A Surface Dynamic and Thermodynamic Analysis of Long-Duration Freezing Rain Events

While even short periods of freezing rain can be hazardous, the most severe impacts tend to occur when it persists for many hours. Because of the latent heat released as rain freezes at the surface, freezing rain has been described as self-limiting, with air temperatures often rising above 0°C shortly after precipitation onset. Previous studies have primarily focused on developing climatologies of all freezing rain observations and the conditions concurrent with them. Here, we specifically concentrate on surface observations of long-duration (six or more hours) freezing rain events over North America.

As with freezing rain in general, long-duration events occur most frequently from southeastern Canada into the northeastern United States. An analysis of the longest-duration events at each station shows a broader geographic distribution, with local maxima over Oklahoma and surrounding states – a region with relatively low annual freezing rain frequencies. Observations of wind, precipitation, and temperature during these events help to elucidate their regional and local characteristics. During long-duration freezing rain events over northeastern North America, temperatures increase on average 2-5°C from the time of onset to the end of the event. In the US Great Plains, however, long-duration events tend to be associated with slight temperature decreases on the order of 1°C. An examination of surface winds during these cases reveals that in the Great Plains, freezing rain events most often occur under northerly winds and cold outflow from an Arctic anticyclone. Though events are relatively rare in this region, this concurrent cold-air advection helps sustain freezing rain for long periods when events do occur. In other regions, wind patterns reveal topographic features that support conditions that mitigate the warming due to latent heat release. For example, northeasterly channeled winds down the St. Lawrence River Valley during long-duration events in Quebec provide an important mechanism for the replenishment of cold air at the surface. We also explore the differences in conditions between long- and short-duration freezing rain events. This analysis may be useful to forecasters in discriminating between environments favorable for only one or two hours of freezing rain and those which support sustained and potentially damaging icing events.