Case study of synoptic background and microphysical characteristics for a rare needle snow process

A needle snow process lasting 10 hours occurred in Weihai,east of Shandong Province on February 21, 2024. The snowfall amount reached blizzard level, which was rare. In this paper, synoptic background and microphysical characteristics of the needle snow process were analyzed by the comprehensive observation data of dual polarization radar, precipitation weather instrument, ground automatic station, sounding,ERA5 reanalysis data and quasi-vertical profiles(QVP) method. The causes of needle snow were discussed. The results show that: (1) The needle snow process occurred under the background of large-scale rain and snow in China. During the needle snow period, freezing rain changed to ice pellets in the southern part of Shandong Province, and ice pellets changed to sheet or branch snow in the central and northern parts. The influencing system was backflow situation, with strong northeast wind below 925hPa and strong southwest wind above 700hPa.(2) The cloud top height of needle snow is about 500hPa, and the temperature below 600 hPa is always maintained at-6～-3℃ when needle snow occurs, which is also the main characteristic of needle snow to distinguish it from other snowfalls, such as ice pellets, freezing rain and plate crystal.(3) The diameter of needle crystal particles is 3～4mm, the maximum is 8mm, the final falling velocity is mainly below 2m/s, and the particle number concentration is two orders of magnitude higher than sleet. The snowfall intensity has a certain relationship with the size and particle number concentration of snowfall particles. The diameter of heavy snowfall particles with hourly snowfall of more than 1mm is larger and the particle number concentration is higher.(4) Reflectance factor Ze is generally 20～30dBZ, polarization correlation coefficient ρHV decreases, differential reflectivity ZDR is as high as 0.8～1.0dB, and the high value area of differential propagation phase shift KDP is concentrated below 1km during heavy snowfall.(5) Supercooled water is abundant during needle snow, and there is secondary production of Ice, which leads to high ice crystal particle number concentration.