Abstract In depth examination of the extreme precipitation over Kerala during 15–17th August 2018 – triggering major flood episode in the peninsular Indian state - have been made via the National Centre for Medium Range Weather Forecasting (NCMRWF) Unified Model. A rare alignment of an intense low pressure area (LPA) over head Bay of Bengal (BoB) and a couple of typhoons over South China Sea and West Pacific, driven by anomalous sea surface conditions along the south-east Asian coastal belts, kept the low level jet meander from central Arabian Sea to West North Pacific (WNP) and remain relentless throughout the episode. Positive velocity potentials (VP, at 850 hPa) were seen prevailing for the region and some of these VP fringes unevenly crossed over to Kerala - suggestive of an uneven wind convergence – and associated rainfall necessarily not dictated by orography alone. A single system of negative stream functions (SF) suggested the prevalence of a conveyor belt like flow – termed, the ‘Remotely Aligned Intense Tropical Circulations’ (RAITC) – helping bring ‘an additional supply of moisture’ from WNP to Kerala; the cyclones over the WNP provided large amounts of moisture to the upper air and while some of these cyclones were weakening, a part of this moisture got injected to the south-westward flow and was then propelled further by the BoB LPA. While the NCUM global forecasts could capture the general circulation pattern for the intense precipitation over Kerala in the medium range, the spatial distribution of the heavy spells couldn’t be captured well in day-7 forecast; the strength and movement of the BoB LPA as well as the alignment and intensity modulation of the multiple tropical circulations showed some deviations. The convection permitting regional version of the model, NCUM-R (4 km grid length), exhibited lesser frequency for the light rainfall events, while over-predicting the frequency for the extreme precipitations.

中文翻译：

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2018 年 8 月喀拉拉邦（印度）极端降水和洪水事件的 NWP 观点

摘要 通过国家中期天气预报中心 (NCMRWF) 统一模型，对 2018 年 8 月 15 日至 17 日期间喀拉拉邦的极端降水（引发印度半岛的重大洪水事件）进行了深入研究。受东南亚沿海地带异常海面条件的驱动，孟加拉湾 (BoB) 上空的强烈低压区 (LPA) 和南海和西太平洋上空的几场台风罕见地排列在一起，使从阿拉伯海中部到西北太平洋 (WNP) 的低空急流蜿蜒曲折，并在整个事件中保持无情。正速度电位 (VP, 在 850 hPa) 被视为该地区盛行，其中一些 VP 边缘不均匀地跨越到喀拉拉邦——暗示着不均匀的风汇聚——并且相关的降雨不一定仅由地形决定。单一的负流函数 (SF) 系统表明，像流动一样的传送带普遍存在——称为“远程对齐的强烈热带环流”(RAITC)——有助于将“额外的水分供应”从 WNP 带到喀拉拉邦；WNP 上空的气旋为高空提供了大量水分，虽然其中一些气旋正在减弱，但部分水分被注入西南气流，然后被 BoB LPA 进一步推动。虽然 NCUM 全球预报可以捕捉喀拉拉邦上空强降水的一般环流模式，但 第7天预报未能很好地捕捉到强法术的空间分布；BoB LPA 的强度和运动以及多个热带环流的排列和强度调制显示出一些偏差。模型的区域版本 NCUM-R（4 公里网格长度）对流允许区域版本的小雨事件的频率较低，同时高估了极端降水的频率。