2011年浙江省出现了自1999年以来最典型、 最强的梅雨降水集中期， 连续4次强降水过程造成旱涝急转。本文利用NCEP再分析资料对2011年浙江梅汛期前后大尺度环流背景进行了分析并与历史同期进行了对比， 同时又对4次强降水过程的梅雨锋结构进行了诊断分析。结果表明： （1）中高纬环流急转前后都具有很大的经向度， 但由急转前的“三槽三脊”转变为“两槽一脊”， 梅汛期为单阻形势， 贝加尔湖阻塞高压前的西北气流为梅雨提供了冷空气条件。梅汛期印缅槽和西太平洋副热带高压较急转前明显加深\, 加强， 偏强的印缅槽和西太平洋副热带高压有利于在副热带高压西北侧建立持久\, 稳定的水汽通道， 西南气流与偏东气流在浙江构成准纬向切变， 使得大量暖湿气流辐合上升， 青藏高压北侧的偏北大风造成高层强烈辐散， 这都为暴雨提供了良好的动力和水汽条件。（2）暴雨主要出现在梅雨锋前沿， 梅雨锋区的上升运动与南北两支下沉气流相配合， 北支携带冷空气向梅雨锋输送， 南支与梅雨锋区上升气流构成经向垂直反环流， 加强了锋区的上升运动。（3）4次暴雨过程梅雨锋都为相当位温密集带， 在对流层低层垂直方向上近似直立分布。由于受冷空气影响， 第一、 三、 四次过程梅雨锋区具有较明显的温度梯度， 低层锋区向北倾斜； 相反， 没有冷空气的作用， 第二次过程锋区无温度梯度， 梅雨锋向南倾斜。

The most typical and strongest precipitation processes during Meiyu period since 1999 wasoccurred in Zhejiang province in 2011,and  the sharp turned from drought to flood caused by four continuous heavy precipitation processes during Meiyu period. Using the objective analysisdata, large scale circulation backgroundbefore and after Meiyu periods are analyzed and contrasted at the same period in the history. At the same time, the different strcture of Meiyu front in the four heavy precipitation processes were compared. The results show that: (1) Before and after the alternationof the middle-high latitude circulation has obvious meridionality. The circulation situation is three-trough and three-ridge before the alternation turning to two-trough and one-ridge after the alternation.There is only one blocking high in Meiyu period. Northwesterlyariflowin front of the Lake Baikal blocking high provides cold air  for Meiyu. The India-Burma trough and the west Pacific subtropical high in low latitude become stronger than the pre-alternation, which helps to build continuous steady water vapor channel on the northwest side of the subtropical high. Southwesterly and easterly are joined together and form quasi-zonal shear near Zhejiang Province, which makes abundant warm and wet air current convergent and rise in this area. Northerly on the north side of Qinghai-Tibetcausedhigh creates forceful divergence on upper level. All above situations offer the good dynamic and moisture conditions for heavy rain. (2) There is heavy precipitation in front of Meiyu front. Ascending motion in the Meiyu frontal zone coordinates with two branches of subsidence, the north one delivers cold air to Meiyu front, while the southone and the updraft in Meiyu frontal zone shapes counter meridional vertical circulation. It also reinforces ascending motion in the Meiyu frontal zone. (3) The Meiyu front is always equivalent potential temperature intensive ribbonand nearly upright at low level of troposphere. Since the affect of cold air, there is obvious horizontal temperature gradient in Meiyu frontal zone with northward bias of lower level frontal zone during the first, third and fourth rainstorm. On the contrary, no cold air in the second rainstorm process, there isno temperature gradient in frontal zoneand the Meiyu front tilts toward south.