An ecoregional assessment of freezing season air and ground surface temperature in the Mackenzie Valley corridor, NWT, Canada

- Assesses a common air-freezing threshold as an indicator of sufficient ground freezing
- Characterizes latitudinal and temporal trends in temperature indices across ecoregions
- Air temperature trend of 0.75 °C/N° in the air, while surface temperatures increase by 0.9 °C/N°
- The use of a common air-freezing threshold appears inappropriate across all ecoregions analyzed

Data from a network of air and ground surface temperature monitoring sites along a transect from the low arctic tundra to the taiga plains and the midboreal forest were utilized to explore latitudinal and temporal trends in various derived temperature indices. Parameters examined include air and ground surface mean annual temperatures, freezing degree-days, as well as freezing n-factors, and timing of the onset and completion of freezing at the ground surface. Temperature data show an N-S trend of about 0.75 °C increase per degree of latitude in the air, while ground surface temperatures increase by 0.9 °C per degree of latitude. Local ranges around the regional trend are between about ± 1 °C to ± 2 °C in the air and about ± 2 °C to ± 6 °C at the ground surface, with the range increasing northward for both sets of data.The appropriateness of using a common air-freezing index threshold value as an indicator of sufficient ground freezing for winter construction and transportation activities was investigated by determining the accumulated surface freezing degree-days (FDDS) on the date the air-freezing index reached 300 degree-days. This FDDS value was highly variable and in 20% of cases was less than 10. These low values reflect a slower rate of the active layer freeze-back, especially for more southerly study sites where the value of the freezing n-factor is low (0.1-0.3) and the active layer is thick (1-1.5 m). In the north, analysis shows a substantial range in the freezing n-factor, the greatest range in active layer freeze-back and the highest range in near-surface ground temperatures, suggesting that a single air temperature index is perhaps not appropriate for all sites within the ecoregions examined.