Analysis on mesoscale system evolution and environmental field characteristics of a warm-sector heavy rainfall event in summer in the southeast of Fanjing Mountains

Abstract

Using routine upper-air and surface weather observations, satellite images, Doppler weather radar products, the observations from regional automatic weather stations and NCEP 6-hourly reanalysis data with 1°×1° resolution, we have conducted an analysis of the mesoscale system evolution and environmental field characteristics of a warm-sector heavy rainfall event occurred in the southeast of Fanjing Mountain from July 3 to 4 in 2016. Results are as follows. (1) This event, whose main influencing systems are westerly trough at 500 hPa and warm shear line at 850 hPa, occurred in the upper trough area to the northwest of the subtropical high, the south side of low-level warm shear, the left front of low-level jet and the divergence zone at 200 hPa. As a result, this event is a warm-sector extreme heavy rainfall event in Guizhou without the affecting of obvious cold air in the ground level. (2) This warm-sector heavy rainfall event is directly caused by four continuous convective cloud clusters, in which the severe precipitation occurred near the center of the convective cloud clusters and in the area of big values of TBB gradient at their back side. (3) The heavy rain was caused by the mixed precipitation echoes of cumuliform clouds. Favorable environmental conditions causing such a high efficient precipitation include warm clouds, deep wet layer, low-level abundant water vapor transport, abnormal low level of free convection (LFC) and level of condensation lifting (LCL), and "thin high" distribution of convective effective potential energy with moderate intensity. (4) The zones with great value of water vapor flux and the water vapor convergence centers are concentrated in southeast side of Fanjing Mountain because of the blocking effect of terrain. The easterly wind in the boundary layer turned south in front of the mountain and met the southerly airflow in the rainstorm area, forming an east-west stable mesoscale convergence line. The convections were triggered, merged, strengthened and then moved eastward near the convergence line, which is the important cause for the heavy rain forming. The meso-and micro-scale dynamical forcing by windward slope and the trumpet-shaped topography are favorable to the transport and uplift condensation of water vapor in the boundary layer.

Keywords

• warm-sector heavy rainfall
• mesoscale convective system
• warm shear line
• train effect
• terrain effect