Understanding mechanisms for trends in Sahelian squall lines: Roles of thermodynamics and shear
Understanding mechanisms for trends in Sahelian squall lines: Roles of thermodynamics and shear
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了解萨赫勒飑线趋势的机制：热力学和切变的作用
DOI：
10.1002/qj.3955
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发表时间：
2021
影响因子：
8.9
通讯作者：
Bickle M
中科院分区：
文献类型：
--
作者：
Bickle M
在西非萨赫勒地区，飑线主宰降雨，有证据表明，近几十年来，它们的强度有所增加。更强的风切变可能是这一趋势的关键驱动因素，并可能随着气候变化而继续加强。然而，全球数值模式很难捕捉到切变对有组织对流的作用，使得萨赫勒地区降雨强度变化的预测不确定。为了调查最近和未来可能的环境变化的影响，并隔离热力学效应的剪切效应，理想化的飑线模拟初始化与代表当今的配置文件：该配置文件，然后修改使用趋势再分析和气候预测。切变的增加导致风暴强度和降水量的增加，但热力学变化的影响主导切变的影响。用未来廓线进行的模拟产生了较短寿命的风暴，这可能是由于理想模式中对流抑制增加以及缺乏大尺度辐合或天气变率。基于对流不稳定空气和水分的相对流入的理论模型被发现可以准确地预测风暴的整体特征，包括平均降雨率和区域平均最大垂直速度，解释切变的作用。然而，该模型不是一个预测工具，因为降雨量取决于风暴速度，这仍然是一个自由参数。这项研究表明了切变对长期降雨趋势的重要性，并强调了气候模型需要包括切变的影响，以捕捉极端降雨的变化。
Squall lines dominate rainfall in the West African Sahel, and evidence suggests they have increased in intensity over recent decades. Stronger wind shear may be a key driver of this trend and could continue to strengthen with climate change. However, global numerical models struggle to capture the role of shear for organised convection, making predictions of changing rainfall intensities in the Sahel uncertain. To investigate the impact of recent and possible future environmental changes, and to isolate thermodynamic effects from shear effects, idealised squall line simulations were initialised with a profile representative of the present day: this profile was then modified using trends from reanalyses and climate projections. Increased shear led to increased storm intensity and rainfall, but the effects of the thermodynamic changes dominated the effects from shear. Simulations initiated with future profiles produced shorter‐lived storms, likely due to increased convective inhibition and the absence of large‐scale convergence or synoptic variability in the idealised model. A theoretical model based on the relative inflow of convectively unstable air and moisture was found to predict bulk characteristics of the storms accurately, including mean rain rates and area‐averaged maximum vertical velocities, explaining the role of shear. However, the model is not a prognostic tool as rainfall is dependent on the storm speed, which remains a free parameter. The study shows the importance of shear to long‐term rainfall trends and highlights the need for climate models to include effects of shear to capture changes in extreme rainfall.
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