Abstract
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.