Tornadoes in Chiba Prefecture on 11 December 1990

Abstract

On the evening of 11 December 1990, two supercell storms hit the Chiba Prefecture, southeast of Tokyo, and spawned two tornadoes in Mobara and Kamogawa. The Mobara tornado caused the most severe tornado damage since 1960 in Japan over a damage swath of 6.5 km in length and 500 m in average width. A detailed damage survey revealed that the tornado moved north-northeastward at a speed of about 16 m s-1. The maximum wind speed near the ground, estimated from damage to structures, was more than 78 m s-1. The storms were initiated in the warm sector of a developing extratropical cyclone, about 6-7 h prior to the tornadogenesis. They moved straightforwardly northeastward at a speed of about 16 m s-1 throughout their life cycles including their supercell phases. The mesocyclone in the Mobara storm had been detected by a single-Doppler radar for 44 min. Vertical vorticity of the mesocyclone amplified to 2 × 10-2 s-1 almost simultaneously between 1 and 5 km AGL, about 20 min prior to the tornadogenesis. -from Authors