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Abstract Freezing rain is highly disruptive to society, ecology, and transportation; however, the driving physical processes governing variability and trends in freezing rain have not been extensively studied. This work investigated the temporal and spatial variability of freezing rain occurrence across eastern North America, using ERA5 reanalysis data from 1941 to 2020. After validation of freezing rain frequencies over time using 132 station sites, relationships between freezing rain and several modes of natural variability that influence North American temperature and hydroclimate are investigated across subdomains. Additionally, random forest regression analysis was used to examine nonlinear relationships between freezing rain and natural variability. Results indicate that within the eastern continental United States, freezing rain is more common with a weaker Aleutian low/anomalous ridge, or a negative Pacific–North American pattern—linked to Alaskan/North Pacific ridging—and cold air intrusion into continental North America. The long-duration modes (e.g., decadal oscillations) generally exert weaker influence at the studied time scales but can modulate the effect of some of the other modes. When evaluating these patterns using random forest, nonlinearities become apparent, particularly with the Arctic Oscillation and El Niño–Southern Oscillation, where highly positive and negative values are linked to more freezing rain in some domains. The analysis also revealed a generally limited signal for significant trends in freezing rain over time, though northernmost domains show increases. The fact that notable trends in freezing rain frequency (particularly decreasing trends in the south) have not yet occurred considering global climate change indicates that the role of natural climate variability is the dominant driver of historical variations. Significance StatementThe purpose of this study was to examine long-term trends in freezing rain across central and eastern North America. The study first evaluated frequencies over time, noting that there are limited overall trends (increasing and decreasing). One of the main reasons for this is due to the important role that natural climate variability plays in modulating the occurrence of freezing rain events. Natural climate variability that promotes higher atmospheric pressure over the Pacific sub-Arctic regions appears to be strongly indicative of favorable conditions for freezing rain, particularly in the United States. This work extends our knowledge of the important factors that drive seasonal to multiyear freezing rain variability.
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Characteristics, Evolution, and Formation of Cold Air Outbreaks in the Great Plains of the United States
Abstract Wintertime cold air outbreaks (CAOs) in the Great Plains of the United States have significant socioeconomic, environmental, and infrastructural impacts; the events of December 1983 and February 2021 are key examples of this. Previous studies have investigated CAOs in other parts of North America, particularly the eastern United States, but the development of CAOs in the Great Plains and their potential subseasonal-to-seasonal (S2S) predictability have yet to be assessed. This study first identifies 37 large-scale CAOs in the Great Plains between 1950 and 2021, before examining their characteristics, evolution, and driving mechanisms. These events occur under two dominant weather regimes at event onset: one set associated with anomalous ridging over Alaska and the other set associated with anomalous pan-Arctic ridging. Alaskan ridge CAOs evolve quickly (i.e., on synoptic time scales) and involve stratospheric wave reflection. Conversely, Arctic high CAOs are preceded by weak stratospheric polar vortex conditions several weeks prior to the event. Both categories of CAOs feature anomalous upward wave activity flux from Siberia, with downward wave activity flux over Canada seen only in the Alaskan ridge CAOs. The rapid development of the Alaskan ridge CAOs, also linked with a North Pacific wave train and anomalous wave activity flux from the central Pacific, suggests that these events could be forced by tropical modes of variability. These findings present evidence that different forcing mechanisms, with contrasting time scales, may produce distinct sources of predictability for these CAOs on the S2S time scale.
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- Award ID(s):
- 1946093
- PAR ID:
- 10397996
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 35
- Issue:
- 14
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- 4585 to 4602
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1175/JCLI-D-21-0772.1
