Effect of low-level eastward airflow on formation of severe precipitation on the east side of Fanjing Mountain

Using conventional observation data, intensive automatic weather station observation data, radar products, NCEP 1°×1° reanalysis data and other data, we have analyzed the four severe precipitation events on the east side of Fanjing Mountain in Tongren, Guizhou. These events were under the influence of low-level eastward airflow. Our analysis focuses on the formation mechanism of severe precipitation under the combined effect of low-level eastward airflow and topography, and established a conceptual model of severe precipitation on the east side of Fanjing Mountain under the influence of low-level eastward airflow. The main results are as follow. (1) The upper-level trough, the low-level shear line and the surface mesoscale convergence line are the key weather systems affecting the severe precipitation on the east side of Fanjing Mountain. (2) The shallow low-level eastward airflow plays a key role in the severe precipitation on the east side of Fanjing Mountain. When the u component of low-level airflow decreases with height, the ascending airflow at the windward slope terrain converges. When the v component of the airflow increases with height, a horizontal vortex tube being perpendicular to the orientation of mountains is generated at the windward slope terrain, where two vortex loops form due to the effect of topographic force. The vertical component of vorticity strengthens the updraft near the foot of the mountain in which is conducive to the generation of severe precipitation. (3) There are three mechanisms for the formation of the severe precipitation zone on the east side of Fanjing Mountain, that is, rainfall superposition effect formed for the convection that is trigged many times at the foot of the windward slope, organizational convection triggered by surface mesoscale convergence line itself, and "train effect" formed with the eastward motion of echo along surface mesoscale convergence line. The severe precipitation zones generated by the three mechanisms mentioned above are consistent with the orientation of surface mesoscale convergence line.