Reasearch on the water cycle around the Baiu front and its year-to-year variation in association with the seasonal march of the Asian monsoon

初夏出现了一个重要的副热带锋区--梅雨锋，它通过强烈的水汽输送给东亚地区带来了巨大的降水。本文利用1998年（GAME/IOP年）的观测资料等，研究了在亚洲季风系统的季节性推进过程中，这种水循环是如何建立和维持的。(1)1998年中国梅雨期，长江流域6月下旬至7月下旬发生特大洪水。虽然厄尔尼诺现象已经转变为拉尼娜现象，但季风西风并未侵入西太平洋地区，西太平洋地区的对流活动在7月以后也没有明显增强。这些洪水事件的发生可能伴随着对流区的季节内转换，而对流区的季节内转换正是在这种异常的季节转换下发生的。(2)梅雨锋上细小的中α低压有时是在雨区组织成中α云系后形成的。这些个例说明了中国干旱、半干旱地区陆面-大气相互作用过程在梅雨锋α中尺度低压演变中的重要作用。(3)虽然低层偏南风对华南或华中锋区的水汽输送早在3、4月就已发现，但此时偏南风系统位于斜压区（成熟期在副热带高压区）。(4)通过对1993年日本地区夏季极端凉爽潮湿、1993 ~ 1995年秋季降水锋面活动以及1997/1998年日本地区冬季转暖的对比分析，揭示了全球季节持续性异常在区域尺度异常天气中的重要作用。

A significant subtropical frontal zone called the Baiu (Meiyu) front appears in early summer, and brings huge precipitation in East Asia by the intense moisture transport toward the front. The present study investigated how such water cycle around the Baiu front is established and sustained in the seasonal march of the Asian monsoon syslem, based on the observational data in 1998 (the GAME/IOP year), and so on. The main results are as follows.(1) In 1998 Meiyu in China, the extreme floods occurred around the Changjiang River Basin in late June and latc July. Although the El Nino had turned to the La Nina, the monsoon westerly did not intrude into the western Pacific region and the convective activity there was not enhanced so much even after July. Those flood events were suggested to be caused accompanied by the intraseasonal transition of the convective areas there under such abnormal seasonal transition.(2) fine meso-α-lows on the Meiyu front in China were sometimes formed after the organization of the rainfall area into the meso-α-cloud systems. The present cases illustrate an interesting role or the land surface-atmospheric interaction processes around the arid or semiarid areas in China in the evolution of a meso-α-scale low on the Meiyu front.(3) Although the moisture transport by the low-level southerly wind to the frontal zone around the south or central China is found already in March or April, the southerly wind system then is located in the baroclinic zone (in the mature Baiu/Meiyu season, it is in the subtropical high area).(4) The analyses for the 1993 extremely cool and wet summer around Japan, the autumn rainfall frontal activities in 1993 to 1995, and the transition into the warmer winter in 1997/1998 around Japan (for comparison) suggest the important role of the persistent global anomaly from season to season in the regional-scale abnormal weather conditions.