Structure and Dynamics of Convective Clouds that Cause Wind Hazards

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The structure and dynamics of convective clouds that produce tornadoes and downbursts in relatively humid regions like Japan was studied by means of a numerical simulation and by analyzing observational data derived from Doppler radars. In order to clarify the characteristics of the environment for tornado-producing convective clouds, a tornado database between 1961 and 2000 in Japan was compiled, and so-called bulk parameters that characterize meso-scale environment was examined based on the upper air sounding data of Japan Meteorological Agency between 1988 and 1999.1. Tornado-producing stormsOn the evening of 19 September 1990, at least three tornadoes occurred in Kanto District in association with Typhoon 9019. Our Doppler observation revealed that nine mini-supercells occurred over the District and their detailed structure was clarified. A numerical simulation in which the upper air sounding at Tateno at 2100JST was used as a horizontally uniform environment reproduced the observe … More d characteristics of the mini-supercell surprisingly well. The simulated storm exhibited a generation change every an hour. The vertical vorticity of the mesocyclone was found to be produced principally by tilting the horizontal vorticity in the environmental wind. Another Doppler radar observation of a mini-supercell that spawned two tornadoes in Chiba prefecture on 5 July 1996 detected clearly its generation change.2. Downburst-producing stormsA storm that caused damage-producing downbursts in Misato town in Saitama prefecture on 8 September 1994 was analyzed by using observational data. A numerical simulation in which the upper air soundings at Maebashi and Tateno at 1500JST was used as a horizontally homogeneous environment reproduced the observed characteristics of the storm well. The downburst was found to be initiated by the loading of graupel and hail at the upper level, and then intensified principally by the melting of graupel and hail near the OC level and secondly by evaporation of rain drops. Less