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

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Chris McCray '15
Thursday, November 3, 2016, 12:30-1:20 p.m. in ASAC 319

Chris McCray ’15 will be here in person to speak about some of his graduate research related to freezing rain he’s working on at McGill University.

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 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 shows a broader geographic distribution, with local maxima in the number of 18+ h events over Oklahoma and surrounding states – a region with relatively low annual freezing rain frequencies. Conditions during long-duration events vary greatly between regions of the continent. In northeastern North America, temperatures tend to increase on average 2-5°C from event start to end, while in the Great Plains temperatures actually decrease on the order of 1°C under strong cold-air advection. These changes are associated with very different phase changes, with events in Oklahoma often beginning as rain and transitioning to freezing rain. The reverse pattern occurs in the Northeast. 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.