Moist Available Potential Energy, Multiscale Energetics and the Multiscale Dynamics Governing an Extremely Devastating Rainstorm

Abstract

An unprecedented record‐breaking extreme hourly rainfall event (201.9 mm/hr) occurred on 20 July 2021, at the Zhengzhou Station, China. In previous studies, a meso‐‐scale convective storm is found to be the direct process causing the rainfall; also found is the impact of processes on other scales (synoptic‐ and meso‐scales), in enhancing moisture supplement, wind convergence, vertical wind shear, etc. How these processes, which lie on distinct scales, may interact, and/or, how the interaction may play a role, remain elusive. Motivated by the seminal theoretical work of Lilly (1986, https://doi.org/10.1175/1520‐0469(1986)043<0113:tseapo>2.0.co;2 ), we hence explore this aspect of the storm dynamics, using the functional analysis tool, multiscale window transform (MWT), and the theory of canonical transfer. We first extend the MWT‐based multiscale energetics to an atmosphere with moisture so as to deal with processes like rainstorm, then apply MWT to reconstruct the fields onto three scale windows, namely, the synoptic‐scale, meso‐ β ‐scale, and meso‐ γ ‐scale windows. The reconstructed meso‐‐scale process is shown to characterize the storm. Contribution to its growth from other scales is hence measured by the canonical energy transfers from the ambient. It is found that, beneath 700 hPa, locally the system is dominated by a primary barotropic instability and a secondary barotropic instability: both the synoptic and meso‐ β ‐scale processes fuel the storm through canonical kinetic energy (KE) transfers. But above 700 hPa, the transfer direction is reversed, forming a KE sink. By calculation, the anisotropicity of the storm wind accounts for the reversal. In comparison to previous studies, we highlight the importance of the energy extracted from the ambient through barotropic instabilities, which contribute directly to the record‐breaking rainfall in the lower layer.