Large- and Meso-α-scale Characteristics of Meiyu/Baiu Front Associated with Intense Rainfalls in 1-10 July 1991

摘要

The large- and meso-α-scale features of the active Meiyu/Baiu front 1-10 July 1991 are studied by utilizing mainly the Geostationary Meteorological 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 subtropical anticyclone. The intense precipitation zone of -500km width extends from the eastern foot of the Tibetan Plateau to the western North Pacific. The frontal zone is characterized by intense precipitation, low-level jet stream, nearly moist neutral stratification and strong gradient of equivalent potential temperature θe.The large-scale confluence/convergence in the frontal zone sustains strong gradient of θe against the sink of θe due to the convective transport. The differential advection of θe generates the convective instability against the stabilizing effect of the convection, and thus, the moist neutral stratification is sustained during the period of intense convective rainfalls. The strong condensation heating is one of the factors to sustain 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 acceleration along the northwestern periphery of the westward protruding subtropical 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 in the frontal zone by inducing the ascent motion, the cold and dry advection.The largest diurnal variation of convective clouds is found in 90-100°E. Within the heavy rainfall zone, such significant diurnal variation is not seen, whereas the eastward passage of meso-α-scale cloud systems is evident. They form in the heavy rainfall area, and develop into meso-α-scale frontal depressions during propagation along the frontal zone, where the significant baroclinicity is seen within the nearly moist neutral layer in the lower troposphere.