Two types of Tibetan Plateau vortex genesis in June and the associated mechanisms

Abstract

The Tibetan Plateau vortex (TPV) is a key system triggering rainfall over the Tibetan Plateau (TP) during the boreal summer. The TPV genesis mechanisms are complicated and its classification is a great challenge. This study attempts to elucidate these aspects. By introducing the standardized index of 24-h increment of equivalent potential temperature Delta_{24h} theta_{{text{e}}} at 500 hPa, all the TPV cases generated in June between 1980 and 2016 are classified as either positive or negative. Composite analysis is subsequently applied to the extremes of these two types, i.e., the first and last fifth-percentile cases with extremely negative and positive standardized Delta_{24h} theta_{{text{e}}}, respectively. Results indicate that 70% of them occur in relatively warmer and wetter environments, with diabatic heating dominating the positive type and the dynamic effect of large-scale circulation dominating the negative type. For the extremely positive cluster, the geopotential-height increment over the TP exhibits a negative/west–positive/east dipole, which enhances the southerly flow over the western TP, while forming surface water–vapor convergence. Consequently, strong condensation heating occurs near the sub-cloud level, resulting in the development of potential vorticity below and eventually TPV genesis. For the negative cluster, local shear lines at 500 hPa and upstream troughs at 250 hPa occur at the TPV genesis location. In conjunction with anomalous westerlies, positive potential vorticity is generated in situ due to zonal advection. The retardation caused by the Kunlun Mountains on the impinging westerly flow associated with side-boundary friction also contributes to TPV genesis southeast of the mountains.