- Freezing rain events have important and dangerous impacts on the function of nearly all sectors regionwide
- The frequency of these events varies depending on the location
- While some conditions have become less favorable for creating freezing rain events, others have become more favorable and increased in strength over time
- While future precipitation trends are extremely difficult to predict, practitioners should still prepare for the effects of prolonged freezing rain events in the future
What is Freezing Rain, and How Does it Form?
Freezing rain occurs when frozen precipitation falls through a warm layer of air, causing the precipitation to melt and change from solid to liquid. However, because the surface where it lands is below freezing, the liquid precipitation freezes on contact, creating a dangerous icy layer. Typically, the severity of these events is not related to the intensity (i.e. volume of precipitation) of the event, but rather the duration.
Freezing rain events can be caused by multiple atmospheric patterns that have specific sets of conditions:
- Frontal occlusion, or when a cold front overtakes a warm front, can trap cold air near the Appalachian Mountains.
- Low lying valleys typically contain cold air, and when warm, moist air flows over this cold air, freezing rain events often occur.
- Strong pressure gradients caused by the proximity of cyclones, or centers of low pressure, and anticyclones, or centers of high pressure to one another, increasing advection and causing stronger winds. This is also called the cyclone-anticyclone pattern.
- Easterly flow from the Atlantic ocean holds cold air masses against the Appalachian mountains while warm, moist air masses flow over it. This is also called cold air damming.
Importance of Freezing Rain Events
Because freezing rain creates an often invisible layer of ice on roadways and other aspects of modern transportation infrastructure, these events can make travel dangerous. Additionally, the weight of the accumulated ice on various structures can lead to downed trees and powerlines, occasionally causing communities to lose power for extended periods of time. Overall, these events, while somewhat rare in occurrence, have the potential to cause billions of dollars in damage and create great risks to communities and human life. 1 2 3 4 5
Observed and Projected Changes in Freezing Rain Patterns
As the climate warms, winters in the Great Lakes region are warming faster than other seasons. We already observe a decrease in snow in the southern part of the Great Lakes Basin, as precipitation that would have, historically, been snow is falling as rain. Therefore, the rain-snow boundary is migrating north. Because freezing rain occurs at temperatures near freezing, it is plausible to expect freezing rain events to be more likely in regions that previously experienced snow. Other changes in the location of winter storms would also have an effect on freezing rain events.
Though the Great Lakes region experiences freezing rain events, the frequency of those events varies depending on geographical location. South-central Canada, comprising Ontario and Quebec, observed a 78% increase in the frequency of freezing rain by 2014, compared to the 1976-1985 baseline. Long Island, NY experienced an even larger increase in freezing rain frequency of more than 150% by 2014 compared to the 1976-1985 baseline. The regions of New Jersey, Pennsylvania, and New York with the exception of Long Island, have experienced decreases in the frequency of freezing rain events.
Observed cyclone-anticyclone patterns between 1997 and 2014 have shifted northward, as compared to 1979-1996. The cyclone part of this pattern has seemed to intensify, and the anticyclone has remained similar in strength. These events have increased from 25.2% to 13.8%, respectively. The cold air damming and cold air trapping patterns have changed very little between these two time periods, and the number of freezing rain events occurring with the frontal occlusion pattern has decreased by 13.6%.
These observations suggest that, while some freezing rain-producing patterns are decreasing, the patterns that are increasing (i.e. the cyclone-anticyclone pattern) typically last longer, leading to greater potential for property damage and loss of life over time.
Due to the complex microphysical processes and dynamics involved, future precipitation events, especially those related to freezing rain, are difficult to predict. Small changes in these dynamics can lead to large differences in precipitation type, and accurate modeling of the future is beyond the state of the science.
In summary, modeling future freezing rain trends remains beyond our capacity. However, the previously observed trends should motivate practitioners to consider and plan for increased impacts of freezing rain events in the future. Under projected climate conditions for the Great Lakes region, a future scenario considering freezing rain events that may be likely, is a general decrease in the frequency of freezing rain events, with a further concentration of freezing rain events during the winter months. A warming cold-season climate does not mean that freezing rain will not occur in the future, but may be concentrated in events that are strong and more associated with local conditions as opposed to large-scale weather systems.
- Bernstein, B. C., 2000: Regional and local influences on freezing drizzle, freezing rain, and ice pellet events. Weather and forecasting,15 (5), 485–508
- Changnon, S. A., and K. E. Kunkel, 2006: Severe storms in the midwest. ISWS Informational/Educational Materials 2006-06.
- Davis, N., A. N. Hahmann, N.-E. Clausen, and M.ˇZagar, 2014: Forecast of icing events at a wind farm in sweden. Journal of Applied Meteorology and Climatology,53 (2), 262–281
- Lott, N., and T. Ross, 2006: Tracking and evaluating us billion dollar weather disasters, 1980-2005. Asheville, NC: NOAA National Climatic Data Center
- Murphy, C., and Coauthors, 2020: Adapting existing energy planning,simulation, and operational models for resilience analysis. Tech.rep., National Renewable Energy Lab.(NREL), Golden, CO (UnitedStates).