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Occurrence of Warm Freezing Rain: Observation and Modelling Study
  • Zhengqi Lu,
  • Yongxiang Han,
  • Yangang Liu
Zhengqi Lu
Nanjing University of Information Science and Technology
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Yongxiang Han
Nanjing University of Information Science and Technology

Corresponding Author:[email protected]

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Yangang Liu
Brookhaven National Laboratory (DOE)
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Freezing rain has been normally considered to be composed of supercooled raindrops when the near surface air temperature is below freezing. However, according to a statistical survey of freezing rain events in China over the last two decades (from 2000 to 2019), we find that there were 656 cases occurring at near surface air temperature greater than 0℃ (hereafter warm freezing rain and denoted by WFR), which account for 7% of the total freezing rain events. To explain this phenomenon, a theoretical model is established by relaxing the equilibrium assumption to consider the transient heat exchange between raindrops and the surrounding atmosphere. Sensitivity analysis of the model shows that the temperature lag of raindrops to atmosphere is the main cause of WFR. The direction of raindrop temperature departure from the equilibrium depends on the sign of the temperature lapse rate Г, and the magnitude of the temperature lag is determined by the raindrop diameter D, Г, and relative humidity RH. An increase of D, an increase of Г, and a decrease of RH enhance the lag of raindrop temperature and thus the occurrence of the WFR events. Further simulations of 4 ideal and 25 real sounding profiles reveal that WFR events can form by the “melting of solid hydrometeors” or “supercooled warm rain process” when considering the temperature lag between raindrops and the atmosphere. With the assumption of initial raindrop diameter of 2mm, together the observed Г and RH, the model can diagnose more than 95% of WFR events.
16 Mar 2022Published in Journal of Geophysical Research: Atmospheres volume 127 issue 5. 10.1029/2021JD036242