Rainstorms in Zhejiang mainly occur in the flood season(February-October),during which heavy rainfall is common.The focus of the present study was two flood season rainstorm processes that occurred in Zhejiang in 2013,one belonging to the heavy spring rains hereafter referred to as the 4.29 rainstorm,based on its date of occurrence(29 April 2013) and the other being a typical Mei-yu rainstorm(hereafter referred to as the 6.6 rainstorm,using the same notation as previously mentioned).Based on NCEP analysis data,the circulation,thermal,dynamic and water vapor conditions of these rainstorm processes were analyzed.Through comparative analysis,the mechanistic similarities and differences were studied,providing a theoretical basis for further work on local rainstorm forecasting.The results can be summarized as follows:(1)During the 4.29 rainstorm,the high latitude circulation at the height of 500 hPa in East Asia was characterized by a western high and eastern low pattern.The main system of impact was the westerly trough at high latitudes together with a lower-level vortex,with stronger cold air,and a more southerly northward position of warm air,which induced southerly rain.A double block was present in the high-latitude circulation of the 6.6 rainstorm,and a low vortex was the main influence from the upper to lower levels,with weak cold air,and southwesterly flow further north,thus resulting in a northerly storm.(2)During the 4.29 process,an upper-level strong westerly jet existed,with a positive vertical circulation and low-level convergence corresponding to high-level divergence.During the 6.6 rainstorm,the high-level westerly wind was weak,with a more complex divergence field of a divergence zone during the early stage of precipitation from the near-surface to the lower levels,and a convergence zone in the middle levels,which transferred to weak convergence until the later stage of precipitation near the ground and with no apparent vertical circulation.(3)The source of water vapor for the two rainstorm processes was different:the water vapor for the 4.29 rainstorm process was mainly provided by the southwest jet stream,with the main source regions being the Bay of Bengal,the South China Sea and the western Pacific.The divergence distribution of water vapor flux was convergence in the lower layers,corresponding to upper-level scattered divergence.In the 6.6 rainstorm,the water vapor come from the southwest and southeast,with the main source regions being the Bay of Bengal,the South China Sea,the western Pacific,and the East China Sea.There was low-level divergence in the early stages of the divergence distribution of water vapor flux and convergence in the middle stages.In the later stages,however,there was consistent convergence under the middle level.The strengthened vapor flux convergence above the near-surface level played an obvious role in the rainfall intensity increasing.(4)An east-west oriented front with a strong cold air intrusion existed in the 4.29 process,while a northwest-southeast front with a weak cold air intrusion was apparent in the 6.6 process,both of which were advantageous for the release of baroclinic potential energy,and could promote convection development.