A DIAGNOSTIC STUDY TO THE MULTI-SCALE CHARACTERISTICS OF A MEIYU FRONtheAVY RAIN-FALL PROCESS IN 1998

In this paper a typical Meiyu front process with heavy rainfall,from June 12th to June 15th in 1998 has been analyzed,which was the most intense event in the Yangtze River vareachesley in the rainy season.The result s show that the Meiyu heavy rainstorm was brought outunder a combination of three space-scale weather systems.The favorable large-scale background field is the Double-Block Highs pattern in Asia-Europe mainland at 500 hPa.Ural and Okhotsh (sea) were controlled by a blocking high,respectively.The Okhotsh(sea)blocking high blocked off the cold air from moving east ward easily,hence the cold air from polar was piled up in the west of Lake Baikal.At the same time,the Western Pacific subtropical High (WPSH) located anomalously southward at lower latitudes provided favorable environment for the warm and moist air moving northward.And the warm,moist air metwith the cold air from the north in the middle and lower reaches of the Yangtze River.It is a favorable environment for the Meiyu pattern.While the synoptic scale trough provided dynamic conditions for the heavy rainf all.In the downstream of the trough,there existed a narrow belt,which had convergence,positive vorticity in the low-levels,coupled with divergence and negative vorticity in the upper-levels.Itis a pronounced property of the Meiyu front.On meso-α-scale,the vertical structure was characterized by strong convergence in the low-levels and divergence in the upper-levels,which induced strongly ascending motion.At the same time,in the upper-levels,there existed a secondary-circulation forced by upper-level jet stream south to the system.Such a 3D structure on meso-α-scale provided necessary conditions for heavy rainf all,including dy namic conditions,moisture convergence and instabilit y.Thus,a heavy rainf all was unavoidable.