What Allows Some Freezing Rain Events to Persist for Many Hours? A Focus on Dynamic and Thermodynamic Processes

Freezing rain can have devastating impacts, from dangerous road conditions to tree and power line damage leading to long-lasting power outages. Because of its typically light intensity, the severity of a freezing rain event is closely related to its duration. Most freezing rain events persist only for a few hours, as the diabatic processes associated with phase changes in the warm and cold layers act to destroy these layers. This has led some authors to describe freezing rain as a self-limiting process. Predicting the maintenance of the precise vertical temperature profile necessary for freezing rain events remains a major forecast challenge. In this presentation, we examine the thermodynamic and synoptic-dynamic characteristics impacting freezing rain event duration over the United States and Canada using surface and upper-air observations from 1979–2016. Upon identifying freezing rain events at 579 surface stations, we perform a comparison of the conditions that have produced long-duration (6 or more hours of freezing rain) events with those that have led to less-severe short-duration (2-4 hour) events. We identify statistically significant, regionally coherent differences between the two event types, with a focus on three geographic regions.

In the northeastern United States/southeastern Canada, where long-duration freezing rain events occur most frequently, onset surface and cold-layer temperatures are significantly colder at long-duration event onset than at short-duration event onset. This allows warming from warm-air advection aloft and the release of latent heat of fusion when rain freezes at the surface to persist for longer periods before the cold layer erodes and precipitation transitions to rain. This is also true in the southeastern United States, where freezing rain is commonly associated with Appalachian cold-air damming. There, evaporative cooling also appears to be important in maintaining below-0°C surface temperatures for a prolonged period, and long-duration events are associated with significantly drier low-level air than short-duration events at onset. In the south-central United States, onset cold layers are nearly identical for short- and long-duration events, as low-level cold-air advection occurs during events and offsets any warming resulting from the release of latent heat of fusion at the surface. Instead, longer-duration events there are associated with deeper and warmer onset warm layers aloft. Through an improved understanding of the conditions leading to persistent freezing rain, we hope to provide forecasters with additional information to help them better predict these potentially damaging events.