Changes in Sahel Land-Surface Characteristics and Climate Variation

9706403雪萨赫勒地区有两种截然不同的水文现象：持续干旱，这很可能部分是由地表变化引起的；地表水平衡有显著的季节性变化。将进行NCEP GCM与生物圈模式(SSIB)的耦合模拟，该模式已经根据HAPEX和SEBEX数据进行了校准和验证，以调查地表条件对萨赫勒地区水文过程和区域气候变异性的影响，特别是土地覆盖对萨赫勒长期干旱模式的影响。模拟将使用来自实地测量和卫星观测的土地属性数据和土地覆盖数据，以评估区域和当地湿度来源在确定降雨量方面的综合情况。将探讨植被分布的异质性对陆地-大气相互作用的影响。随后将开发一种方法，将地表参数聚合到GCM网格框或次网格框中。这项研究将集中于土地条件如何改变降雨量、ET、土壤水分和径流的季节模式。结果将通过与观测到的降雨量、地表温度，特别是河流流量的比较来证实。河流径流模型将用于将陆面模型在GCM的每个网格单元中产生的径流值转换为流域出水口。将进行诊断分析，以确定萨赫勒降水的来源，以及每个蒸发源对萨赫勒降水及其异常的相对贡献。还将注意研究陆地表面过程对AFJ动力学和波传播的影响。陆面效应的重要性将针对不同的陆地强迫异常模式进行测试，其中包括不同程度的陆地表面退化和一个GCM格子框中不同百分比的植被覆盖变化。

9706403 Xue There are two distinct hydrological phenomena in the Sahel: a persistent drought, which is very likely to have been partially caused by the land surface change, and a significant seasonal variation in the surface water balance. Simulations that couple a NCEP GCM with a biosphere model (SSiB), which has been calibrated and validated against HAPEX and SEBEX data, will be conducted to investigate influences of land-surface conditions on hydrological processes and regional climate variability in the Sahel, especially the impact of land cover on long-term Sahel drought patters. The simulations will use land property data and land cover data from field measurements and satellite observations to evaluate the integration between regional and local moisture sources in determining rainfall. The impacts of heterogeneity of vegetation distribution on land-atmosphere interactions will be explored. It will be followed by the development of a methodology to aggregate land-surface parameters to GCM grid boxes or sub-grid box. The study will focus on how land condition changes seasonal patterns in rainfall, ET, Soil water, and runoff. The results will be confirmed through comparisons with observed changing rainfall, surface temperature, and in particular, the river discharge. A river routing model will be used to convert the runoff values produced by the land surface model in each grid cell of a GCM to the basin outlet. The diagnostic analyses will be made to determine sources of Sahel precipitation, and relative contributions of each evaporation source to the rainfall and its anomalies in the Sahel. Attention will also be paid to study the impacts of land surface processes on AFJ dynamics and wave propagation. The significance of the land surface effects will be tested with respect to various patterns of land forcing anomalies, which include different degrees of land surface degradation and different percentage of vegetation cover changes in one GCM grid box.