Large-scale background and maintenance mechanism of the extreme rainfall in summer 2020 over East Asian

A severe flooding hit central-eastern China along the Yangtze River to southern of Japan in summer 2020. The floods were induced by extreme rains, and the associated dynamic and thermodynamic conditions are investigated. The large-scale southwestward-moving anomalous quasi-biweekly (QBWO) circulation was essentially the fluctuation of cold air mass related to the tropospheric polar vortex over the mid-high latitude Eurasian in boreal summer. The southwestward-moving of cold air mass from mid-high latitudes converged on the northeastward-transporting of warm and moist air by the strong anomalous anticyclone over the western North Pacific, providing important large-scale circulation support for the extreme rainfall. Eastward zonal transport of a large amount of water vapor from the Bay of Bengal and Indo-China and northward transport from the South China Sea provided the background moisture supply. The quasi-biweekly anomalies of potential of vertically integrated water vapor flux played an important role in maintaining the subseasonal variability of rainfall. The diagnosis of moisture tendency budget shows that the enhanced moisture was primarily attributed to the moisture convergence. Further analysis of time-scale decomposition indicates that the convergence of background mean specific humidity by the QBWO flow played dominant roles in contributing to the positive moisture tendency. In combination with adiabatic ascent over the rainfall region induced by the warm temperature advection, the boundary layer moisture convergence strengthened the upward transport of water vapor to moisten the middle troposphere, favoring the persistence of the extreme rainfall. The vertical moisture transport associated with boundary layer convergence was of critical importance in causing low-level tropospheric moistening. By comparison, the horizontal moisture advection played a secondary important role in maintaining the quasi-biweekly oscillation of extreme rainfall in 2020.