Abstract
Abstract. The 0 ∘C temperature threshold is critical for many meteorological and hydrological processes driven by melting and freezing in the atmosphere, surface, and sub-surface and by the associated precipitation varying between rain, freezing rain, wet snow, and snow. This threshold is especially important in cold regions such as Canada, because it is linked with freeze–thaw, snowmelt, and permafrost. This study develops a Canada-wide perspective on near-0 ∘C conditions using hourly surface temperature and precipitation type observations from 92 climate stations for the period from 1981 to 2011. In addition, nine stations from various climatic regions are selected for further analysis. Near-0 ∘C conditions are defined as periods when the surface temperature is between −2 and 2 ∘C. Near-0 ∘C conditions occur often across all regions of the country, although the annual number of days and hours and the duration of these events varies dramatically. Various types of precipitation (e.g., rain, freezing rain, wet snow, and ice pellets) sometimes occur with these temperatures. Near-0 ∘C conditions and the reported precipitation type occurrences tend to be higher in Atlantic Canada, although high values also occur in other regions. Trends of most temperature-based and precipitation-based indicators show little or no change despite a systematic warming in annual surface temperatures over Canada. Over the annual cycle, near-0 ∘C temperatures and precipitation often exhibit a pattern: short durations occur around summer, driven by the diurnal cycle, and a tendency toward longer durations around winter, associated with storms. There is also a tendency for near-0 ∘C surface temperatures to occur more often than expected relative to other temperature windows at some stations due, at least in part, to diabatic cooling and heating that take place with melting and freezing, respectively, in the atmosphere and at the surface.
Highlights
In cold regions such as Canada, both environmental processes and socio-economic activities are significantly impacted by temperatures near 0 ◦C
As this study focuses on the identification of conditions near 0 ◦C at the surface along with the associated precipitation, hourly surface temperature and the manual precipitation type observations across Canada were retrieved from the archive
We note that WMO Solid Precipitation Intercomparison Experiment (SPICE) broke precipitation down into three categories based on surface temperature (T ): snow < −2 ◦C, mixed precipitation −2 ◦C ≤ T ≤ 2 ◦C, and rain > 2 ◦C (Nitu et al, 2018); this approach was similar to that used by Yang et al (1995, 1998)
Summary
In cold regions such as Canada, both environmental processes and socio-economic activities are significantly impacted by temperatures near 0 ◦C. Periodic transitional episodes from below to above 0 ◦C (or vice versa) can have adverse effects including mid-winter ice jams and related flooding (e.g., Beltaos et al, 2006; Lindenschmidt et al, 2016), freeze–thaw damage to infrastructure (e.g., Kraatz et al, 2019), unseasonal frosts (e.g., McKenney et al, 2014), and recreation impacts (skiing, avalanches; e.g., Moen and Fredman, 2007; Laute and Beylich, 2018). If these periods are associated with precipitation If near-0 ◦C occurrences change, this will have subsequent impacts on all of these issues
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