Causes and characteristics of a persistent rainstorm event in warm front of a cold vortex

Based on conventional meteorological observational data, precipitation data from automatic observational stations, and National Centers for Environmental Prediction(NCEP) FNL reanalysis data, we diagnostically analyzed the causes and characteristics of a persistent rainstorm event due to a cold vortex in Northeast China from August 6 to 8, 2019.The results indicated that a warm front that remains stable and makes frontogenesis causes the persistent rainstorm event.Heavy precipitation occurs in the south of the 850 hPa front areas, with a pattern of an east-westward belt, corresponding to the maximum frontal zone.The middle-level atmosphere has weak unstable convection, which favors the maintenance of heavy precipitation.The region with a high value of positive potential vorticity in the upper layer extends down to the lower layer, leading to the increase of the vorticity in the middle and lower layers and the formation of a columnar structure of positive vorticity ahead of the warm front.Moreover, it induces cyclonic circulation in the front area near the surface and promotes the occurrence of low-pressure systems.A strong convergence zone at the low-level frontal area is located on the windward slope.The interaction of frontal convergence and topographic forced uplift makes the low-level strong convergence zone remain in the same area for 3 days.In the weakened stage of the eastward movement of the cold vortex, warm and moist air carried by a typhoon moves northward, resulting in the northward movement of the front area and the maintenance of heavy precipitation.In the stages of formation and maintenance of the cold vortex, T_850-500≥25 ℃, K≥35 ℃, and a certain convective effective potential energy are a good indication of the occurrence of heavy precipitation.The water vapor input along the eastern boundary of the rainstorm area is the most critical, accounting for half of the total water vapor input.Although the total water vapor input from the southern boundary is not quite large, it concentrates in the first rainstorm day and contributes most of the water vapor for the rainstorm on August 6, 2019.