer precipitation, especially in the Yellow River Basin （YRB）. One set employed prescribed FVC and LAI which have no interannual variations based on the climatology of vegetation distribution; the other with FVC and LAI derived from satellite observations of the International Satellite Land Surface Climate Project （ISLSCP） for 1987 and 1988. The simulations of the two experiments were compared to study the influence of FVC, LAI on summer precipitation interannual variation in the YRB. Compared with observations and the NCEP reanalysis data, the experiment that included both the effects of satellite-derived vegetation indexes and sea surface temperature （SST） produced better seasonal and interannual precipitation variations than the experiment with SST but no interannual variations in FVC and LAI, indicating that better representations of the vegetation index and its interannual variation may be important for climate prediction. The difference between 1987 and 1988 indicated that with the increase of FVC and LAI, especially around the YRB, surface albedo decreased, net surface radiation increased, and consequently local evaporation and precipitation intensified. Further more, surface sensible heat flux, surface temperature and its diurnal variation decreased around the YRB in response to more vegetation. The decrease of surface-emitting longwave radiation due to the cooler surface outweighed the decrease of surface solar radiation income with more cloud coverage, thus maintaining the positive anomaly of net surface radiation. Further study indicated that moisture flux variations associated with changes in the general circulation also contributed to the precipitation interannual variation.