Preliminary analysis of mesoscale feature and causes for rainstorm caused by the warm front frontogenesis in Northeast China

Based on conventional observation data, satellite images, automatic observational data of precipitation and NCEP/NCAR reanalysis data (0.25°×0.25°), the mesoscale characteristics of a rainstorm caused by warm front frontogenesis were analyzed, and the cause of the rainstorm was preliminarily discussed. The major conclusions were as follow. The rainstorm occurred under the condition of the strengthening and westward-stretching subtropical high and the influence of typhoon activity. Water vapor transport on the periphery of the subtropical high pressure provided abundant water vapor conditions for rainstorms. The increase of the lower southwest wind led to the warm front, strengthened the convergence and uplift of the warm front, and caused a large area of rainstorm weather. There existed the conditional symmetric instability (CSI) in the frontogenetical area. The frontogenetical effect and the release of CSI strengthened the oblique upward flow along the frontal surface and the frontal secondary circulation. The developments of slantwise convection by CSI further triggered the convective instability. The convective instability led to a large area of vertical ascending motion, and the precipitation was significantly enhanced. The sounding analysis showed that the atmosphere of warm front cloud belt was in an unstable state, which was advantageous to the convection development with short time heavy rainfall. The rainstorm was produced by the β mesoscale finger cloud. The mesoscale cloud had the characteristics of backward propagation, which was caused by the rebirth and merging of convective clouds. Heavy precipitation was mainly composed of warm cloud precipitation, high precipitation efficiency and heavy rain. The warm front was stable and less active, and the convective cell caused by warm front was repeated in the same region, and traveled along the warm front from west to east to form the train effect. The strong echo activity of the rainstorm center had a high precipitation efficiency exceeding 4 hours (maximum reflectivity of the radar echo exceeding 45 dBz), resulting in long duration of heavy rainfall and large precipitation accumulation.