Large- and meso-alpha-scale characteristics of Meiyu/Baiu front associatedwith intense rainfalls in 1-10 July 1991

The large- and meso-alpha-scale features of the active Meiyu/Baiu front 1-1 0 July 1991 are studied by utilizing mainly the Geostationary Meteorologica l Satellite (GMS) IR data and ECMWF re-analysis data. The intense Meiyu/Baiu frontal precipitation occurs over the Yangtze River Basin in association with the westward elongation of the North Pacific subt ropical anticyclone. The intense precipitation zone of similar to 500 km wi dth extends from the eastern foot of the Tibetan Plateau to the western Nor th Pacific. The frontal zone is characterized by intense precipitation, low -level jet stream, nearly moist neutral stratification and strong gradient of equivalent potential temperature theta e. The large-scale confluence/convergence in the frontal zone sustains strong gradient of theta e against the sink of theta e due to the convective trans port. The differential advection of theta e generates the convective instab ility against the stabilizing effect of the convection, and thus, the moist neutral stratification is sustained during the period of intense convectiv e rainfalls. The strong condensation heating is one of the factors to susta in the ascent motion in the frontal zone. The strong low-level convergence in the frontal zone is accompanied by the northward strong ageostrophic wind, which is associated with the strong acc eleration along the northwestern periphery of the westward protruding subtr opical anticyclone. The intrusion of the mid-latitude disturbances into the frontal zone, which occurs in the vicinity of the cut off vortex, enhances the precipitation i n the frontal zone by inducing the ascent motion, the cold and dry advectio n. The largest diurnal variation of convective clouds is found in 90-100 degre es E. Within the heavy rainfall zone, such significant diurnal variation is not seen, whereas the eastward passage of meso-alpha-scale cloud systems i s evident. They form in the heavy rainfall area, and develop into meso-cr-s cale frontal depressions during propagation along the frontal zone, where t he significant baroclinicity is seen;within the nearly moist neutral layer in the lower troposphere.