Observation Characteristics and Synoptic Mechanisms of Typhoon Lekima Extreme Rainfall in 2019

Observation characteristics of extreme heavy rainfall with its thermodynamic structure evolution and water vapor transmission of super-typhoon Lekima in 2019 is comprehensively diagnosed and analyzed, in terms of automatic weather station data, FY-2G TBB and the microwave hygrometer channel inversion data of FY-3D MWHSⅡ, radar networking data and NCEP FNL 1° by 1° analysis data. Results show that heavy rain covers most parts of East China when Lekima heading north, its extreme rainfall (process rainfall amount more than 350 mm) occur in eastern Zhejiang and central Shandong, with the maximum rainfall point of 833 mm and 612 mm, respectively. The average rainfall of the whole province ranks first or second in the history of process rainfall in Zhejiang and Shandong, and daily rainfall values of 21 national stations are new historical records. The interaction of the typhoon, subtropical high, mid-latitude westerly trough and the abundant water vapor transport of the strong southeast low-level jet (over 25-35 m·s^-1 in speed) along the coast of East China provide favorable environmental conditions for the long-term maintenance of Lekima and the occurrence of extreme heavy rainfall as the typhoon northward. The extreme heavy rainfall in eastern Zhejiang is mainly caused by the development of powerful typhoon body, its deep vertical vortex system (over 60×10^-5 s^-1 in vorticity) and the strong upward movement breaking through the tropopause, as well as the dense deep convection system (TBB of -80--72 K) with high-efficiency rainfall and latent heat feedback in the eye-wall area of typhoon. The extreme heavy rainfall in the middle part of Shandong is closely related to the evolution of Lekima's asymmetric structure and the invasion of cold air during the typhoon heading northward. The extremity of rainfall comes from the combined action of the long-distance heavy rainfall originated from the easterly inverted trough and a long time "frontal properties" rainfall. The inverted trough frontogenesis, the convergence of southeast low-level jet and the easterly wind provide good dynamic and water vapor conditions for the long-distance rainstorm. Three main spiral rain belts in the north of the typhoon move anticlockwise, new convective systems constantly induce on warm side and merge in the inverted trough area, leading to train effect on the windward slope of terrain in central Shandong. With the continuous invasion of 500 hPa dry and cold air from the lower layer, a θ_se frontal zone inclining westward with height is formed near 118°E on the west side of typhoon. The warm and humid flow climb causes the second stage of long-time stable rainfall during Lekima's arrival in Shandong and the slow circle round in the Laizhou Bay.