Relative contribution of soil moisture and snow mass to seasonal climate predictability: a pilot study

Land surface hydrology (LSH) is a potential source of long-range atmospheric predictability that has received less attention than sea surface temperature (SST). In this study, we carry out ensemble atmospheric simulations driven by observed or climatological SST in which the LSH is either interactive or nudged towards a global monthly re-analysis. The main objective is to evaluate the impact of soil moisture or snow mass anomalies on seasonal climate variability and predictability over the 1986–1995 period. We first analyse the annual cycle of zonal mean potential (perfect model approach) and effective (simulated vs. observed climate) predictability in order to identify the seasons and latitudes where land surface initialization is potentially relevant. Results highlight the influence of soil moisture boundary conditions in the summer mid-latitudes and the role of snow boundary conditions in the northern high latitudes. Then, we focus on the Eurasian continent and we contrast seasons with opposite land surface anomalies. In addition to the nudged experiments, we conduct ensembles of seasonal hindcasts in which the relaxation is switched off at the end of spring or winter in order to evaluate the impact of soil moisture or snow mass initialization. LSH appears as an effective source of surface air temperature and precipitation predictability over Eurasia (as well as North America), at least as important as SST in spring and summer. Cloud feedbacks and large-scale dynamics contribute to amplify the regional temperature response, which is however, mainly found at the lowest model levels and only represents a small fraction of the observed variability in the upper troposphere.     

2004年南海夏季风的爆发及中南半岛陆面-过程的可能影响Ⅱ： 数值试验

在对南海夏季风的爆发及中南半岛陆面过程的可能影响进行了诊断分析的基础上,应用MM5/NOAHLSM模式,研究了中南半岛陆气相互作用对2004年南海夏季风爆发过程的可能影响。结果发现:在南海夏季风爆发前,中南半岛南海地区低层气温差确实出现低值,甚至负值;尽管短期内中南半岛土壤湿度和降水的变化没有引起季风爆发日期的改变,但对季风爆发的强度有影响。土壤湿度和降水变化引起的干异常可导致地表感热通量的增大和地表温度的升高,致使中南半岛与南海之间低层的温差异常(负温差)减小,季风爆发强度减弱;不同的是,湿异常可引起季风爆发强度增强。这一结果说明,在南海夏季风爆发前期,中南半岛上空对流活动和降水异常及其引起的土壤湿度的异常变化在一定程度上会影响到季风爆发的过程。文章还比较了不同温湿地表条件下低层大气状态的差异和地表能量、水分平衡过程的不同,分析了陆气相互作用对季风活动产生影响的物理机制。     

Recent Advances in Dynamical Extra-Seasonal to Annual Climate Prediction at IAP／CAS

Recent advances in dynamical climate prediction at the Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP/CAS) during the last five years have been briefly described in this paper. Firstly,the second generation of the IAP dynamical climate prediction system (IAP DCP-II) has been described,and two sets of hindcast experiments of the summer rainfall anomalies over China for the periods of 1980-1994 with different versions of the IAP AGCM have been conducted. The comparison results show that the predictive skill of summer rainfall anomalies over China is improved with the improved IAP AGCM in which the surface albedo parameterization is modified. Furthermore, IAP DCP-II has been applied to the real-time prediction of summer rainfall anomalies over China since 1998, and the verification results show that IAP DCP-II can quite well capture the large scale patterns of the summer flood/drought situations over China during the last five years (1998-2002). Meanwhile, an investigation has demonstrated the importance of the atmospheric initial conditions on the seasonal climate prediction, along with studies on the influences from surface boundary conditions (e.g., land surface characteristics, sea surface temperature).Certain conclusions have been reached, such as, the initial atmospheric anomalies in spring may play an important role in the summer climate anomalies, and soil moisture anomalies in spring can also have a significant impact on the summer climate anomalies over East Asia. Finally, several practical techniques(e.g., ensemble technique, correction method, etc.), which lead to the increase of the prediction skill for summer rainfall anomalies over China, have also been illustrated. The paper concludes with a list of criticalre quirements needed for the further improvement of dynamical seasonal climate prediction.     

Regional climate simulation with a high resolution GCM: surface hydrology

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

欧亚土壤湿度异常对北半球大气环流的显著影响

用44 a ERA40再分析资料的土壤湿度和大气环流变量场, 研究持续性的欧亚大陆土壤湿度异常对后期北半球大尺度大气环流的反馈作用。首先,运用经验正交函数分解去除ENSO遥相关及趋势影响后,分析了欧亚大陆中高纬度土壤湿度变率主要模态的季节变化特征,及相对应主分量时间序列显示的土壤湿度异常的衰减时间,结果表明土壤湿度异常的主要模态在全年都表现出很好的连续性。其次,对不同季节的连续3个月的月平均土壤湿度和500 hPa高度场进行滞后最大协方差分析,研究欧亚地区中高纬度土壤湿度异常与北半球大气环流异常之间的线性耦合。第一最大协方差模态的结果表明:全年的主导信号是大气强迫土壤湿度的变化,但在冬季和夏季,大气中类似于负位相北极涛动的环流型与之前月份(最长达4个月)土壤湿度的持续变化显著相关。最后,基于土壤湿度变率中心的回归分析也证实了秋季和春季欧亚土壤湿度,特别是北非副热带,欧亚内陆和西伯利亚地区的土壤湿度异常,分别与其后的冬季和夏季的大气环流显著相关。欧亚大陆土壤湿度异常超前大气环流的信号,将有助于改善冬季和夏季北半球季节气候预报能力。     

山东半岛夏季降水异常的环流型及影响因子分析

利用1961—2016年华东地区106个气象观测站的日降水数据和再分析资料,分析引起山东半岛夏季降水异常的大气环流型及其与前期下垫面因子（海温和土壤湿度）的关系,结果发现：1）当孟加拉湾出现西南风异常,日本列岛以南和贝加尔湖西南侧地区分别呈反气旋和气旋式环流异常时,加强了向山东半岛的水汽输送,配合区域大气上升运动异常最终导致山东半岛夏季降水偏多;反之,当孟加拉湾出现西北风异常,日本列岛以南和贝加尔湖西南地区分别呈气旋和反气旋式环流异常时山东半岛降水偏少。2）孟加拉湾和北太平洋中部关键区的对流层整层位势高度与下垫面海温自春季持续至夏季存在显著正相关,当两个地区的整层位势高度均呈正异常时,分别对应夏季孟加拉湾的强西风气流和日本列岛以南的反气旋环流异常。3）区域土壤湿度异常引起的感热和潜热通量异常,可能是引起贝加尔湖关键区位势高度和山东半岛局地对流异常的原因：贝加尔湖西南地区土壤湿度偏大时,其上空对流层位势高度为负异常;山东半岛地区土壤湿度偏大时,其上空对流层大气出现异常上升运动。4）利用关键区春季下垫面因子（海温和土壤湿度）建立山东半岛夏季降水的统计预测模型,留一交叉检验的距平同号率达到75%。这些结果可为山东半岛夏季降水预测提供重要参考。     

陆面过程模式对土壤含水量初值的敏感性研究

利用IAP94陆面过程模式,采用淮河流域能量与水份循环试验(HUBEX)期间两种下垫面(森林和旱田)、不同季节(5月、8月和11月)的观测资料,研究了模式对土壤含水量初值的敏感性.结果表明:对于森林和旱田下垫面,当土壤含水量减少时,地表净辐射均略有减少,同时潜热通量减少而感热通量增加.另外,模式对土壤含水量初值的敏感性有较明显的季节差异,相对而言在晚春和夏季较强,而在秋季明显减弱.这说明春夏季节的土壤含水量初值在淮河流域区域气候的模拟和预测中尤其值得关注.     

Improved Representation of Land-surface Heterogeneity in a Non-hydrostatic Numerical Weather Prediction Model

This study focuses on the relevance of accurate surface parameters, in particular soil moisture, and of parameterizations for heterogeneous land surfaces, for the prediction of sensible and latent heat fluxes by a mesoscale weather forecast model with horizontal grid resolution of 7 km. The analysis is based on model integrations for a 30-day period, which are compared both to flux measurements obtained from the LITFASS-2003 field experiment and to high-resolution-model (1-km grid spacing) results. At first, the relevance of improved parameter sets and input data compared to usual operational practice for an accurate prediction of near-surface fluxes is shown and discussed. It is demonstrated that an observation-based land-surface assimilation scheme leads to an improved soil moisture analysis, which is shown to be essential for the realistic simulation of surface fluxes. Secondly, the implementation of two efficient parameterization strategies for subgrid-scale variability of the surface, the mosaic and the tile approach, is presented. Using these methods, the simulations are in better agreement with measurements than simulations with simple aggregation methods that use effective surface parameters. Integrations with the mosaic approach reproduce high resolution simulations very well and more accurately than simulations with the tile method. Finally, the high resolution simulations are analyzed to justify and discuss the approximations underlying both methods.     

Influence of soil moisture on the Sahelian climate prediction II

Summary The sensitivity of short-term climate to soil moisture parameterization has been investigated using the Florida State University Global Spectral Model. This is done through the implementation of a simple and realistic soil moisture availability in a prognostic equation for the surface temperature.The results from two seasonal simulations between 11 May and 31 August 1979 are discussed with an emphasis on the Sahelian regions. These experiments indicated that most of the significant changes that resulted from the new parameterization occurred at and near the surface. Above the surface, land surface effects were strongly diffused and for most of the cases were not conclusively discernible. At the surface, however, soil moisture appeared to be a parameter of large influence. Important modulations in the surface temperature were obtained. The time dependence of the soil moisture availability has led to an evolution of the surface heat sources in the model resulting in an apparent northward propagation of the centers of maximum temperature as the rainfall season progressed over the north African continent from May to August. This has had an important effect on the distribution of the primary variables and showed significant departures from the control.The interaction introduced by the new scheme between the physical processes parameterized in the model, has resulted in a differential modification of the rainfall distribution, and the changes in the hydrological cycle have remarkably affected the summer Sahelian rainfall.With 20 Figures     

Contribution of realistic soil moisture initial conditions to boreal summer climate predictability

Seasonal climate forecasts mainly rely on the atmospheric sensitivity to its lower boundary conditions and on their own predictability. Besides sea surface temperature (SST), soil moisture (SM) may be an additional source of climate predictability particularly during boreal summer in the mid-latitudes. In this work, we investigate the role of SM initial conditions on near-surface climate predictability during ten boreal summer seasons using three complementary ensembles of AMIP-type simulations performed with the Arpège-Climat atmospheric general circulation model. First we have conducted an assessment of the SM predictability itself through a comparison of simple empirical SM models with Arpège-Climat. The statistical and dynamical models reveal similar SM prediction skill patterns but the Arpège-Climat reaches higher scores suggesting that it is at least suitable to explore the influence of SM initialization on atmospheric predictability. Then we evaluate the relationships between SM predictability and some near surface atmospheric predictability. While SM initialization obviously improves the predictability of land surface evaporation, it has no systematic influence on the precipitation and near surface temperature skills. Nevertheless, the summer hindcast skill is clearly improved during specific years and over certain regions (mainly north America and eastern Europe in the Arpège-Climat model), when and where the SM forcing is sufficiently widespread and strong. In this case, a significant impact is also found on the occurrence of heat waves and heavy rains, whose predictability at the seasonal timescale is a crucial challenge for years to come.     

The sensitivity of the tropical circulation and Maritime Continent precipitation to climate model resolution

The dependence of the annual mean tropical precipitation on horizontal resolution is investigated in the atmospheric version of the Hadley Centre General Environment Model. Reducing the grid spacing from about 350 km to about 110 km improves the precipitation distribution in most of the tropics. In particular, characteristic dry biases over South and Southeast Asia including the Maritime Continent as well as wet biases over the western tropical oceans are reduced. The annual-mean precipitation bias is reduced by about one third over the Maritime Continent and the neighbouring ocean basins associated with it via the Walker circulation. Sensitivity experiments show that much of the improvement with resolution in the Maritime Continent region is due to the specification of better resolved surface boundary conditions (land fraction, soil and vegetation parameters) at the higher resolution. It is shown that in particular the formulation of the coastal tiling scheme may cause resolution sensitivity of the mean simulated climate. The improvement in the tropical mean precipitation in this region is not primarily associated with the better representation of orography at the higher resolution, nor with changes in the eddy transport of moisture. Sizeable sensitivity to changes in the surface fields may be one of the reasons for the large variation of the mean tropical precipitation distribution seen across climate models.     

A land data assimilation system using the MODIS-derived land data and its application to numerical weather prediction in East Asia

Land Data Assimilation Systems have been developed to generate the surface initial conditions such as soil moisture and temperature for better prediction of weather and climate. We have constructed Korea Land Data Assimilation System (KLDAS) based on an uncoupled land surface modeling framework that integrates high-resolution in-situ observation, satellite data, land surface information from the WRF Preprocessing System (WPS) and the MODIS land products over the East Asia. To present better surface conditions, the KLDAS is driven by atmospheric forcing data from the in-situ rainfall gauges and satellite. In this study, we 1) briefly introduce the KLDAS, 2) evaluate the meteorological states near the surface and the surface fluxes reproduced by the KLDAS against the in-situ observation, and then 3) examine the performance of the mesoscale model initialized by the KLDAS. We have generated a 5-year, 10 km, hourly atmospheric forcing dataset for use in KLDAS operating across East Asia. The KLDAS has effectively reproduced the observed patterns of soil moisture, soil temperature, and surface fluxes. Further scrutiny reveals that the numerical simulations incorporating the KLDAS outputs show better agreement in both the simulated near-surface conditions and rainfall distribution over the Korean Peninsula, compared to those without the KLDAS.     

Relevance of soil moisture for seasonal climate predictions: a preliminary study

In the framework of the Global Soil Wetness Project (GSWP), the ISBA land-surface scheme of the ARPEGE atmospheric general circulation model has been forced with meteorological observations and analyses in order to produce a two-year (1987–1988) soil moisture climatology at a 1^°×1^° horizontal resolution. This climatology is model dependent, but it is the climatology that the ARPEGE model would produce if its precipitation and radiative fluxes were perfectly simulated. In the present study, ensembles of seasonal simulations (March to September) have been performed for 1987 and 1988, in which the total soil water content simulated by ARPEGE is relaxed towards the GSWP climatology. The results indicate that the relaxation has a positive impact on both the model's climatology and the simulated interannual variability, thereby confirming the utility of the GSWP soil moisture data for prescribing initial or boundary conditions in comprehensive climate and numerical weather prediction models. They also demonstrate the relevance of soil moisture for achieving realistic simulations of the Northern Hemisphere summer climate. In order to get closer to the framework of seasonal predictions, additional experiments have been performed in which GSWP is only used for initialising soil moisture at the beginning of the summer season (the relaxation towards GSWP is removed on 1st June). The results show a limited improvement of the interannual variability, compared to the simulations initialised from the ARPEGE climatology. However, some regional patterns of the precipitation differences between 1987 and 1988 are better captured, suggesting that seasonal predictions can benefit from a better initialisation of soil moisture.     

Validating a regional climate model’s downscaling ability for East Asian summer monsoonal interannual variability

Performance of a regional climate model (RCM), WRF, for downscaling East Asian summer season climate is investigated based on 11-summer integrations associated with different climate conditions with reanalysis data as the lateral boundary conditions. It is found that while the RCM is essentially unable to improve large-scale circulation patterns in the upper troposphere for most years, it is able to simulate better lower-level meridional moisture transport in the East Asian summer monsoon. For precipitation downscaling, the RCM produces more realistic magnitude of the interannual variation in most areas of East Asia than that in the reanalysis. Furthermore, the RCM significantly improves the spatial pattern of summer rainfall over dry inland areas and mountainous areas, such as Mongolia and the Tibetan Plateau. Meanwhile, it reduces the wet bias over southeast China. Over Mongolia, however, the performance of precipitation downscaling strongly depends on the year: the WRF is skillful for normal and wet years, but not for dry years, which suggests that land surface processes play an important role in downscaling ability. Over the dry area of North China, the WRF shows the worst performance. Additional sensitivity experiments testing land effects in downscaling suggest the initial soil moisture condition and representation of land surface processes with different schemes are sources of uncertainty for precipitation downscaling. Correction of initial soil moisture using the climatology dataset from GSWP-2 is a useful approach to robustly reducing wet bias in inland areas as well as to improve spatial distribution of precipitation. Despite the improvement on RCM downscaling, regional analyses reveal that accurate simulation of precipitation over East China, where the precipitation pattern is strongly influenced by the activity of the Meiyu/Baiu rainfall band, is difficult. Since the location of the rainfall band is closely associated with both lower-level meridional moisture transport and upper-level circulation structures, it is necessary to have realistic upper-air circulation patterns in the RCM as well as lower-level moisture transport in order to improve the circulation-associated convective rainfall band in East Asia.     

陆面热力因子应用于中国夏季降水预测的初步试验

基于对中国东部夏季降水与欧亚大陆土壤温度和全球海表温度的相关分析,选取不同关键区的土壤温度和海表温度作为夏季降水的预测因子。利用1961-1990年的资料,分别以土壤温度作为第1组预测因子,海表温度作为第2组预测因子,综合海表温度与土壤温度因子作为第3组预测因子,使用改进的典型相关分析和集合典型相关分析法对中国东部夏季降水场进行预测,建立了相应的预测模型。然后,利用1991—2010年的资料进行了独立样本预测试验。在独立样本预测试验中,综合海表温度与土壤温度因子建立的模型比只用海表温度进行预测的各项预测评分高,说明加入土壤温度因子后预测效果有所提高。基于陆面热力因子的预测模型对夏季降水有一定的技巧,而综合海温与陆面热力因子的预测模型对中国东部夏季降水有较高的预测能力。     

Regional‐scale surface hydrologic simulations from global climate models: A case study

Abstract

Current understanding of the regional nature of global changes in the climato‐logical regime of the earth is limited. General circulation climate models (GCMs) cannot provide consistent and detailed information on the regional patterns of precipitation, soil moisture and runoff that are required by water resource planners. A case study is presented that couples a limited area model, with high spatial resolution and realistic land‐surface parametrization, to a global climate model. Results for July are presented for the continent of Australia.     

EFFECTS OF SPRING SOIL MOISTURE IN THE INDOCHINA PENINSULA ON SUMMER PRECIPITATION OVER THE SOUTH CHINA SEA AND ITS SURROUNDING AREAS: A SIMULATION STUDY FOR 1999

Using the regional climate model RegCM4.4.5, coupled with the land model CLM4.5, we investigated the effects of springtime soil moisture in the Indochina Peninsula on summer precipitation over the South China Sea and its surrounding areas in 1999. Results have indicated that there exists positive correlation between soil moisture and summer precipitation over the western Pacific Ocean and negative correlation between soil moisture and summer precipitation over the eastern Indian Ocean. Summer precipitation in the South China Sea and its surrounding areas responds to springtime soil moisture in the Indochina Peninsula (the northwest region is critical) because general atmospheric circulation is sensitive to the near-surface thermodynamic state. Increased (decreased) soil moisture would result in decreased (increased) local surface temperatures. Latitudinal, small-scale land–sea thermal differences would then result in northeasterly wind (southwesterly wind) anomalies in the upper layer and southwesterly wind (northeasterly wind) anomalies in the lower layer, which strengthen (weaken) monsoon development. As a result, precipitation would enter the Western Pacific region earlier (later), and water vapor over the eastern Indian Ocean would enter the South China Sea earlier (later), causing a precipitation reduction (increase) in the eastern Indian Ocean and increase (reduction) in the Western Pacific.     

东亚中纬度夏季陆面增暖与土壤湿度异常的联系

利用1979—2015年ERA-Interim再分析土壤湿度、云量资料,NCEP/NCAR再分析环流资料和CPC土壤湿度资料,分析了东亚中纬度夏季陆面热力异常的时空分布特征及其与前期春季土壤湿度异常的联系,探讨了前期春季土壤湿度影响东亚中纬度夏季陆面增暖的可能途径。结果表明,东亚中纬度夏季土壤表层温度呈全区一致增暖趋势,其中贝加尔湖及以南地区温度变化最剧烈、增暖最迅速,且1990年代中期前后存在一个明显由冷向暖的年代际转变。进一步分析发现,春、夏季西西伯利亚到贝加尔湖北部地区的土壤湿度与夏季贝加尔湖及以南地区的土壤表层温度在年代际和年际尺度上均存在紧密联系:西西伯利亚到贝加尔湖北部地区土壤湿度异常偏高,通常对应贝加尔湖及南部地区夏季土壤表层温度偏高。西西伯利亚到贝加尔湖北部地区春、夏土壤湿度异常可以引起夏季大气环流异常,从而对东亚夏季中纬度陆面热力异常产生影响:春、夏土壤湿度偏高时,贝加尔湖及其南部地区上空位势高度为正异常,对应为反气旋性异常环流,云量减少,有利于东亚中纬度陆面增暖;反之,则对应为气旋性异常环流,不利于陆面增暖。     

初值对中国东部初夏土壤湿度可预报性影响

土壤湿度是影响天气和气候非常重要的因子之一,但目前针对土壤湿度可预报性的研究报道相对较少。该文在对BCC_CSM模式进行了适合的陆面初始化的条件下,设计了两组在中国东部地区采用不同土壤湿度初值的回报试验研究该地区土壤湿度的可预报性及初值对其可预报性影响问题。试验结果表明:BCC_CSM模式在真实的外场强迫下可以模拟出相对合理的土壤湿度;土壤湿度的可预报性在表层约为3候,随着深度的增加,土壤湿度的可预报性持续时间增加,在中层预报性甚至能达到月尺度以上;初值对于土壤湿度的预报存在影响,在表层影响时间约为2~3候,影响时间随着深度增加;浅层土壤湿度受降水的影响较大,浅层土壤湿度变化滞后降水变化约1~2 d,中层土壤湿度变化与降水变化存在5 d左右的滞后关系。     

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。