What Drives the Intensification of Mesoscale Convective Systems over the West African Sahel under Climate Change?
What Drives the Intensification of Mesoscale Convective Systems over the West African Sahel under Climate Change?
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DOI：
10.1175/jcli-d-19-0380.1
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发表时间：
2020-04-01
影响因子：
4.9
通讯作者：
Tucker, Simon
中科院分区：
文献类型：
--
作者：
Fitzpatrick, Rory G. J.;Parker, Douglas J.;Tucker, Simon
在气候变化下，极端降雨量预计将增加，带来潜在的社会经济风险。然而，增长的幅度还不确定。近几十年来，西非萨赫勒地区的极端风暴频率有所增加，垂直风切变被证明是一个原因。也观察到了更干燥的中层，更强的冷池，以及风暴组织的增加。全球模式没有捕捉到中低对流层风切变或冷池对风暴组织的潜在影响，因为它们将对流参数化。在这里，我们使用第一个允许对流的非洲气候变化模拟来了解热力学和风暴动力学的变化如何影响未来萨赫勒地区的极端降雨量。该模式模拟了到21世纪末与典型浓度路径8.5(RCP8.5)相关的变暖，预计MCS的极端降雨率将增加28%。萨赫勒平均水汽变化遵循Clausius-Clapeyron尺度，但具有区域异质性。降雨率与风暴时间总柱水(TCW)和风暴中垂直速度的乘积成比例。此外，风暴前风切变和对流有效势能都对风暴中的垂直速度有调制作用。在我们的模式中，虽然风切变影响云顶温度，但它与降雨率没有直接关系。在我们的模型中，预计未来TCW的增加是降雨率增加的主要解释。最后，尽管在未来的气候中模拟了更冷的冷池，但我们没有看到近地表风的显著变化，这突出了未来研究允许对流的风暴动力学模型的途径。
Extreme rainfall is expected to increase under climate change, carrying potential socioeconomic risks. However, the magnitude of increase is uncertain. Over recent decades, extreme storms over the West African Sahel have increased in frequency, with increased vertical wind shear shown to be a cause. Drier midlevels, stronger cold pools, and increased storm organization have also been observed. Global models do not capture the potential effects of lower- to midtropospheric wind shear or cold pools on storm organization since they parameterize convection. Here we use the first convection-permitting simulations of African climate change to understand how changes in thermodynamics and storm dynamics affect future extreme Sahelian rainfall. The model, which simulates warming associated with representative concentration pathway 8.5 (RCP8.5) until the end of the twenty-first century, projects a 28% increase of the extreme rain rate of MCSs. The Sahel moisture change on average follows Clausius-Clapeyron scaling, but has regional heterogeneity. Rain rates scale with the product of time-of-storm total column water (TCW) and in-storm vertical velocity. Additionally, prestorm wind shear and convective available potential energy both modulate in-storm vertical velocity. Although wind shear affects cloud-top temperatures within our model, it has no direct correlation with precipitation rates. In our model, projected future increase in TCW is the primary explanation for increased rain rates. Finally, although colder cold pools are modeled in the future climate, we see no significant change in near-surface winds, highlighting avenues for future research on convection-permitting modeling of storm dynamics.