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

一个重要的副热带锋区，称为白云(梅雨)锋出现在初夏，并通过向锋面的强烈水汽输送为东亚带来了巨大的降水。本文根据1998年(GAME/IOP年)的观测资料，研究了在亚洲季风系统的季节进行期中，百雨锋周围的这种水循环是如何建立和维持的。主要研究结果如下：(1)1998年中国梅雨期，长江流域极端洪涝灾害发生在6月下旬和7月。虽然厄尔尼诺已转为拉尼娜，但季风西风并没有侵入西太平洋地区，即使在7月以后，那里的对流活动也没有明显增强。这种异常的季节转换可能伴随着对流区的季节内转换。(2)中国梅雨锋上的细小中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地区干旱或半干旱地区陆面-大气相互作用过程在梅雨锋上中尺度低的演变中所起的有趣作用。(3)虽然在3、4月份已经发现低层南风对中国南部或中部锋区的水汽输送，但南风系统仍位于斜压区(在白云/梅雨成熟季节，它位于副热带高压地区)。(4)对1993年日本周围极端凉爽潮湿的夏季、1993-1995年秋季降水锋面活动进行了分析。而日本周边1997/1998年进入暖冬(作为对比)，则表明全球持续的季节间异常异常在区域尺度异常天气条件中的重要作用。

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.