Abstract:The structure and mechanism of a Jiangnan cyclone with a northern track that occurred during 10-13 May 2014 was analyzed by using conventional observations, NCEP 1°×1° reanalysis data and water vapor images from FY 2E. The results are as follows: (1) The distribution of pressure and rainfall areas is similar to the output of Shapiro Keyser conceptual model. (2) The Jiangnan cyclone occurred under the weather situation of the divergence at 250 hPa, and trough merger and ridge development downward at 500 hPa. (3) The weak warm core existed close to the low center under 850 hPa, but there was no back bent warm front as Shapiro Keyser conceptual model showed. (4) When the positive relativity vorti city tilted westward with height, the cold frontal zone intensified in the western side of surface cyclone and the positive relativity vorticities in the upper troposphere strengthened, the cyclone was rapidly deepening; when the positive relative vorticities at different levels were nearly vertically overlapped and the cold frontal zone in the low troposphere weakened, the cyclone developed slowly. (5) Heavy rain was caused by warm and moisture air, strong upward vertical volecity with two secondary circulation crossing the warm front. (6) It was also found by equation that surface cyclone hardly developed without upward vertical velocity at the initial stage, but it moved rapidly toward northeast as the result of warmer advection and larger differential vorticity advection between the upper level and low level troposphere downstream during cyclogenesis. Then surface cyclone was intensified rapidly by srtong upward vertical velocity, but it moved slowly due to the less warm advection and differential vorticity advection. Surface cyclone was slowly deepening due to weakly upward vertical velocity, and it moved more slowly because of the weakly warm advection and differential vorticity advection.