Comparison of kinetic energy conversion characteristics of two extreme precipitation episodes during persistent unusually heavy rainfall on 17–23 July 2021 in Henan, China

Abstract

A persistent heavy rainfall event, contributed by two extreme episodes (P1 and P2), occurred in Henan province of China from 17 to 23 July 2021. An active mesoscale vortex was present in the lower troposphere, along with a shallow trough moving eastward in the upper troposphere. The budgets of rotational kinetic energy (KR) and divergent kinetic energy (KD) at distinct stages are investigated. The lower-level KR anomaly, related to the mesoscale vortex growth therein, was responsible for P1. However, the upper-troposphere KR was dominant and strong during the P2 stage, associated with an eastward-developing upper-level trough. Prior to both P1 and P2, active inner interaction between rotational and divergent winds presented, manifested as a rapid increase of KVRVD signals. KVRVD and precipitation had similar pattern distribution and consistent evolution tendency. Therefore, KVRVD might be a useful indicator for the occurrence and intensification of rainfall. Expect for two components of winds interaction, the external source feeding kinetic energy growth is also explored. For P1, APE (available potential energy)-> KD-> KR was the key path towards rotational kinetic energy, providing energy for mesoscale vortex growth in lower troposphere, which was favorable for the extreme episode of P1. Nevertheless, for P2 stage, APE acted as a direct energy source towards KR in upper troposphere, with higher efficiency (via APE-> KR) and thus stronger KR. This kind of difference in conversion path depended on the vertically-coupled degree between the lower-level mesoscale vortex and the upper-level trough, in addition to the rotational and divergent wind speed. Their tilting/upright configuration produced strong baroclinic/barotropic process, accelerating energy conversion of APE-> KD/APE-> KR via GD/GR (i.e., generating KD/KR by divergent/rotational winds crossing geopotential height contours, usually as a generator resulting from barotropic/baroclinic process). Therefore, kinetic energy budget may be a valuable tool for identifying key weather systems development and forecasting extreme rainfall episodes.