Cause analysis of a torrential rain event induced by residual vortex
from super severe typhoon Utor in Guangxi

Based on conventional meteorological observation data, NCEP 1°×1° reanalysis data, reanalysis data with resolution 0.75°×0.75° from ECMWF as well as observations from automatic weather stations and Doppler weather radar, we analyze the cause of a torrential rain in Guangxi induced by residual vortex from super severe typhoon Utor (1311) by dynamic and thermodynamic diagnosis. The results show that (1) the encirclement situation formed by continental high, subtropical high over the west Pacific and equatorial high and, the reinforcement and northward movement of the tropical convergence zone, resulted in the residual vortex from Utor moving slowly in the western South China first northward and then southward. (2) Weak vertical wind shear and the long duration of connection between monsoon surge and the residual vortex are favorable to moisture and unstable energy supplementing to the Utor residual vortex, which lead to a long continuance of the vortex. (3) When the residual vortex moved southward at the later stage, it encountered with monsoon surge to obtain latent heat, which is the main reason of the vortex resuscitated and precipitation increased. (4) Analysis of radar data showed that at the southeast of the residual vortex the storm cells formed a shape of helical ribbon and caused the train effect.