2012年华南前汛期降水特征及环流异常分析

2012年华南前汛期于4月第2候开始，6月第5候结束。前汛期降水经历了三个不同的阶段：第一阶段是4月第2候至5月第3候的降水集中期（锋面降水），江南大部和华南大部降水偏多25％以上，第二阶段是5月第4候至6月第2候的少雨期，华南中部和东部降水偏少50％以上，第三阶段是6月第3—5候的第二个降水集中期（季风降水），江南东南部至华南中西部降水偏多50％以上。对各阶段大气环流距平场的分析结果表明：华南前汛期开始后，偏强的乌拉尔山高压脊导致南下的冷空气偏强，偏强的低层副热带高压使得我国南方为整层水汽输送的异常辐合区，两者共同导致华南前汛期第一阶段的锋面降水较常年同期偏多；南海夏季风在爆发后偏弱和西北太平洋副热带高压（以下简称副高）持续3候异常偏北是导致第二阶段前汛期降水明显偏少的主要原因；第三阶段，南海夏季风异常偏强，副高南落并增强，以及孟加拉湾季风槽的偏强使得华南前汛期此阶段的季风降水偏多。

Analysis of the Precipitation Feature and General Circulation Anomaly During the Pre Flood Season in South China in 2012

The pre-flood season (also named as early rainy season) in South China in 2012 begins in the 2nd pentad in April, and ends in the 5th pentad in June. The pre-flood season has experienced three different periods. The first one is the frontal precipitation period from 2nd pentad in April to 3rd pentad in May, with precipitation over 25% more than normal in most Jiangnan area and most South China. The second period is the dry period from 4th pentad in May to 2nd pentad in June, with precipitation over 50% less than normal in central and eastern South China. The last one is the summer monsoon precipitation period during 3rd-5th pentads in June, with precipitation over 50% more than normal in southeastern Jiangnan area and central-western South China. Analysis of the atmospheric circulation anomalies during each period of the pre-flood season in South China in 2012 indicates that after the pre-flood season onset, more frontal precipitation during the first period is caused by the increased high ridge over the Ural Mountains and the anomalous convergent moisture flux in South China due to the intensified low-level western Pacific subtropical high (WPSH); during the dry period, the South China Sea summer monsoon (SCSSM) is weaker and the WPSH abnormally shifts northward for three pentads. In the last summer monsoon precipitation period, the anomalously intensified SCSSM, the southward WPSH, as well as the enhanced monsoon trough over the Bay of Bengal favor anomalously more precipitation in South China.