What caused the shift of heavy snowfall to freezing rain during the two-day elevated convection in February 2024 over Shandong peninsula of China?

From 19 to 20 February 2024, precipitation caused by elevated convection occurred in Shandong peninsula for two consecutive days, but there were significant differences in phases (heavy snowfall on the 19 February 2024 and freezing rain on the 20 February 2024). In this paper, utilizing multisource meteorological data from conventional meteorological observation, three-dimensional lightning locator, dual polarization radar and ERA5 reanalysis, the differences of their thermodynamic mechanism were compared and their conceptual models were proposed. The results shows: (1) The precipitation on the 19 and 20 February 2024 occurred under the background of similar circulation: That is, there was a deeply sphenoid cold air pad above the ground with a temperature below 0℃, forming a low-level strong inversion layer. The climb of the southwest jet along the cold air pad in front of the southern trough over 700 hPa not only provides abundant water and dynamic conditions, but also its intensity change is an important factor of formatting melt layer. (2) There are a variety of instability mechanisms in this process: In the lower layer, there is a conditional symmetrical instability zone near the interface between cold and warm air, and the warm and humid airflow rises obliquely on the cold pad, and in the middle layer, above the symmetrical instability zone, with the southwestern jet pushing northward twice, the conditional instability zone is established near 500 hPa, and the tilted rising flow into this area triggers elevation convection. The vertical circulation caused by local dynamic frontogenesis is also an important influencing mechanism of the extreme disaster weather by elevated convection. (3) The blizzard on 19 February 2024 was caused by frontogenesis dynamic process, and the blizzard belt was parallel with the frontogenesis belt. On the 20 February 2024, the warm advection caused the front dissipation near 700 hPa, and the frontogenesis of the warm front in the lower layer, which not only caused the cold pad to gradually become thinner, but also formed a thicker melting layer, so most of the hydrometeors melted into raindrops, forming freezing rain. The distribution is closely related to the low-level warm front: The freezing rain is near the warm frontogenesis, the snowfall zone distributes more northern, and the ice particle belt is between them. (4) The differences in precipitation phases are caused by both differences in environmental fields and differences in cloud microphysical structures. The microphysical characteristics of precipitation particles in clouds, the liquid water content in the melting layer, and the thickness, strength, and duration of the melting and freezing layers all have significant impacts on the precipitation phase. On the 19 February 2024, the convection developed higher, and the 30 dBz extended to the height above −20—−10°C, and multiple snowfall clouds continuously act with high snowfall efficiency. During heavy snowfall, the polarization features of radar observation indicate that Z_DR is −1—0.5 dB, CC＞0.98, and K_DP does not exceed 1°/km, showing the characteristics of uniform heavy snowfall. There was a significant melting layer bright band in radar reflectivity during freezing rain on the 20 February 2024, and the CC was less than 0.9 (0.7—0.9) and the gradient was larger, while the Z_DR below this height increased to 1—3 dB, corresponding to water droplets or ice particles with larger particle size.