Climatology of ice pellets formed through complete melting of solid precipitation over southern Quebec

Abstract:

The severity of winter storms when the temperature is near 0°C strongly depends on the phase of precipitation. Distinguishing between freezing rain and ice pellets is difficult because they are formed under similar atmospheric conditions. For example, up to 30 mm of freezing rain was predicted over southern Quebec, Canada on 11 January 2020. Instead, 10 hours of ice pellets and an accumulation of 5 cm of solid precipitation were reported, with lower impacts than may have occurred had the predicted freezing rain fallen. The most common formation mechanism for ice pellets involves snowflakes falling through a layer with temperatures slightly >0°C and partially melting. Upon descent into a near-surface cold layer, the remaining ice initializes particle refreezing. Several studies have also identified ice pellets that formed following complete melting of snowflakes, with refreezing potentially occurring, for example, via contact nucleation when raindrops strike active ice nuclei or snow crystals in the refreezing layer. However, the frequency of ice pellets formed through complete melting remains unknown.

In this presentation, we identify ice pellet events in southern Quebec from 1979-2020 using surface observations. We then retrieve vertical temperature and moisture profiles for these events using ERA-5 reanalysis and explore the characteristics of these profiles, including the depths and temperatures of the melting and refreezing layers. Using this information, we partition freezing rain events into Type A (partial melting) and Type B (complete melting). We focus on the conditions distinguishing Type A and B events to determine potential mechanisms involved in their formation. We find that Type A events represent the majority of ice pellet observations in southern Quebec, while the rarer Type B events are associated with colder refreezing layers and tend to be more prolonged. This work contributes to an improved understanding of the near-surface atmospheric conditions differentiating freezing rain and ice pellets during winter storms.