Numerical Investigation of the Impact of Vegetation Indices on the Variability of West African Summer Monsoon
Numerical Investigation of the Impact of Vegetation Indices on the Variability of West African Summer Monsoon
复制标题
植被指数对西非夏季风变化影响的数值研究
DOI：
--
复制
发表时间：
2007
期刊：
影响因子：
--
通讯作者：
I. Poccard
中科院分区：
文献类型：
--
作者：
Weiping Li;Weiping Li;Y. Xue;I. Poccard;I. Poccard
利用卫星产品和简化的简单生物圈陆面模式（SSiB）进行了一系列数值试验，研究了叶面积指数（LAI）和植被覆盖度（FVC）对1987和1988年西非夏季风的气候影响。SSiB以离线模式以及与国家环境预报中心环流模式（GCM）耦合模式运行。在控制和试验中分别采用了基于有限的地面调查和卫星遥感的LAI/FVC，与控制试验相比，GCM和卫星实测LAI/FVC的离线试验在15°N以南的热带西非地区都产生了较高的土壤湿度和较低的地表温度，最大偏差出现在12°N附近。这导致正纬向温度梯度最大值北移。在对流层低层相关的东风切变的结果在非洲东风急流和夏季降雨带在西非向北移动，这部分纠正了干燥的偏差在GCM控制运行。此外，GCM试运行模拟了一个相对较强的西非夏季风在1988年，与北移的非洲东风急流，一个更强的热带东风急流，和更多的降雨量比1987年，与观测一致。然而，由于卫星测得的LAI/FVC在1987年和1988年之间的差异很小，该模型未能产生降水的年际变化大的观察。还进行了水平衡分析，以研究影响降水变化的主导过程。确定了水汽通量辐合和地面蒸散的相对贡献。GCM和离线结果的比较表明，CEOP数据在非洲气候研究中有可能得到进一步的应用。
The climate impacts of leaf area index (LAI) and fractional vegetation cover (FVC) on the West African summer monsoon in 1987 and 1988 were investigated through a series of numerical experiments conducted with satellite products and the Simplified Simple Biosphere land surface model (SSiB). The SSiB was run in the offIine mode as well as in the coupled mode with the National Centers for Environmental Prediction General Circulation Model (GCM). Prescribed monthly LAI and FVC from a look-up table based on limited ground surveys and those derived from satellite remote sensing were employed in the control and test runs, respectively.Compared with the control runs, both the GCM and offline test runs with satellite measured LAI/FVC produce higher soil moisture and lower surface temperature in tropical West Africa to the south of about 15°N, with the maximum deviations being located around 12°N. This leads to the northward shift of the maximum of positive latitudinal temperature gradient. The associated easterly shear in the lower troposphere results in the African easterly jet and summer rainfall band in West Africa shifting to the north, which partially corrects the dry bias in the GCM control runs. In addition, the GCM test run simulates a relatively stronger West African summer monsoon in 1988, with a northward shifted African easterlyjet, a stronger tropical easterlyjet, and more rainfall than in 1987, consistent with observations. However, due to small differences in satellite measured LAI/FVC between 1987 and 1988, the model fails to produce interannual variations of precipitation as large as seen in the observation. Water balance analysis has also been carried out to investigate the dominant processes affecting the changes in precipitation. The relative contributions of moisture flux convergence and surface evapotranspiration are identified. The comparisons between GCM and offline results in this study demonstrate a possible future application of CEOP data in African climate study.