A new perspective on the orographic effect of the windward slope on the multi-scale eastward-Moving southwest vortex systems

Abstract

The multi-scale convection activities are highly prone to induce the local severe rainstorms within Yangtze River Basin (YRB), and the orographic effect of the windward slope located within the Second-Step Terrain Region (SSTR) is widely confirmed to have significant roles in strengthening the multi-scale Eastward-Moving Southwest Vortex (EMSV) systems that belongs to a typical type of the convection activities, this paper is thereby focused on revealing the orographic effect of the windward slope on the multi-scale EMSV systems from a new perspective of Potential Vortex (PV) framework. Results show that the different synoptic circulations involved in the multi-scale EMSV systems can be effectively decomposed into the meso-scale balanced circulation and the local-scale perturbed circulation based on the Piecewise Potential Vortex Inversion (PPVI) technique. Then these different structure circulations demonstrate dissimilar modification under the topographical force of the windward slope within SSTR. The good agreements among the positive PV anomalies, the cyclonic perturbed circulation and the local heavy precipitation are explicitly discovered around the windward slope region, which suggests that the positive PV anomalies induced by the windward slope can reinforce the local precipitation via the regulation of the cyclonic perturbed circulation. Further investigation of the linkage between the windward slope and the positive PV anomalies reveals that the reinforcement of the latent heat release that is favorable for the genesis of the positive PV anomalies relies more on the strengthening of the updraft driven by the orographic lifting effect than the increase of the horizontal convergence of the low-level wind driven by the orographic blocking effect around the windward slope region.