Mechanisms of Short-Duration Heavy Rainfall in the Western Pacific Subtropical High Area: An Analysis of Two Rainstorms of 2018 in Hunan Province, China

This study examines two short-duration heavy rainfall cases of 2018 in Hunan province, China, using observations and the NCEP FNL analyses. Short-duration heavy rainfall occurring in the western Pacific subtropical high (WPSH) area is one of the major natural hazards during spring and summer over the middle reaches of the Yangtze River basin. Rainstorms in the two cases selected showed high echo-top height but low storm centroid elevation, resembling the "low-echo centroid" structure. Weak vertical shear was observed in the mid-lower troposphere, thereby leading to a short life cycle of storms. Before the heavy rainfall events, in addition to substantial changes in convective available potential energy (CAPE) and convective inhibition (CIN), both cases were characterized by a lack of troughs and jet streams in the middle and upper troposphere over the middle reaches of the Yangtze River basin. A proper configuration of the WPSH extending to the middle reaches of the Yangtze River basin along with low-level synoptic systems (e.g. a surface convergence line or the cold air invading) favoured moisture supply and lifting mechanisms for convection and the rainstorms. During the hot afternoon hours, the thermals forming at the surface further enhanced convection and the rainstorms.