2003年梅雨期干冷空气的活动及其对梅雨降水的作用

Activity of Dry Cold Air and Its Impacts on Meiyu Rain During 2003 Meiyu Period

利用NCEP/NCAR再分析资料和卫星云图,分析了2003年梅雨期暴雨过程中干冷空气侵入和演变的特征以及干冷空气侵入对暴雨发生、发展和维持的作用,得到以下结论:梅雨期中高纬度环流的演变和调整与暴雨过程密切相关,亚洲北部阻塞高压的建立和维持,使得500 hPa高度上西风锋区南压,导致干冷空气南下,极涡活动的异常偏东和加强有助于冷空气向江淮流域输送,有利于江淮流域暴雨的发展。在江淮流域梅雨期间,干冷空气的活跃与暴雨过程相对应。干冷空气来源于中高纬度和中高层,当它南下或下降到达江淮流域以北或上空时,与北上的暖湿气流交汇,使得暴雨产生和发展。中低层30°N以北的正环流的下沉支是干冷空气的输送“载体”;江淮流域以北的对流层中低层的北风对淮河流域对流层的形成和发展起着重要作用,同样,高层北风所产生的高空辐散叠置于低层辐合区之上,有利于江淮流域暴雨的发生发展。而且,干冷空气的侵入有利于干层的形成和维持,干层的存在加强了暴雨过程的对流性不稳定,对暴雨的加强和发展起重要作用。干冷空气侵入是梅雨湿度锋形成和维持的一个重要动力和热力原因。卫星云图上分析发现,干冷空气的侵入轨迹为一条明显的暗区,干冷空气南下侵入到黄河以南后,分为两支,其中一支“干侵入2”从西南方向侵入,与冷锋云系相互作用,对暴雨的发生和发展起着关键的作用,这与Browning等强调的“干侵入”不尽相同。

By employing the NCEP/NCAR reanalysis data and satellite cloud image, the characteristics of the dry cold air activity, the dry cold air intrusion and evolution, as well as its impacts on the torrential rain during the 2003 Meiyu period are diagnosed. It is shown that there exists a dose relationship between the evolution of the circulation over the mid-high latitudes and the torrential rain process, and the establishment and maintenance of the blocking anticyclone over Northern Asia lead to the southward movement of the westerly frontal zone and the dry cold air. The abnormal eastward movement and the enhancement of the polar cyclone transfer the dry cold air to the Changjiang River and the Huaihe River basins and thus enhance the torrential rain over the area. It is also revealed that the northerly activities are in coherence with the torrential rain process, and the strong dry cold air accompanies the heavy torrential rain. Once the dry cold air originated from the mid-high latitudes and the mid-high level meets the moist warm air over the Changjiang River and the Huaihe River basins, the torrential rain develops. The descending air branch of the direct circulation to the north of 30° N is the main carrier of the dry cold air. The northerly in the mid-low level over the north of the Changjiang River and the Huaihe River basins plays an important role in the formation and the development of the convective layer, meanwhile, the northerly in the mid-high level also acts as a significant part in the enhancement of the divergence in the upper level. The convergence in the lower level and the divergence in the upper level are in favor of the development of the torrential rain. The results demonstrate that the dry cold air intrusion contributes to the formation and maintenance of the dry level, which enhances the convective instability during the development of the torrential rain process. The dry cold air intrusion is one of the important dynamical and thermal causes for the formation and maintenance of moist front during the Meiyu period. From the cloud image, it can be found that the dry cold air intrusion is an obvious dark zone and it separates into two branches while intruding into the south of the Yellow River, and one of them named as “the dry cold air intrusion 2” intrudes from southwest and interacts with the cold front cloud system, which plays a key role in the development of the torrential rain, however, it is not the same concept as the “dry intrusion” emphasized by Browning （reference）.