Journal of Geophysical Research: Atmospheres A Mechanism for the Southward Propagation of Mesoscale Convective Systems Over the Bay of Bengal
Journal of Geophysical Research: Atmospheres A Mechanism for the Southward Propagation of Mesoscale Convective Systems Over the Bay of Bengal
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地球物理研究杂志：大气是孟加拉湾中尺度对流系统向南传播的机制
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通讯作者：
E. Stokoe
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作者：
Kevin A. Whitehead;E. Stokoe
孟加拉湾上空向赤道传播的降水事件在许多以前的观测研究中已有记载。建议的传播机制包括平均表面对流层中层风切变驱动的对流正交的低对流层风和重力流产生的出对流从对流开始的每日变化的陆地-海洋环流分散南部。在这项研究中，我们使用的天气研究和预报模式能够解决中尺度对流系统在南亚夏季风季节进行高分辨率模拟。结果表明，该中尺度系统具有飑线状结构，并伴有前导线/尾随层状结构。饱和下沉气流和跳跃上升气流形成的对流后部，显示了重力流状结构。尾流射流在传播方向上产生水平动量倾向。研究表明，对流中心的移动速度比对流层中层风快，并且与表面附近的后部入流急流的速度相同。这些系统也被证明是紧密耦合的昼夜陆面加热循环。我们进行额外的模式模拟不同的水平分辨率和积云参数化的列入。积云参数化模式不能模拟这一南传中尺度系统的上下气流对和重力流结构。我们发现，需要高分辨率的模式来解决上升气流和下沉气流对和积云参数化假设打破了这样高的分辨率。在模拟这些中尺度系统时，专门使用云微物理学变得至关重要。
Equatorward propagating precipitation episodes over the Bay of Bengal have been documented in many previous observational studies. Proposed propagation mechanisms include mean surface to midtropospheric wind shear driving the convection orthogonal to the lower tropospheric winds and the gravity currents generated by outflow from convection initiated by the diurnally varying land-ocean circulations dispersing south. In this study, we perform high-resolution simulations using the Weather Research and Forecast model capable of resolving mesoscale convective systems during the South Asian summer monsoon season. This mesoscale system is shown to have squall line-like structure with leading line/trailing stratiform. The rear inflow due to saturated downdraft and jump updraft indicates a gravity current-like structure. The rear inflow jet produces horizontal momentum tendencies in the direction of propagation. The center of convection is shown to move faster than the midtropospheric winds and at the same speed as that of the rear inflow jet near the surface. These systems are also shown to be tightly coupled to the diurnal land surface heating cycle. We perform additional model simulations with varying horizontal resolution and with the inclusion of cumulus parameterization. A model with cumulus parameterization is unable to simulate the updraft-downdraft pair and the gravity current structure of this southward propagating mesoscale system. We find that high model resolution is needed to resolve the updraft-downdraft pair and cumulus parameterization assumptions break down at such high resolutions. Using cloud microphysics exclusively becomes essential in simulating these mesoscale systems.