Formation and maintenance of the Mei-yu front disturbance over China

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To clarify the formation and the maintenance processes of the Mei-yu frontal disturbance over China, the following synoptic analyses have been conducted.(1)Formation processes of the lower layer cold air mass to the east of the Tibetan Plateau that cause the Mei-yu rainfall events over China.(2)Diversity in the synoptic condition that causes the activation of the Mei-yu front to the south of the Yangtze River basin.(1):The cold air mass to the east of the Tibetan Plateau tends to appear 〜35゜ N or 〜40゜ N of 〜110゜ E. The cold air mass that appears 〜35゜ N is formed between the low pressure system around 25゜ N and the high pressure system around 40゜ N that migrated from the north of the Tibetan Plateau. Vertical wind shear in the zonal flow of lower troposphere begins to increase in 〜30゜ N of eastern fringe of the Plateau two days before the appearance of the cold air mass. The downward and upward air motion in the lower layer, which considered to be a secondary circulation due to the wind … More shear increase, appear in the north and the south side of the shear zone, respectively The upward air motion exists in the upper layer of the downward current area, and the condensation heating is admitted there. It indicates that the cloud and precipitation are generated in upper troposphere. On the other hand, the evaporation cooling is admitted in the lower layer downward current area. The lower layer cold air mass is generated due to the cooling by evaporation from the cloud droplets and the raindrops. On the other hand, the cold air mass that appears in 〜40゜ N is formed along with the baroclinic wave from the north of the Tibetan Plateau. After the passage of the trough, the air mass to the west of the trough is cooled further by evaporation from the cloud droplet and the raindrop in the downward current area, and the low temperature region was formed there.(2):The formation process of the rainfall zone to the south of the Yangtze River basin is various. It can be classified into the following cases. (1) The cases when the propagation of the baroclinic wave from the higher latitude circulation is observed, (2) the cases when the strengthen in the subtropical anticyclone in the South China Sea and southwesterty wind in the southern part of China is observed, and (3) the cases when the eastward migration of low pressure system that originated from the Tibetan Plateau is observed. Among these, the major process is (1) that acounts for about 60 percent of the total cases. Less