Analysis of Multiphase Precipitation During a Strong Cold Wave at End of March 2020
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Abstract:
Based on conventional observation data, dual-polarisation radar data, and raindrop spectrometer data, different phases of rain, sleet, and pure snow caused by a rare cold wave that occurred in Changzhou on 28 March 2020 are analyzed. The results show that: (1) The impact of this intense cold wave weather happened in the winter and spring alternating season at the end of March. There was a severe drop in cooling weather in Changzhou (the temperate drop in early spring ranked second in history), and light rain, light sleet, or light snow occurred successively. During the cold wave, the temperature fell sharply twice. The first drop (8.7 ℃ in 6 h) was caused by the weak cold air at the surface. The second cooling period (2 h drop of 5.0 ℃ during this time) occurred primarily due to a combination of high altitude trough, mid to low-level shear, and surface cold high pressure, as well as strong cold advection intrusion and the absorption of evaporative (or sublimative) heat by an apparent dry layer at 850 hPa. This triggered a sharp decrease in temperature at the mid and low levels during precipitation, causing the entire layer to become a cold layer and remained low. This provided favourable conditions for the rain to turn into snow, resulting in rapid changes in the precipitation phase. (2) The dual-polarisation radar determined that the three precipitation phases displayed different polarisation variable characteristics. When the precipitation phase changed to sleet, Z increased, Z DR increased significantly, C C showed an evident low-value zonal distribution, and KDP increased slightly. During the transition from sleet to pure snow, the mixed region of rain and snow phase composed of Z>30 dBz, Z DR >1 dB, C C <0.95 was gradually pressured south, and then became Z>22 dBz, Z DR <1 dB,C C >0.98 pure snow, and K DP decreased slightly. It was also observed that dual-polarisation radar had more reference significance for monitoring precipitation in different phase states than single polarisation radar. (3) The rain phase consisted of high-concentration small particles and low-concentration large particles. The raindrop speed-scale spectrum presented a slanted band, which conformed to the Brandes experience curve, characterised by a large falling velocity and small particle diameter. In the stage of sleet, the number concentration decreased, the particle size range broadened, Dm increased significantly, and the maximum diameter approached 6 mm. The range of raindrop velocity and particle size fell between rain and pure snow. In the pure snow stage, at the beginning of the snow, the snow intensity was small, and the number concentration decreased significantly. As the snow intensity increased, the number concentration increased, nearing 2.2 m-3mm-1. The spectral width of particles widened rapidly, and there were more large particles. The concentration of small particles was larger, whilst the concentration of large particles was smaller.
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Supported by:Beijing E-Tiller Technology Development Co., Ltd.
Supported by:Beijing E-Tiller Technology Development Co., Ltd.