Abstract: Based on the conventional and intensive observation data,FNL (Final Analysis) reanalysis data,as well as the TBB (Temperature of Black Body) observed by the FY-2E satellite,the associated thermodynamics,dynamic and instability mechanisms,warm front frontogenesis and the symmetric instability for the occurrence and evolution of two rare heavy rainfalls at the beginning of May over Heilongjiang province were analyzed.The results suggest that the influencing systems of the two heavy rainfalls are the upper trough that cuts into the cold vortex.The difference of the location at which the new cold cortex is formed leads to a different heavy rainfall area.During the two heavy rain processes,there is a secondary circulation perpendicular to the warm front.The secondary circulation is helpful to strengthen the ascending motion and strengthen the rainfall intensity.During the two heavy rain processes,the characteristics of the atmosphere are thermal stability.The low layer cold zone before the warm front is dry and cold.It acts as a cold wedge.Warm air rising above the cold wedge along the front is the main cause for the persistent rainfall.The difference is that in the cold air below the fronts of the 2016 rainstorm,the range and thickness of the sinking motion are larger.The joint action of the cold wedge and the terrain cause the upward motion mainly happen in the upper and middle layers of the frontal zone.Two rainstorms occur in the warm frontogenesis.Rainfall and frontogenesis accompany with each other.The rainfall intensity reaches the strongest level during the period that the frontogenesis is the strongest.When the frontogenesis starts to weaken,the rainfall intensity also starts to weaken till reaching zero.The moist convective instability of the middle troposphere is conducive to the development and maintenance of rainfall.There is a mesoscale rain belt paralleling with the warm front.

Key words: Spring heavy rainfall, Frontogenesis, Symmetric instability, Cold vortex, Secondary circulation