Long-Duration Freezing Rain Events over North America: Regional Climatology and Thermodynamic Evolution

Abstract

Freezing rain can cause severe impacts, particularly when it persists for many hours. In this paper, we present the climatology of long-duration (6 or more hours) freezing rain events in the United States and Canada from 1979 to 2016. We identify three focus regions from this climatology and examine the archetypal thermodynamic evolution of events in each region using surface and radiosonde observations. Long-duration events occur most frequently in the northeastern United States and southeastern Canada, where freezing rain typically begins as lower-tropospheric warm-air advection develops the warm layer aloft. This warm-air advection and the latent heat of fusion released when rain freezes at the surface erode the cold layer, and freezing rain transitions to rain once the surface temperature reaches 0°C. In the southeastern United States, a larger percentage of events are of long duration than elsewhere in North America. Weak surface cold-air advection and evaporative cooling in the particularly dry onset cold layers there prevent surface temperatures from rising substantially during events. Finally, the south-central United States has a regional maximum in the occurrence of the top 1% of events by duration (18 or more hours), despite the relative rarity of freezing rain there. These events are associated with particularly warm/deep onset warm layers, with persistent low-level cold-air advection maintaining the cold layer. The thermodynamic evolutions we have identified highlight characteristics that are key to supporting persistent freezing rain in each region and may warrant particular attention from forecasters tasked with predicting these events.

An Exceptional Case of Freezing Rain in Bucharest (Romania)

“…For example, some stations have reports for limited periods or the reports are missing overnight [22,38]. To address these issues, recent climatological studies of freezing rain in the United States were based only on data from stations that had reports for at least 80% of the annual hours for at least 10 out the 15 years studied [38] or for at least 30 out of 38 years studied [22]. When these criteria are applied to the three stations from Bucharest only Bȃneasa is identified as suitable for analysis.…”

“…A similar distribution was observed in the case of the freezing rain days with a maximum in December (31 freezing rain days, representing 38% of all freezing rain days). 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 To account for the severity of freezing rain events in the United States and Canada, the duration of the event was considered as proxy [22]. Long-duration events being those in which at least six hours of freezing rain were reported.…”

“…This occurs in environments in which the number of active ice nuclei is very low. Freezing rain can also occur even if a warm layer is not present [19][20][21][22]. For example, in a study based on 10 years of observations from weather stations in the continental United States, 25% of all freezing rain cases occurred without a warm layer [20].…”

Regional Thermodynamic Characteristics Distinguishing Long- and Short-Duration Freezing Rain Events over North America

“…Processes such as warm-air advection (WAA) in the warm layer and cold-air advection (CAA) or evaporative cooling in the cold layer are necessary to offset these selflimiting effects for prolonged freezing rain to occur (e.g., Lackmann 2011). As a result, freezing rain events tend to be short-lived, with 1-h events most common and an exponential decrease in frequency with increasing duration (Cortinas et al 2004;McCray et al 2019).…”

“…In McCray et al (2019), we examined freezing rain events at surface stations in the United States and Canada and used event duration as a proxy for severity, with long-duration (LD) events being those with $6 h of freezing rain. Our climatology of these events highlighted three focus regions ( Fig.…”