Synoptic background and integrated observational features of a rare needle-snow event

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 (Z_e) was generally within 20—30 dBz, differential reflectivity (Z_DR) reached up to 0.8—1.0 dB, and the high value area of differential propagation phase shift (K_DP) 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.