1998梅雨锋的结构特征及形成与维持

应用1998年6月中旬的分析资料, 对1998年6月16~17日暴雨时段的梅雨锋结构及梅雨锋的形成与维持进行了诊断分析, 确认了暴雨发生时期梅雨锋结构的一些普遍特征, 如在对流层低层表现为θ_se锋而不是温度的强烈对比, 梅雨锋区是一个低层正涡度带以及风和水汽的辐合带等。同时揭示了1998年6月暴雨时期梅雨锋结构的典型特征:锋区从近地面可伸展到600 hPa层 (一般认为梅雨锋只存在于800 hPa以下), 近乎垂直、略向北倾; 锋区900 hPa以下是一个大气弱对流不稳定区, 向上气层变为潮湿中性直至400 hPa, 等θ_se线基本上呈垂直分布; 锋区斜压性相当弱等。研究表明, 出现强降水时的梅雨锋结构已经变性或者说它是介于温带锋面结构和ITCZ结构之间的副热带锋系结构。分析还认为, 在强盛的南海季风涌和频繁的西风带扰动组配的大尺度有利背景形势下, 低层空气的水平运动和地转偏差风对1998梅雨锋的形成和维持有明显的正贡献。

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.