Abstract:

Using datasets of the hourly surface observation, upper-air observation, and ERA5 reanalysis, the study analyzes the precipitation phase characteristics and the causes of a freezing rain event in southern Heilongjiang province during the rain-snow weather process on November 8, 2021.Results show that the atmospheric circulation background features an eastward-moving and intensifying upper-level low vortex.The southeastern jet stream transporting warm and moist air provides moisture and energy for the precipitation, forming a warm layer that causes the freezing rain.The stratification over the freezing rain area exhibits a "cold-warm-cold" structure.Ice crystal growth produces snow in the upper cold layer, which melts into rain upon entering the warm layer.As the rain descends through the cold layer near the surface, it remained in a supercooled liquid state, forming freezing rain i.e.a process consistent with the "ice-phase melting" mechanism.The warm advection-induced warming in the mid-to-lower atmosphere is indicative of freezing rain, while the northeasterly in the lower layer contributes to cold advection.Sustained and strong northeasterly tends to enhance cold advection, causing a temperature drop in the cold layer and increasing the freezing rain intensity.Further comparison with a typical freezing rain event in Jilin province reveals that a strong warm layer and pronounced inversion layer significantly contribute to the intensified freezing rain.

Key words: Temperature advection, Warm layer thickness, Ice-phase melting