了解2018年喀拉拉邦洪水：混合Rossby-重力波的作用
Understanding Kerala Floods of 2018: Role of Mixed Rossby-Gravity waves
摘要：
喀拉拉邦是印度西南部的沿海国，在2018年西南季风期间遭受了一系列洪水的蹂躏。该季节的特点是降雨量趋势严重异常，该州北部地区的日平均降雨量偏离高达150毫米。虽然有许多关于加剧喀拉拉邦洪水的水文地质因素的研究，但没有试图深入研究实际上导致这一年异常降水的物理学。这项研究旨在记录导致2018年喀拉拉邦洪水的动力学现象。2018年印度季风期间，热带太平洋700 hPa气压层的天气扰动激发了向西传播的对流耦合混合Rossby-Gravity（MRG）波。它们以两种重要的模式穿越印度洋-一种是周期为20 - 40天的主要慢波（如马登-朱利安振荡），另一种是周期为5 - 8天的次快波。它们的特点是垂直相位传播到对流层上部，是深对流和强降水的前兆。此外，这些波通过介质的传播增强了介质的相对渦度，因此形成的涡旋或环流关于赤道对称。因此，观察到风场的弯曲和热带辐合带的扩大。MRG波，特别是慢模式在风场中引起的辐散，助长了热带对流，并在2018年给喀拉拉邦带来了非常强的降雨。
英文摘要：
Kerala, the south-west coastal state of India, was ravaged by a series of floods during the South-West Monsoon of 2018. The season was marked by severely anomalous rainfall trends, with upto 150 mm of departures from the mean daily precipitation in the northern districts of the State. Although, there were many studies about the hydrogeological factors which aggravated the floods in Kerala, no attempt was made to delve into the physics which actually resulted in anomalous precipitation during the year. This study intends to document the dynamical phenomenon which caused the Kerala Floods of 2018. The westward propagating convectively-coupled Mixed Rossby-Gravity (MRG) waves were excited by the synoptic disturbances of the tropical Pacific at the pressure level of 700 hPa, during the Indian Monsoon of 2018. They travelled across the Indian Ocean in two significant modes -- a predominant slow moving wave of 20-40 days period (as Madden-Julian Oscillation) and a secondary faster wave of 5-8 days period. They are characterised by vertical phase propagation to the upper troposphere, a precursor to deep convection and intense precipitation. Further, the propagation of these waves through a medium enhances its relative vorticity and the gyres or circulations thus formed are symmetrical about the equator. Consequently, the meanders in the wind field and widening of the Intertropical Convergence Zone were observed. The MRG waves, especially the slow mode induced divergence in the wind field, which fueled convection in tropics and brought very heavy rainfall to the State of Kerala in 2018.
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