Dynamical analysis of multi-scale interaction during the “21·7” persistent rainstorm in Henan

Abstract

This study explored the multiscale dynamical processes and energy contributions to the “21∙7” torrential rainfall event (TRE), which hit Zhengzhou on July 20, 2021. Using a localized, instantaneous energetics diagnostic tool: the multiscale window transform (MWT) and MWT-based energy and vorticity analysis (MS-EVA), three scales fields: basic-flow fields (>64 days), intraseasonal oscillation fields (8–64 days) and synoptic-scale-eddy fields (<8 days) were obtained. An inverted trough over “21∙7” TRE area on the scale of 8–64 days and a mesoscale high in front of Typhoon In-Fa (2021) on the scale of <8 days were identified by the MWT. There was persistent energy conversion from available potential energy (APE) to kinetic energy (KE) represented by the buoyancy term in the last 12 h and throughout the occurrence of the exceptional rainstorm. Latent heating was an important source of APE 12 h prior to the rainstorm. In addition to buoyancy conversion, a downscale energy cascade process of KE (barotropic instability) above 500 hPa was another source of KE prior to the rainstorm. During the rainstorm's occurrence, the downscale energy cascade process of APE (baroclinic instability) at middle levels was the major source of APE. KE decreased significantly owing to eddy dissipation and diffusion at middle levels and the divergence of KE fluxes at low levels. The work done by the pressure gradient force was negligible for KE.