APRIL 2004 CORTINAS ET AL. 377 An Analysis of Freezing Rain, Freezing Drizzle, and Ice Pellets across the United States and Canada: 1976±90 JOHN V. C ORTINAS JR.* Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma BEN C. BERNSTEIN National Center for Atmospheric Research, Boulder, Colorado CHRISTOPHER C. ROBBINS NOAA/National Weather Service, Fort Worth, Texas J. WALTER STRAPP Meteorological Service of Canada, Downsview, Ontario, Canada (Manuscript received 20 May 2003, in ®nal form 14 October 2003) ABSTRACT A comprehensive analysis of freezing rain, freezing drizzle, and ice pellets was conducted using data from surface observations across the United States and Canada. This study complements other studies of freezing precipitation in the United States and Canada, and provides additional information about the temporal charac- teristics of the distribution. In particular, it was found that during this period 1) spatial variability in the annual frequency of freezing precipitation and ice pellets is large across the United States and Canada, and these precipitation types occur most frequently across the central and eastern portions of the United States and Canada, much of Alaska, and the northern shores of Canada; 2) freezing precipitation and ice pellets occur most often from December to March, except in northern Canada and Alaska where it occurs during the warm season, as well; 3) freezing rain and freezing drizzle appear to be in¯uenced by the diurnal solar cycle; 4) freezing precipitation is often short lived; 5) most freezing rain and freezing drizzle are not mixed with other precipitation types, whereas most reports of ice pellets included other types of precipitation; 6) freezing precipitation and ice pellets occur most frequently with a surface (2 m) temperature slightly less than 08C; and 7) following most freezing rain events, the surface temperature remains at or below freezing for up to 10 h, and for up to 25 h for freezing drizzle. 1. Introduction as well, since these precipitation types, primarily freezing Freezing precipitation, de®ned by the American Me- rain, have produced fatalities, injuries, and a signi®cant teorological Society's Glossary of Meteorology as freez- loss of property (Bendel and Paton 1981; Forbes et al. ing rain, freezing drizzle, and freezing fog (Glickman 1987; Toth 1988; Rauber et al. 1994; Martner et al. 1992; 2000), can have a devastating effect on many industries, Marwitz et al. 1997; Jones and Mulherin 1998; De- including transportation, energy, agriculture, and com- Gaetano 2000; Irland 2000; Changnon 2003). An ice merce. In addition to freezing precipitation, ice pellets at storm that occurred during January 1998 in the north- the ground often are an indication of freezing precipi- eastern United States and southeastern Canada illustrates tation aloft, which can have a detrimental effect on air- the effect of these storms on society and underscores the craft (Zerr 1997; Bernstein et al. 1998). The effects of need for climatological information to understand the fre- these events on human life and activities are signi®cant quency of these hazards, which may lead to better mit- igation practices. [See Gyakum and Roebber (2001) and Roebber and Gyakum (2003) and for a complete mete- * Additional af®liation: NOAA/OAR/National Severe Storms Lab- orological description of this storm.] oratory, Norman, Oklahoma. a. Impacts of freezing precipitation Corresponding author address: Dr. John Cortinas Jr., NOAA Re- search, R/OSSX5, 1315 East±West Hgwy., Silver Spring, MD 20910. The 1998 ice storm affected more than three million E-mail: [email protected] people in four states and two Canadian provinces, caus- q 2004 American Meteorological Society Unauthenticated | Downloaded 09/30/21 07:01 AM UTC 378 WEATHER AND FORECASTING VOLUME 19 ing 44 fatalities and damage estimated at $3 billion U.S. the ground, several authors have documented icing con- dollars (USD) in Canada and $1.4B USD in the United ditions caused by freezing drizzle and freezing rain aloft States (NOAA 1998). In addition to the obvious total that signi®cantly impacted aircraft performance (Sand disruption of transportation due to ice accumulation on et al. 1984; Pike 1995; Ashenden and Marwitz 1997; the roads, vehicles, and airport runways, Jones and Mul- Bernstein et al. 1999). Although a climatology of su- herin (1998) report that the impact of this storm on the percooled large droplet (SLD) icing conditions aloft cur- public utilities industry and the communications system rently is very dif®cult to create because of the lack of was signi®cant. Heavy ice loads on distribution and historical, direct measurements of these phenomena transmission power lines, as well as on trees and branch- throughout the atmosphere, ground reports of freezing es that eventually broke off and fell onto power lines, precipitation can be used to infer partially the clima- caused a loss of electricity to roughly 1.5 million people tology of SLD icing conditions. Bernstein et al. (1997, during the storm. The communication system in this area 1998) and Bernstein and McDonough (2000) examined also was affected when broadcast and two-way towers pilot reports of in-¯ight icing and research aircraft mi- collapsed; nonwireless phone service to some residential crophysics measurements and found that when aircraft customers was disrupted when trees and branches fell ¯ew in close proximity to surface observations of freez- on phone drops. ing rain, freezing drizzle, and/or ice pellets, signi®cant State and federal forests also were damaged signi®- icing was often found in the lower atmosphere. Surface cantly by the storm. Irland (2000) and DeGaetano reports of ice pellets provide a strong indication of the (2000) report that the storm impacted 25 million acres presence of freezing rain aloft that is formed by the of forest in the United States and Canada and the level classical melting mechanism (Hanesiak and Stewart of tree damage was compared to that from a 1938 hur- 1995). Thus, it is important to include them in an anal- ricane that affected the area (Jones and Mulherin 1998). ysis of freezing precipitation. Most damage occurred to hardwood stands; evergreen stands were able to withstand the ice loading. The b. Previous studies storm's impact on the forest not only destroyed plant life and the habitats of many animals, but it also affected One of the most extensive reviews of freezing pre- industries that rely on the forests. The storm interrupted cipitation studies appears in a U.S. Army report by Ben- logging and hauling for a week or more in some places, nett (1959). Bennett shows that freezing precipitation and losses to the region's maple syrup industry exceeded in the United States occurs east of the Rockies primarily $10 million USD (DeGaetano 2000). between November and March. An area of relatively Although the total elimination of these impacts is not high occurrence (greater than 6 days annually) extends possible for every storm, the development and imple- from northwestern Texas northeastward to New Eng- mentation of mitigation procedures based on a clima- land. Bennett estimates that within this ``glaze belt'' tology of these events may have reduced the amount of most areas will experience storms with ice accumula- damage to property, secondary economic losses due to tions between 0.64 and 1.27 cm (0.25 and 0.50 in.) once power outages, as well as the number of injuries and every 3 yr. In Canada and Alaska, limited data show fatalities. Losses to utilities may be reduced by vigilant that freezing precipitation is reported in up to 10% of tree trimming programs around transmission lines, de- the annual hourly observations across southern Alaska signing transmission lines for higher ice loads, provid- and west-central and southeastern Canada. ing electrical line redundancies in areas prone to ice Since Bennett's report, many regional and national storms, and designing procedures to melt and manually climatologies on freezing precipitation for the United remove accreted ice. Damage to forests may be reduced States and Canada have been published (McKay and by planting trees that can withstand heavy ice loading. Thompson 1969; Baldwin 1973; Gay and Davis 1993; Injuries and deaths may be reduced by an effective pub- Strapp et al. 1996; Robbins and Cortinas 1996; Branick lic education program in areas susceptible to storms that 1997; Bernstein and Brown 1997; Zerr 1997; La¯amme involve freezing precipitation. and PeÂriard 1998; Stuart and Isaac 1999; Cortinas 2000; The amount of property losses and injuries associated Cortinas et al. 2000; Bernstein 2000; Robbins and Cor- with severe ice storms is large. Changnon (2003) tinas 2002; Changnon and Karl 2003). These studies showed that 87 freezing rainstorms in the United States usually have focused on particular regions of North during 1949±2000 caused more than $16.3 billion (in America and/or one type of freezing precipitation. Mc- 2000 USD) in property losses. The highest frequency Kay and Thompson (1969), Strapp et al. (1996), Laf- of these storms occurred in the northeast United States. lamme and PeÂriard (1998), and Stuart and Isaac (1999) The Deep South, however, had the greatest percentage have produced climatographies of freezing rain and driz- of ice storms that produced insured property losses in zle in Canada. Their results show that freezing rain and excess of $1 million USD. Ice storms were responsible drizzle occur at all times of the year, including June, for 60% of the winter-related losses during 1988±95 and July, and August, when freezing drizzle can occur in 20% of all weather-caused injuries (Kocin 1997). the far northern regions. Throughout the rest of the year, In addition to the effects of freezing precipitation near freezing drizzle occurs more frequently than freezing Unauthenticated | Downloaded 09/30/21 07:01 AM UTC APRIL 2004 CORTINAS ET AL.