A 10-Year Statistic Study on the Tornadoes That Occurred in Jiangsu and Zhejiang Province: Composite Background Environment and Linear Trends

According to a 10-year (from 2007 to 2016) statistical analysis on the tornadoes in China, Jiangsu Province and Zhejiang Province, which share many similar geographical characteristics, experienced 159 (ranked first) and 59 (ranked fifth) tornadoes, respectively. Tornadoes within Jiangsu [i.e., the Jiangsu type (JST)] and those within Zhejiang [i.e., the Zhejiang type (ZJT)] featured notable annual, monthly and diurnal variations, which were remarkably different from each other. Both JST and ZJT showed the largest occurrence frequency (~50% on average) in the afternoon of summer, and the background environments before the formation of these tornadoes were composited to focus on their respective universal features. For the JST, it is found that, the upper-tropospheric divergence and positive geopotential-height anomaly, the middle-tropospheric shortwave trough, warm temperature advection and positive temperature anomaly, and the lower-tropospheric strong southwesterly wind, convergence and cyclonic vorticity all acted as favorable conditions for the convective activities within Jiangsu. For the ZJT, their background environment differed from that of the Jiangsu notably, and the conditions for the tornadoes’ formation were overall more favorable for the JST. Linear trend analyses indicated that, both the annual and seasonal (summer) occurrence of the JST tended to decrease significantly, whereas, those of the ZJT were not significant. For the JST, its occurrence frequency showed a low linear correlation to the variation of the surface temperature, instead, its decreasing frequency was more closely related to the significant weakening in the intensity of the vertical shear of the zonal wind and the updraft helicity within Jiangsu.