321 / 2024-09-14 09:52:59

The multiscale dynamics of the extremely devastating hourly rainfall on 20 July 2021 in Zhengzhou, China

multiscale interaciton,heavy rainstorm

An unprecedented record-breaking extreme hourly rainfall event (201.9 mm/h) occurred on 20 July, 2021, at the Zhengzhou Station, China. It coincided with the severe typhoon In-fa (No. 2106) thousands of kilometers away, which hence has been identified as the cause of the extreme event via moisture transportation. By observation the moisture transport was relatively uniform, while the rainfall was a highly localized one. This implies that there must be more profound dynamics lying behind, which we henceforth investigate here, using the functional analysis tool, multiscale window transform (MWT), and the theory of canonical transfer. The fields are first reconstructed onto three scale windows, namely, the synoptic-scale, meso-β-scale, and meso-γ-scale windows. The previously observed meso-β-scale vortex is reconfirmed here, which is generally believed to lead to the meso-γ-scale rainstorm. Here we find, for the first time, it is actually a supercell on the meso-γ-scale window that mainly accounts for the heavy rainfall. The supercell develops in a streamwise environment, with vertical vorticity and updraft coinciding. With such a configuration, previously it was believed no kinetic energy (KE) would exchange between supercells and their environments (Lilly, 1986), but here it is not the case. Beneath 700hpa, the canonical KE transfers from the synoptic and meso-β-scale windows make the dominant KE source of the supercell; but above 700hPa, the transfer direction is reversed, forming a KE sink. By calculation, the anisotropicity of the supercell wind accounts for the KE transfer. This differs our finding from the classical paradigm such as that in Lilly (1986), where an isotropic wind field was assumed.