Microphysical mechanisms driving rain-to-snow transitions under contrasting near-surface thermodynamic and moisture conditions

Winter precipitation events, involving complex transitions between rain and snow, pose a significant forecasting challenge and often lead to severe weather hazards. The difficulty in predicting precipitation type stems from an incomplete understanding of the microphysical processes that govern these transitions under diverse atmospheric conditions. This study investigates two rain-to-snow transition events in western Liaoning, China. Both cases shared a common evolution from rain to snow, with the melting of ice-phase particles aloft serving as the initial source for raindrops. However, they occurred under contrasting near-surface thermodynamic and moisture conditions—one with a sub-freezing, dry sub-cloud layer (Case 1) and the other with a near-0 °C, moist layer (Case 2), leading to distinct transition processes. Ground-based disdrometer observations revealed distinct surface precipitation characteristics driving rain-to-snow transitions. In Case 1, a bimodal velocity-diameter (V–D) distribution indicated the coexistence of rain and snow. In contrast, Case 2 was characterized by a unimodal V–D distribution of dense, rimed particles. High-resolution WRF simulations using the Thompson microphysics scheme successfully reproduced these transitions. The simulations accurately captured the particle size distributions (PSDs) of rain and graupel. A comparative analysis identified distinct microphysical processes as the primary difference between the two cases, governed by the vertical profiles of temperature and humidity. In Case 1, melting initiated at higher altitudes, followed by in-cloud riming. In the subsequent descent through the sub-cloud dry layer, strong evaporation and sublimation cooled the environment and removed liquid water, causing a rapid transition to snow. In Case 2, a deeper warm layer resulted in lower-altitude melting, while high near-surface moisture fueled dominant riming processes within the near-surface layer, prolonging the mixed-phase period with dense hydrometeors. Conceptual models illustrate how near-surface temperature and relative humidity jointly regulate the key microphysical processes such as melting, riming, evaporation, and sublimation.