Observational and Modeling Study on the Mechanisms of Rain–Snow Transition in Western Liaoning

Complex-phase winter precipitation events involving various phases often lead to severe disasters. To address the current research gap in the microphysical mechanisms of rain–snow transition, this study investigates these mechanisms in cases during early winter of 2021–2023 in western Liaoning, using ground-based disdrometer observations and the WRF (Weather Research and Forecasting) model. By analyzing the disdrometer data, the precipitation phases were differentiated based on changes in the velocity–diameter (V–D) spectrum morphology. A power function, V=α×D^β, was employed to fit the V–D relationship for the various stages of rain–snow processes in western Liaoning. The fitting coefficient β indicates the dependence of the terminal velocity of precipitation particles on their diameter. The results showed that the β values ranged from 0.3 to 0.5 for pure rain and from 0.1 to 0.2 for the rain–snow transition stage and were typically less than 0.1 for pure snowfall. These findings offer valuable insights for future improvements to localized models. Building upon the observational insights, the WRF model was used to simulate three selected cases, leading to the development of a conceptual model for November rain–snow transition processes in western Liaoning. The results revealed that near-surface atmospheric temperature and water vapor conditions, particularly the presence and evolution of inversion layers above 0°C (referred to as warm layers), were crucial for determining the surface precipitation phase. Rain formation primarily depended on the complete melting of high-altitude ice-phase particles as they fell through a warm layer or on the coalescence growth of supercooled water droplets at lower altitudes. When the warm layer was present but thin or relatively cold, ice-phase particles partially melted and then refroze or rimed with supercooled water droplets (primarily observed around 2 km altitude), leading to mixed-phase precipitation such as sleet or graupel. When the entire atmospheric column remained below 0°C, deposition and ice-crystal autoconversion above 4 km were the primary mechanisms for snow crystal formation and growth, ultimately resulting in pure snow.