The large scale circulation was favorable for tornado genesis with obvious wind shear and high moisture in the lower levels. The cold air extended to the south as the small trough stretched southward in the surface layer, and the unstable layer with high SWEAT index, high K index and low lifting condensation level (LCL) were necessary environmental conditions for tornado genesis. The bell shape terrain of Pearl River Delta region helped the convergence and local vortex around the surface to genesis.
The variables recorded by the surface automatic weather stations showed strong signals when the tornado passed by. The distribution of 3 s gust and 3 s minimum pressure was northwest-southeast, which was consistent with the tornado swath. The pressure and temperature dropped sharply while the wind speed increased dramatically when the tornado developed, and also the winds cyclonic convergenced around the tornado. The rainfall raised 5 to 10 min before the tornado arrived and the main rainfall zones located to the left of the tornado path indicated that the tornado was situated in the right-rear flank of the supercell. The pressure was higher and temperature was lower after the tornado left than that before the tornado came.
The Tornadic Vortex Signature (TVS), the hook echo and bounded weak echo region were detected when the tornado touched down. The heights of the TVS's top, base and centroid decreased as the storm strengthened and descende to the lowest when the tornado touched down on the ground. The wind shear was much higher than that of the detected mesocyclone, which also increased as well when the tornado touched down.
Key words：Severe tornado; damage survey; hook echo; Foshan
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