高原和山脉地形对长江中下游梅雨形成的影响

The Effects of Tibetan Plateau and Mountains over Southeastern China on the Formation of the Meiyu Rain

长江中下游梅雨(下称梅雨)是西南暖湿季风气流随季节增暖北进过程中的产物,是东亚副热带季风系统中的一个重要组成部分。利用气候平均资料分析和敏感性数值模式试验,研究了青藏高原(下称高原)和江南山脉地形对梅雨形成的影响。分析表明,在对流层低层,江南处于强劲西南急流风速中心的下游,有江南山脉地形对西南急流强迫抬升,具有强烈的风速和水汽辐合,这是梅雨形成的直接原因。敏感性数值试验表明,无高原时该西南急流增强,江南上空气旋性涡度增大,梅雨雨量增加2 mm/d,这反映了夏季高原巨大的感热气泵的抽吸作用使其东侧的江南地区环流的气旋性减弱,上升受到抑制,降水减少;无江南山脉地形时梅雨雨量减少2 mm/d,雨带北移到淮河流域,而加高山脉地形时则梅雨量增大,雨带位置南移,说明山脉地形对西南季风气流有强迫抬升增幅降水作用,对梅雨雨带的形成亦有重要影响。

The Meiyu Rain （MR） to the south of the mid-and lower-reaches of Yangtze River over Southeastern China （SEC） is one of the important components of the East Asia Subtropical Monsoon system. It is the result of the process of the marching northward of warm and wet southwesterly with season warming. Climate mean data analyses and numerical model sensitivity experiments are used to study the effect of Tibetan Plateau and the mountains over southeastern China on the formation of the Meiyu. Data analyses shows, at lower troposphere, SEC is located at the downstream of a southwesterly velocity center and there is mechanically forced rise of the southwesterly flow by the mountains over SEC. As a result, there is strong southwesterly jet wind velocity and moisture convergences over SEC. That is the immediate climatic cause of MR formation. Sensitivity numerical experiments shows, without the Tibetan Plateau （TP）, the southwesterly jet over SEC become stronger, its vorticity increases, and MR increases by 2 mm/d. In other words, with TP standing, its Sensible Heat Air Pump （SHAP） makes the circulation＇s vorticity increasing over its surrounding areas but decreasing over SEC by suppressing the rising over there and thus makes MR decrease. On the other hand, without the mountains over SEC, the MR decreases by 2 mm/d and the rain belt shifts north to the Huaihe River reach. On the contrary, with the mountains lifted by 300 m, MR increases and the rain belt shifts south. These indicating that the mountains over SEC can force the southwesterly rising and magnify MR. So the mountains over SEC also have important effect on the formation of MR. Thus it can be seen, MR is originally produced by the typical east Asia summer monsoon, which is caused by sea-land thermal contrast. MR is also affected heavily by the nearby huge plateau and mountains through their thermal and mechanical forcing. These can help us to understand the origin of the summer monsoon and its rains over East China.