Characteristics Analysis of Mesoscale Convective System Causing the Extreme Flash Rain in Yichang
Based on Doppler radar data, automatic weather station data and conventional observation data, the environmental conditions and mesoscale convective system (MCS) in Yichang from 2016 to 2017 were analyzed. The results showed that there are three synoptic situation configurations of extreme flash rain (EFR): baroclinic frontogenesis category, quasi-barotropic category and low-level warm advection forcing category. In the baroclinic frontogenesis environment, the cold front is slower in the central and west of Yichang. The heavy updraft caused by the front, which is stronger, combined with the warm and humid airflow makes MCS high and thick. The strong vertical shear causes hanging structure of the cell. The instantaneous rainfall intensity is stronger and the duration of convection is longer when the airflows are converged and blocked by the terrain, resulting in intermittent and dispersive EFR. The quasi-barotropic type Ⅰ occurs at the inner edge of the subtropical high. The center of convergence formed by the topography with developing of the southerly in the saddle field triggers and strengthens the convective cell, causing heavy instantaneous rain intensity. Extremely, the weak steering flow and the backward propagation effect of the weakening cell in the piedmont area of the downwind side cause the convection to be streng-thened again in the valley area in front of the mountain, resulting in EFR with short time, very small range and heavy rain intensity. The quasi-barotropic type Ⅱ occurs at the convergence of the warm moist easterly and northerly flow of the boundary layer while the easterly wave moves to west, resulting in heavy precipitation tower deep echoes with ultra-low center of mass. The topographical transition zone on the east side of the mountain merges the northerly wind and the easterly wind several times, causing the EFR with relatively small intensity, larger area and lasting several times. In the warm advection forcing environment, a convergence line is formed with the strengthening of southwestern jet. The deep and strong echoes inclined downstream are arranged along the convergence line. The shear line and convergence line are in accord with the trend of rain band and the “train-effect” appears on the convection line, causing the cell to be regenerated and strengthened, moving downstream in the upstream of the convection line. So the line EFR with discontinuity distribution appears some times in a row on the convection line.