Simulation of vegetation feedbacks on local and regional scale precipitation in West Africa
Simulation of vegetation feedbacks on local and regional scale precipitation in West Africa
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DOI：
10.1016/j.agrformet.2016.03.001
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发表时间：
2016-05-28
影响因子：
6.2
通讯作者：
Webster, Stuart
中科院分区：
文献类型：
--
作者：
Hartley, Andrew J.;Parker, Douglas J.;Webster, Stuart
已提议计划改变西非的土地利用，以防治荒漠化并保护该地区的生物多样性，然而，迫切需要工具来评估这些拟议的变化对当地和区域规模降水的影响。我们使用高分辨率、允许对流的数值天气预报（NWP）模型来研究中尺度对流系统（MCS）的启动和传播如何取决于地表植被覆盖。模拟涵盖了 2006 年 8 月西非季风期间为期 4 天的时间。在模拟的许多方面，有证据表明植被类型对降水位置产生重大影响，而地形和沿海水域的影响最小。在本研究中，植被根据树木（>30%）和草（>30%）植物功能类型的覆盖率进行分类。树-草边界覆盖被定义为在移动的 3 x 3 窗口中出现超过 3 个树和草的网格单元，并使用 3 个网格单元（类似于 12 km）缓冲区进一步放大。我们发现，在整个研究区域（北纬 5 度至北纬 17 度，西经 11 度至东经 9 度），33.8% 的对流起始发生在仅覆盖 28.4% 陆地表面的树草边界上。这明显高于偶然预期 (p = 0.0483)，为植被梯度提供热量和湿度梯度（其大小与土壤湿度相似）的假设提供了支持。此外，我们发现，平均而言，在下午和晚上期间，MCS 下的时间更多地发生在边界覆盖和地形上，其次是树木覆盖，从而支持了土地覆盖类型影响较大传播系统位置的假设。小规模对流单元和大规模 MCS 之间的降水量存在对比模式。平均而言，下午期间草地覆盖范围内积累了更多小规模降水，这表明在边界上方发起的小规模对流倾向于在下午期间偏向植被边界更干燥、更温暖的草地一侧。然而，一旦这些较小规模的对流单元合并在一起形成更大的 MCS，就会观察到最强烈的降水会落在树木覆盖范围内。当强降水（> 10毫米/小时）同时发生在树木、边界和草覆盖物上时，我们发现最高降水率最常见于树木覆盖物（48.4％），最不常见于边界覆盖物（19.9％），表明MCS偏好凉爽、湿润的森林覆盖物。这些结果首次表明，允许对流的 NWP 模型确实表现出与现实世界中观察到的类似的植被响应，因此是评估拟议的未来土地利用变化影响的有用工具。皇冠版权所有 (C) 2016 由 Elsevier B.V. 出版。保留所有权利。
Planned changes to land use in West Africa have been proposed to both combat desertification and to preserve biodiversity in the region, however, there is an urgent need for tools to assess the effects of these proposed changes on local and regional scale precipitation. We use a high-resolution, convection permitting numerical weather prediction (NWP) model to study how the initiation and propagation of mesoscale convective systems (MCS) depends on the surface vegetation cover. The simulations covered a 4-day period during the West African monsoon in August 2006. In many aspects of the simulations, there was evidence of vegetation type exerting a significant influence on the location of precipitation where the influence of orography and coastal water was minimal. In this study, vegetation was classified according to the fractional coverage of tree (>30%) and grass (>30%) plant functional types. Tree-grass boundary cover was defined where more than 3 grid cells of both tree and grass occurred in a moving 3 x 3 window, which was further enlarged using a 3 grid cell (similar to 12 km) buffer. We found that over the whole study region (5N to 17N and 11W to 9E) 33.8% of convective initiations occur over tree-grass boundaries that cover only 28.4% of the land surface. This is significantly more than would be expected by chance (p = 0.0483), providing support to the hypothesis that vegetation gradients provide heat and moisture gradients, of a similar magnitude to that of soil moisture. Additionally, we found that on average, more time under an MCS occurred over boundary cover and orography, followed by tree cover, during the afternoon and evening period, thus supporting the hypothesis that land cover type influences the location of larger propagating systems. Contrasting patterns were found in the quantity of precipitation between smallscale convective cells and larger scale MCS. More small-scale precipitation accumulated, on average, over grass cover during the afternoon period, indicating a tendency for small-scale convection, initiated over boundaries, to prefer the drier and warmer grass side of vegetation boundaries in the afternoon period. However, once these smaller scale convective cells merge together to form larger MCS, a tendency for the most intense precipitation to fall over tree cover was observed. When intense precipitation (>10 mm per hour) occurred simultaneously over tree, boundary and grass cover, we found the highest precipitation rate to be most frequently over tree cover (48.4%), and least frequently over boundary cover (19.9%), indicating a preference of MCS for cooler, more moist forest cover. These results show for the first time that convection-permitting NWP models do exhibit responses to vegetation similar to those observed in the real world, and therefore are useful tools to assess the impacts of proposed future land use changes. Crown Copyright (C) 2016 Published by Elsevier B.V. All rights reserved.