ANALYSIS OF MICRO-SCALE CONVECTIVE SYSTEM IN TYPHOON "SEPAT"

A tornado occurred in Wenzhou Longgang village before the landing of typhoon SEPAT. The character of circulation background and process of its lifecycle were studied,using sounding,radar data,as well as numerical simulation. According to the synoptic background,the air of the region where tornado occurs was very unstable and bore with strong low level wind shear. According to Radar data,there had been weak meso-scale cyclone and weak low level wind shear,before tornado broke out. We also find two cyclones merged and became stronger when the tornado formed,which may be one of the important cause of the tornado. By numerical simulation we find strong low level wind shear and water vapor convergence,and environmental helicity and storm relative environmental helicity changed abruptly when tornado occurred.