TORNADOES IN CHIBA PREFECTURE ON 11 DECEMBER 1990

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 x 10(-2) s-1 almost simultaneously between 1 and 5 km AGL, about 20 min prior to the tomadogenesis. About 4 min before the tomadogenesis a small mesocyclone formed in the south edge of the major mesocyclone, and this new mesocyclone produced the tornado. The Kamogawa storm was in supercell phase for more than 2 h. A mesocyclone was detected both by surface wind records and by the Doppler radar. The vorticity of the mesocyclone amplified and weakened at least two times. Traces of surface pressure, temperature, and precipitation rate at Tateyama Observatory in the storm path, about 20 km southwest of Kamogawa, showed that the center of the mesocyclone was located in cooler air behind the gust front and the major precipitation preceded the mesocyclone. A barometer located near the center of the tornado damage path in Kamogawa recorded two pressure dips, indicating that the centers of the mesolow and the tornado were about 5 km apart.