1981—2012年西北东部夏季降水不同强度雨日变化及其环流特征的对比分析

A comparative analysis of the atmospheric circulation in summertime rainy days with different precipitation intensity in eastern Northwest China during 1981-2012

利用1981—2012年中国西北东部198个台站的逐日气象观测数据和ERA-Interim再分析资料,分析了近32年中国西北东部夏季不同强度降水的雨日和雨量的变化特征,并对比了产生不同强度降水的大气环流系统的异同。结果表明,西北东部夏季降水以小雨和中雨为主,二者占夏季雨日的90%以上、占夏季雨量的70%左右。小雨日数占总雨日的比率在空间上自东南向西北递增;中雨、大雨、暴雨和(特)大暴雨日数的比率自东南向西北递减。夏季小雨、中雨日数和降水量均呈减少的趋势,21世纪初的减少速率慢于20世纪80—90年代;暴雨则呈略微增多的趋势。通过对1981和2003年的个例分析发现,乌拉尔山阻塞高压、蒙古气旋和西北太平洋副热带高压增强西伸时,有利于北方干冷气流和南方暖湿气流在西北东部交汇,降水较多;反之则降水较少。在冷、暖空气均增强的背景下,若冷、暖气团的交界线偏南,西北东部主要受北方冷气团控制,南边界的水汽输送较微弱,易发生小雨;若冷、暖气团的交界线偏北,西北东部主要受到南方暖湿气团的影响,南边界的水汽输送和辐合较强,多出现强降水,降水强度取决于水汽的强度和上升运动的高度。

Using a daily precipitation dataset that consists of observations at 198 stations in eastern Northwest China and ERA Interim reanalysis data during 1981- 2012, the tendencies of rainy days and total amount of summer precipitation of different types are investigated. Moreover, the similarities and differences in atmospheric circulation corresponding to precipitation of different types are compared. Major results show that light rain and moderate rain are the two major types of precipitation in the summer, which account for more than 90% of summer rainy days and about 70% of total amount of summer precipitation. Spatially, the proportion of light rain days increases from southeast to northwest, while the proportion of moderate rain days, heavy rain days and torrential rain days clecreases from southeast to northwest. Days of light rain and moderate rain as well as the amount of total precipitation during these days both experienced a significant decline during the study period. Large decline is found in the 1980s and 1990s, while rainy days and total amount of precipitation have slowly declined in recent several years. On the other hand, days and amount of torrential rain showed a slightly increasing trend. Analysis of several cases in 1981 and 2003 indicates that when the Ural blocking high and the Mongolian cyclone strengthened while the suhtropical high extended westward, cold airflow from the north and warm airflow from the south both intensified. Such a pattern was favorable for the convergence of cold and warm airmasses in eastern Northwest China, leading to increases in precipitation. It is also found that under the condition when both cold airflow and warm airflow enhanced, light rain was mainly controlled by the cold airflow from the north while the boundary line between cold and warm airmasses was located to the south and the water vapor transport in the southern boundary was relatively weak. Moderate rain, heavy rain and torrential rain were mainly influenced by warm and humid air from the south when the boundary line was located to the north, and the water vapor transport and moisture con vergence in the southern boundary strengthened. In this circumstance, rainfall intensity was dependent on the input of moisture flux and the height the ascending motion could reach.