Abstract:
Using data from Doppler weather radar, intensive automatic weather stations and NCEP reanalysis , we have analyzed the differences of environmental conditions in producing rainfalls in Guangxi by typhoon Kai-tak (1213) and Nesat (1117), which had similar tracks, focusing on their low-level environment fields, radar reflectivity and contribution of moist potential vorticity (MPV) etc. Results show that there is a strong southeasterly jet on the east side of Kai-tak, and a strong moist convergence center tends to be around the typhoon center where there is the intense rainfall, while the moisture convergence zone formed by typhoon Nesat mainly locates in the area where northeasterly was met by southeasterly, and rain fell in a broader area. There is a narrow and long strong mesoscale radar echo band in the back of typhoon Kai-tak and Nesat after they made landfall the northern Viet Nam. Although the spiral rain band around Nesat stretched over a wider range and lasted for a longer period. There were the multiple airflow branches near the ground around Nesat because of the intrusion of cold air that is more favorable to the formation of mesoscale vortex and airflow convergence zone, which is believed to be the main reason that leads to a broader area of severe precipitation in Nesat than in Kai-tak. The rain area induced by two typhoons well corresponds to the negative value centers of MPV. Their severe precipitation centers are situated where negative moist barotropc term VMPV1 is overlapped by positive baroclinic term VMPV2. Since the negative VMPV1 value in the rain zone generated by Kai-tak is smaller than that by Nesat, the rainfall caused by Kai-tak is stronger than that by Nesat.