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——白玉（梅雨）锋，并通过向锋面的强烈水汽输送给东亚地区带来了大量降水。本文以1998年（GAME/IOP年）等年的观测资料为基础，探讨了在亚洲季风系统的季节推进过程中，白球锋周围的水循环是如何建立和维持的。主要结果如下：(1) 1998年梅雨，6月下旬和7月下旬长江流域发生特大洪水。虽然厄尔尼诺已经转变为拉尼娜，但季风西风并没有侵入西太平洋地区，即使在7月之后，该地区的对流活动也没有明显增强。在这种异常的季节转换下，这些洪水事件的发生伴随着对流区的季内转换。(2)中国梅雨锋面上细小的中尺度α-低压有时是在降雨区组织成中尺度α-云系统后形成的。这些例子说明了中国干旱或半干旱区地表-大气相互作用过程在梅雨锋中α-尺度低气压演变中的重要作用。(3)虽然低空南风向华南或华中锋面输送水分在3月或4月就已经出现，但此时的南风系统位于斜压区（成熟的白玉/梅雨季节位于副热带高压区）。(4)对1993年日本周边极冷极湿夏季、1993 ~ 1995年秋季降水锋面活动和1997/1998年日本周边向暖冬过渡（作为对比）的分析表明，持续的全球季节异常在区域尺度异常天气条件中发挥了重要作用。

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.