Comparative analysis of potential vorticity between persistent rainstorm and extreme intense rainfall events during the Yangtze-Huaihe Meiyu period

In this study, based on the rainfall measurements from weather stations over China and atmospheric reanalysis products from the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) during the period of 1979-2020, the dynamic mechanisms and differences of persistent rainstorm (PRS) events and extreme intense rainfall (EIR) events over the Yangtze-Huaihe Meiyu domain (YMD) are revealed from the perspective of potential vorticity (PV)-forced vertical motion. According to the improved definitions of PRS and EIR events, 24 PRS events and 24 EIR cases are identified over the YMD during the Meiyu period from 1979 to 2020. Composite analyses for the two types of events demonstrate that the most intense rainband of PRS events is mainly located in the Yangtze River and over the southern regions of it, while for the EIR, the most intense rainband is located in the Yangtze River and over the northern regions of it. The PRS events are found to be closely related to tropical atmospheric intraseasonal oscillation, during which the upper-tropospheric South Asian high extends more eastward, while the northwestern Pacific subtropical anticyclone in the lower and middle troposphere shifts more westward. Thus, the dry and cold air with high-PV around the upper-tropospheric westerly jet located more southward latitudes tends to intrude equatorward and downward, converging with the warm and moist air from the southwest in the lower and middle troposphere to form Meiyu front. However, the EIR events are more dependent to a greater extent on the upper-tropospheric divergence on the southern side of the westerly jet located more northward latitudes and PV-forced downward-intruding cold air. The quantitative diagnoses of PV budget for EIR events show that before and during the peak of intense rainfall, the net negative PV tendency in the upper troposphere is mainly dominated by the negative vertical PV advection, while the positive PV tendency in the middle and lower troposphere is mainly caused by the PV generation due to the vertically non-uniform diabatic heating and vertical PV advection. The vertical velocity decomposition of a typical EIR event further demonstrates that the component of ascending velocity forced by the vertical increase of horizontal PV advection plays an important role in triggering the EIR event.