Papers in Meteorology and Geophysics

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   A torrential rainfall occurred in and around the Tokai District in Japan on 11-12 September 2000, of which maximum rainfall amount was more than 500mm/48h. The synoptic-scale environment and the related mechanism of the torrential rainfall were diagnosed using the operational observation data and the global objective analysis dataset provided by the Japan Meteorological Agency.
   In the peak period of the torrential rainfall, a moist absolutely unstable layer was observed in the lower troposphere, which is inferred to have been generated by lifting of a convectively unstable layer. The magnitude of negative horizontal advection of equivalent potential temperature at 700 hPa was comparable with that of positive advection at 925 hPa, which implies that they equivalently contributed to maintaining the lower-tropospheric convective instability during the torrential rainfall.
   The upward forcing which would have lifted the convectively unstable layer was explained in relation to the geostrophic frontogenesis in the lower troposphere. In the beginning of the torrential rainfall, frontogenesis due to confluent deformation occurred near the Tokai district, associated with a typhoon to the south, a low to the north, and the subtropical high to the east of Japan. Next, a midtropospheric disturbance moved eastward along 40°N and approached to the frontogenetic region, contributing to a decrease of geopotential height to the north of the rainfall region and subsequently to an increase of the frontogenesis due to shear deformation. The region and the period of the shear frontogenesis corresponded well to those of the peak of the torrential rainfall in the Tokai District. An increase of the southerly wind speed due to the decrease of the geopotential height field in the baroclinic zone was contributed to the further increase of convective instability and of frontogenesis.

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