Terrestrial Snow

C. Derksen, R. Brown, and L. Wang

Climate Research Division, Environment Canada

October 7, 2009

Overview

Northern hemisphere terrestrial snow cover exhibits a high degree of intra- and inter-seasonal variability in spatial extent, covering up to 47 million km² in winter, and receding to as low as 4 million km² in summer. Across the Arctic and sub-Arctic, snow cover is a defining characteristic of the environment, covering the landscape for up to 9 months of the year. Unlike liquid precipitation, snowfall is stored on the land surface, redistributed by wind, and metamorphosed by various physical processes before the spring melt period. For a comprehensive perspective on terrestrial snow it is necessary to consider snow cover extent (SCE: the area covered by snow), snow cover duration (SCD: how long snow is on the ground), snow water equivalent (SWE: the amount of liquid water stored in the form of snow), and snow melt timing/duration (when and for how long the snow melts).

Various satellite and ground-based measurements are available to characterize these parameters across the northern hemisphere, and assess the 2007/08 and 2008/09 snow seasons relative to the historical record. Seasonal SCD departures (difference from the 1988-2007average) were computed from the NOAA snow extent data record maintained at Rutgers University (http://climate.rutgers.edu/snowcover/). SWE is more difficult to monitor than SCD due to high spatial variability, a sparse surface observation network across the Arctic, and uncertainties in satellite datasets. The Canadian Meteorological Centre (CMC) has produced a daily global snow depth analysis (~35 km resolution) since 1998 by combining the available ground observations with a snow model (Brasnett, 1999). The main snow melt onset date across the pan-Arctic land mass was derived from satellite scatterometer measurements via QuikSCAT, using the algorithm of Wang et al. (2008).

2007/08

The 2007/08 snow season was characterized by a shorter than average snow cover season over the central Canadian Arctic, most of Europe, and eastern Siberia, with a longer than average snow season in central China and the mid-latitudes of North America (Figure S1a). Time series of fall and spring SCD anomalies (departures divided by the standard deviation over the 1988-2007 period to give an indication of relative magnitude) across the North American and Eurasian sectors of the Arctic (north of 60N) are illustrated in Figure S1b. In fall, the 2007/08 snow cover season was close to the 1988-2007 average in both regions while in spring, the North American Arctic had the earliest disappearance of snow since the start of the NOAA record which dates to 1966.

Figure S1. (a) Snow cover duration (SCD) departures (with respect to 1988-2007) for the 2007/08 snow year and (b) Arctic seasonal SCD anomaly time series (with respect to 1988-2007) from the NOAA record for the first (fall) and second (spring) halves of the snow season. Solid lines denote 5-yr moving average. (c) Maximum seasonal snow depth anomaly for 2007/08 (with respect to 1998/99-2007/08) from the CMC snow depth analysis. (d) Terrestrial snow melt onset anomalies (with respect to 2000-2008) from QuikSCAT data derived using the algorithm of Wang et al. (2008).

Maximum seasonal snow depth anomalies for 2007/08 determined from the CMC analysis are shown in Figure S1c. Below-average snow accumulation occurred across the Himalayas, Europe and northern China; above-average snow depth occurred across central North America and Siberia.

The main snow melt onset date for 2008, relative to the 9-year QuikSCAT record (2000 to 2008), confirms regional early melt onset over the North American Arctic that matches the unusually early dates of snow disappearance identified in the NOAA record (Figure S1d).

2008/09

As for the 2008/09 snow season, there was a shorter than normal snow cover season across a large portion of eastern Siberia, with strong negative anomalies in the North American sector confined to the Canadian Arctic Islands (Figure S2a). When separated for fall and spring, and averaged regionally, negative SCD anomalies were evident across Eurasia in the fall, and the entire Arctic in spring (Figure S2b). The 2009 spring Arctic snow season can therefore be characterized as shorter than normal due primarily to an early disappearance of snow cover in spring.

The shorter than average snow season in 2008/09 occurred in spite of slightly deeper than average snow depth in many parts of the Arctic (Figure S2c). Snow melt onset anomalies from QuikSCAT observations show that the initial timing of snow melt was near normal, or slightly later than normal across large regions of western Siberia, northern Europe, and the Canadian tundra (Figure S2d). Collectively, these datasets suggest that although melt was initiated near the average time, it was of sufficient intensity to rapidly remove the snowpack across large regions of the Arctic.

Figure S2. (a) Snow cover duration (SCD) departures (with respect to 1988-2007) for the 2008/09 snow year and (b) Arctic seasonal SCD anomaly time series (with respect to 1988-2007) from the NOAA record for the first (fall) and second (spring) halves of the snow season. Solid lines denote 5-yr moving average. (c) Maximum seasonal snow depth anomaly for 2008/09 (with respect to 1998/99-2007/08) from the CMC snow depth analysis. (d) Terrestrial snow melt onset anomalies (with respect to 2000-2009) from QuikSCAT data derived using the algorithm of Wang et al. (2008).

References

Brasnett, B. 1999. A global analysis of snow depth for numerical weather prediction. Journal of Applied Meteorology. 38: 726-740.

Kitaev, L., E. Førland, V. Razuvaev, O. Tveito and O. Krueger. 2005. Distribution of snow cover over Northern Eurasia. Nordic Hydrology. 36: 311-319.

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