STRUCTURE AND FORMATION OF MEI-YU FRONT IN 1998

The structures, formation and maintenance of the Mei-Yu front over the torrential rain period of the middle June 1998 were diagnosed with analysis data. Some general structural characteristics of Mei-Yu fronts are confirmed, such as the strong contrast of θ_se rather than temperature, the positive velocity, the wind and moisture convergence over the lower troposphere within the Mei-Yu front zone, etc. Especially, it is revealed that during the torrential rain period, the Mei-Yu front may extend vertically to 600 hPa from the near surface, which is higher than the general thinking of 800 hPa, and the front nearly doesn't tilt with height; the stability conditions of the front zone turn into the moist neutral above 900 hPa from the weak convective unstable under 900 hPa; the baroclinicity is very weak within the Mei-Yu front zone. It is pointed out the nature of the Mei-Yu front's structures has changed into the sub-tropical style, something between extratropical fronts and ITCZ during the torrential rain period. The diagnoses also show that horizontal air motions and geostrophic departures over the lower-troposphere have obvious positive contributions to the formation and maintenance of the Mei-Yu front in 1998.