Cloud System Families in the Meiyu-Baiu Front Observed during 1-10 July 1991

摘要

This paper studies the cloud systems in the stationary Meiyu-Baiu front observed in 1-10 July 1991, using the Geostationary Meteorological Satellite IR data and ECMWF re-analysis data. The Meiyu-Baiu frontal cloud zone consists of a few trains of cloud systems with a length of ∼2000 km, which is recognized as the “cloud system family”. Each family consists of a preceding subsynoptic-scale cloud system, and a few meso-α-scale cloud systems aligned along the trailing portion of the sub-synoptic-scale cloud system. During the 10-day period, 4 sub-synoptic-scale cloud systems, accompanied by Meiyu-Baiu frontal depressions, are formed with an interval of 2-3 days, in association with the short-wave westerly troughs. The sub-synoptic-scale depressions indicate the peak value of vorticity more than 6× 10-5 s-1, at both 850 and 500 hPa. The sub-synoptic-scale cloud system consists of convective clouds in its southern portion, and wide spread clouds in the northeastern portion. A zone of potential vorticity appears in the trailing portion of the sub-synoptic-scale depression. The cold and dry advection occurring to the back of the depression sustain thermal and moisture gradient in the narrow frontal zone after the passage of the depression. A few meso-α-scale cloud systems develop along the trailing portion of the sub-synoptic-scale depression. The main part of the meso-α-scale cloud system consists of the active convective clouds. Some of the cloud systems show features of oval shaped cloud cluster, while others indicate features of leaf shaped cloud system with a cloud band in its trailing portion. These meso-a-scale cloud systems are categorized into three groups based on the horizontal extension of the cloud area, duration and the magnitude of the relative vorticity associated with the cloud system. Type-A meso-α-scale cloud systems are identified for∼2 days, and are accompanied by positive vorticity of ∼4 × 10-5 s-1 in the lower and middle troposphere. Type-B meso-a-scale cloud systems are identified only for∼1 day, and are accompanied by positive vorticity of ∼2 × 10-5 s-1 mainly in the lower troposphere. Type-C meso-α-scale cloud systems are generated during the daytime and decay during the nighttime. The wavelength estimated by Eddy’s baroclinic instability model for the unsaturated frontal zone coincides with the wavelength of the observed sub-synoptic-scale cloud system, while that for the moist frontal zone is consistent with the scale of Type-A meso-a-scale cloud systems. The cloud system families are most clearly observed in 110-140°E, where the Pacific subtropical anticyclone, and the cold low in the northern latitude, provide favorable conditions for the development of cloud systems.