Occurrence of Warm Freezing Rain: Observation and Modeling Study

Abstract

Freezing rain has been normally considered to be composed of supercooled raindrops when the 2 m air temperature (hereafter Tas ${T}_{as}$) is below freezing. However, according to a statistical survey of freezing rain observations in China from 2000 to 2019, we find that there were 656 events that occurred at Tas ${T}_{as}$ greater than 0°C (hereafter warm freezing rain and denoted by WFR), which account for 7% of the total freezing rain observations. Additionally, nearly 3% (266 observations) of freezing rain events occurred when the near‐surface wet‐bulb temperature was greater than 0°C. The modeling and sensitivity experiments on the nonequilibrium raindrop temperature (hereafter Tr ${T}_{r}$) show that the temperature difference between raindrops and the atmosphere is the main cause of WFR. The magnitudes of the normalΔTar ${\Delta}{T}_{ar}$ (difference between raindrop temperature Tr ${T}_{r}$ and air temperature Ta ${T}_{a}$) and normalΔTwr ${\Delta}{T}_{wr}$ (difference between Tr ${T}_{r}$ and wet‐bulb temperature Tw ${T}_{w}$) are determined by the raindrop diameter D, temperature lapse rate Г, and relative humidity RH. Increases of D and Г, and a decrease of RH enhance normalΔTar ${\Delta}{T}_{ar}$ and normalΔTwr ${\Delta}{T}_{wr}$ and thus the occurrence of WFR. Further simulations of 4 idealized and 370 real sounding profiles reveal that either the Ta ${T}_{a}$ or the Tw ${T}_{w}$ cannot properly distinguish the WFR events. When considering the temperature difference between raindrops and the atmosphere, the WFR can form by the “melting of solid hydrometeors” or “supercooled warm rain process.” This study can also deepen our understanding of the conditions of WFR and freezing rain formation at different altitudes.

Drivers of rising monthly water temperature in river estuaries

“…Although it has been shown in glacier‐covered watersheds, the mean elevation of the watershed is a factor that decreases water temperature (Fellman et al 2014), a similar trend was observed in the analysis for the Japanese archipelago, where glaciers are virtually non‐existent. This may depend on higher elevations with lower temperatures and rainfall temperatures (Lu et al 2022). Another important finding is that the relief amount in the watershed also affects the water temperature in river estuaries at a distance from the headwaters.…”

Influence of key parameters of ice accretion model under coexisting rain and fog weather

“…As high-voltage transmission lines are usually erected in high-altitude mountainous areas, they are highly vulnerable to freezing rain (FR) and supercooled fog (SF); the cooling droplets condensed on the cable or snow slush frozen on the cable will cause ice coating on cable, which is usually called as "wire icing". Wire icing will not only affect the normal operation of the transmission network but also cause serious insulator flashover, cable breaking after galloping, damage to the fittings and insulators, tilt and collapse of poles towers, and other accidents (Adhikari and Liu, 2019;Deng et al, 2012;Lu et al, 2022). Since the deployment of China's 330 and 500 kV highvoltage transmission lines, wire icing disasters have increased and seriously affected the safe and stable operation of the power grid (Wang and Jiang, 2012;Hu et al, 2016;Huang et al, 2021).…”