一次罕见针状雪过程的天气背景和综合观测特征分析OA

A 10 h needle-shaped snow process occurred in Weihai,east of Shandong Province,on 21 February 2024.The snowfall amount reached the blizzard level,which is a rare occurrence.In this paper,the synoptic background and microphysical characteristics of the needle-shaped snow process are analyzed based on comprehensive observations of dual polarization radars,precipitation weather instruments,ground automatic stations,soundings,ERA5 reanalysis data and Quasi-Vertical Profiles(QVP)method.The causes of the needle-shaped snow are discussed.The results are as follows:(1)The needle-shaped snow process occurred under the background of large-scale rain and snow in China.During the needle-shaped snow period,freezing rain turned into ice particles in southern Shandong Province,and ice particles transformed into sheet or branch snow in central and northern Shandong Province.The influencing system was a return-flow situation,with strong northeasterly winds below 925 hPa and strong southwesterly winds above 700 hPa.(2)The cloud top height of the needle-shaped snow event was about 500 hPa,and temperature below 600 hPa maintained at-6—-3℃when the needle-shaped snow occurred.This is also the main characteristic that distinguishes needle-shaped snow from other types of snowfall such as ice pellets,freezing rain and plate crystals.(3)The diameter of needle-shaped crystal particles was 3-4 mm,the maximum was 8 mm,the final falling velocity was largely below 2 m/s,and the particle number concentration was two orders of magnitude higher than that of sleet.The snowfall intensity had a certain relationship with the size and particle number concentration of snowfall particles.The diameter of heavy snowfall particles with hourly snowfall greater than 1 mm was larger and the particle number concentration was higher.(4)Reflectance factor(Ze)was generally within 20-30 dBz,differential reflectivity(ZDR)reached up to 0.8-1.0 dB,and the high value area of differential propagation phase shift(KDP)was concentrated below 1 km during heavy snowfall period.(5)Supercooled water was abundant during the needle-shaped snow process,and there existed secondary production of ice,which led to a high ice crystal particle number concentration.