Study on the seasonal cycle of climate and water cycle around the frontal zone and its modulation in East Asia associated with abnormal weather conditions

这一研究项目旨在整合对东亚气候和水循环系统的各种过程及其变异性的新认识，以预测异常天气条件和气候变化对全球变暖等全球尺度因素的响应，并特别关注其季节性周期的“调制”。本项目的主要研究成果如下：1.在中国的大陆部分，“湿热陆面”以及副热带高压地区的低层偏南风，对梅雨锋以南的极湿气团的维持和梅雨锋北侧的水汽入侵起着重要作用。此外，在淮河流域(长江以北)梅雨季成熟前，饱和比湿比湿差一度减小，导致湿度场g…增大梅雨前线更是光芒四射。在日本群岛西部的九州，在成熟的白云季节，高达20%的总降雨量来自地面天气图上白云锋面的温暖部分，巨大的水汽通量正向白云锋面输送。还指出，这种情况下的降水一般是在以百乌暴雨为特征的中尺度降雨系统下进行的。另一方面，虽然朝向日本群岛周围锋区的季节平均低层偏南风在秋雨季节不如百雨季强烈，但作为月平均场或亚月变化的年际变化，赤山季节也经常出现强南风。同样有趣的是，九州西北部的气候降雨量在8月底左右出现了一个小的最大值，就在日本群岛的明月季节开始之前。8月底前后的降水高峰期基本上与赤潮锋面暖区的过程有关。但在1985年后，这类年份的出现频率有所下降。因此，本研究说明了副热带锋暖扇面(BEU/AKISAMAME锋面)的大气过程对于了解那里的降雨气候及其变异性的重要性。对夏季降水的分析表明，1980~2000年，日本和中国的白云/梅雨锋附近的降水呈增加趋势，蒙古和菲律宾的夏季降水呈减少趋势。模型研究和观测分析表明，Rossby波通过西伯利亚或青藏高原的传播也起着重要作用。对夏季靠近日本的鄂霍次克高压的动力过程也进行了重新分析。还对不同季节进行了分析，例如，澄清了以下几点。(1)与冬季西伯利亚高压发展的蒙古周边发达温带气旋出现频率增加有关，4月初日本群岛全境出现快速升温。(2)副热带高压环流的变化和当时季节平均场的变化对台风在5、6月份接近日本群岛的情况有很大影响。(3)盛夏和秋日不同季节的平均基准场也是影响10月中秋0423附近降水分布的重要因素。在总结研究成果的基础上，对本项目的后续研究计划进行了讨论。较少

This research project aims to integrate new understandings of the various processes on climate and water cycle systems in East Asia and their variability for the basis of predicting the abnormal weather condition and climate change as responded to global-scale factors, such as global warming, with special attention to the "modulation" of its seasonal cycle. The main findings of this project are as follows.1. Over the continental part of China, the "hot wet land surface", as well as the strong low-level southerly wind in the subtropical high area, plays an important role in the maintenance of the very humid air mass to the south of the Meiyu front and the invasion of moisture toward the northern side of the Meiyu front. In addition, the deficiency of specific humidity from the saturation specific humidity once decreases around the Huaihe River Basin (just to the north of the Changjiang River Basin) just before the mature Meiyu season there, resulting in the enhancement of the humidity g … More radient across the Meiyu front.2. In Kyushu, the western part of the Japan Islands, up to about 20% of the total precipitation in the mature Baiu season there was brought in the warm sector of the Baiu front on the surface weather maps, where the huge moisture flux is passing toward the Baiu front. It is also noted that the precipitation in such situation was generally under the meso-scale rainfall systems characterizing the Baiu heavy rainfalls. On the other hand, although the seasonal mean low-level southerly wind toward the frontal zone around the Japan Islands was not so strong in the Akisame season (autumn rainy season) as in the Baiu, the strong southerly wind frequently appeared also in the Akisame season as the year-to-year variation of monthly mean field or sub-monthly variation. It is also interesting that the climatological precipitation showed a small maximum around the northwestern part of Kyushu around the end of August, just before the onset of the Akisame season in the Japan Islands. The rainfall peak around the end of August was basically associated with the processes in the warm sector of the Akisame front. However, appearance frequency of such years decreased after ~1985. As such, the present study illustrates the importance of the atmospheric processes in the warm sector side of the subtropical front (the Baiu/Akisame front), for understanding the rainfall climate and its variability there.3. Analyses of the summertime precipitation revealed the increasing trend around the Baiu/Meiyu front in Japan and China, and the decreasing trend around Mongolia and Philippines, during the years from 1980 to 2000. Model studies as well as observational analyses showed that the Rossby wave propagation through Siberia or Tibetan Plateau also plays an important role. The dynamical processes on the Okhotsk high near Japan in summer were also re-examined.4. Analyses on the various seasons were also made and clarified the followings, for example. (1) Associated with the increase in appearance frequency of the developed extratropical cyclone around Mongolia where the Siberian high develops in winter, rapid temperature rise occurs in the whole region of the Japan Islands at the beginning of April. (2) Condition in which the typhoons approache the Japan Islands even in May or June is greatly affected by the variation of the subtropical high cell coupled with the seasonal mean fields then. (3) The different seasonal mean basic fields from midsummer or Akisame season was also an importantfactor characterizing the rainfall distribution at the approach of Ty0423 in October, the mid-autumn. By summarizing these results, the post research plan of this project was also discussed. Less