Abstract:Based on the data of conventional observation, automatic weather station and Doppler weather radar, this paper analyzes the activity characteristics and environmental condition of the typhoon-tornadoes in the Pearl River Delta. The results show that tornadoes occur from June to October and the time is concentrated in the period from 10:00 BT to 20:00 BT. Tornadoes occur between 1.3 h and 21.3 h after typhoon landfall. A strong preference for tornado occurrence is in the northeast quadrant with respect to land-falling typhoon center. High risk periods of tornado genesis in the Pearl River Delta correspond to the typhoon center locating between Zhanjiang of Guangdong and southeast Guangxi or Beibu Gulf. The synoptic situation of upper-level divergence, low-level convergence, and superimposition of strong southeasterly jets at mid- and low-layer over the Pearl River Delta are conducive to the weather background of tornadoes. The common environmental conditions for strong or weak tornado genesis appear to be low LCL, strong deep-layer and low-level vertical wind shear (VWS) and high storm relative helicity (SRH). The major difference between weak and strong tornado cases is that the latter has stronger deep-layer and low-level VWS and greater SRH. The significant diversities are VWS at 0-1 km and SRH between tornadic and nontornadic environmental conditions under the similar typhoon tracks. With the higher values of VWS and SRH, the possibility of supercells and mesocyclones increases, thus tornadoes are most likely to be detected. Tornado storms are mini supercell storms with low centriod. Strong or moderate mesocyclones are likely to be detected through the radial velocity data of the CINRAD/SA radar at low level, and significant tornadic vortex signature (TVS) may be found in the center of mesocyclones. The tornadoes lie in the interior of the hook echo or near TVS. Compared with the supercell tornadoes in the westerlies, mesocyclones of typhoon tornadoes are smaller in scale and lower in stretching height.