Changes in Sahel Land-Surface Characteristics and Climate Variation

在萨赫勒地区有两种不同的水文现象：一种是持续的干旱，这很可能部分是由陆地表面变化引起的，另一种是地表水平衡的显著季节性变化。NCEP GCM与生物圈模型（SSiB）的耦合模拟（该模型已根据HAPEX和SEBEX数据进行校准和验证）将研究萨赫勒地区陆地表面条件对水文过程和区域气候变率的影响，特别是土地覆盖对萨赫勒地区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来评估区域和当地水分来源在确定降雨量方面的整合。探讨植被分布异质性对陆-气相互作用的影响。随后将开发一种方法，将陆地表面参数聚合到GCM网格盒或子网格盒中。该研究将重点关注土地状况如何改变降雨、ET、土壤水分和径流的季节性模式。结果将通过与观测到的变化的降雨量、地表温度，特别是河流流量的比较来证实。河流路径模型将用于将地表模型在GCM的每个网格单元中产生的径流值转换为流域出口。将进行诊断分析，以确定萨赫勒降水的来源，以及每个蒸发来源对降雨的相对贡献及其在萨赫勒的异常情况。此外，还将重点研究地表过程对AFJ动力学和波传播的影响。在不同的土地强迫异常模式下，包括不同程度的地表退化和不同百分比的植被覆盖变化，我们将对地表效应的意义进行检验。

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.