Analysis of the formation mechanism of phase differences caused by elevated convection in Shandong for two consecutive days in winter

From February 19 to 20, 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 19th and freezing rain on the 20th). 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 19th and the 20th 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°C, 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 convective instability zone is established near 500hPa, 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 19th was caused by frontogenesis dynamic process, and the blizzard belt was parallel with the frontogenesis belt. On the 20th, 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) On the 19th, the convection developed higher, and the 30dBZ extended to the height above -20°C~-10°C, and multiple snowfall clouds continuously act with high snowfall efficiency, resulting in a large amount of short-term snowfall. During heavy snowfall, the polarization features of radar observation indicate that ZDR is -1~0.5dB, CC>0.98, and KDP does not exceed 1°·km-1, showing the characteristics of uniform heavy snowfall. There was a significant melting layer bright band in radar reflectivity during freezing rain on the 20th, and the CC was less than 0.9 (0.7~0.9) and the gradient was larger, while the ZDR below this height increased to 1~3dB, corresponding to water droplets or ice particles with larger particle size.