Basic climatic elements 231 Snow cover

Most of the Arctic land and sea ice surface has a snow cover for at least 6-8 months of the year. For a number of reasons snow cover is a key climatic variable. These include: (1) its high albedo (reflectivity in solar wavelengths), typically 0.80 to 0.90 for new snow; (2) the insulating effect it has on the underlying tundra or sea ice; and (3) its

Figure 2.15 Average number of weeks of snow cover over the Northern Hemisphere, based on the NSIDC blended weekly product for 1972-2001 (courtesy of M.J. Brodzik, NSIDC, Boulder, CO).

role in storing precipitation over terrestrial regions; this is released as river discharge during spring and summer. As we have already seen, this impacts strongly on the vertical structure of the Arctic Ocean and hence the sea ice cover.

Figure 2.15 shows the generalized distribution of snow cover duration over the northern continents. The impact of latitude is obvious - in northern Canada, Alaska and Asia, snow is typically present 30-40 weeks per year. Terrestrial snow cover classes have been developed according to climatic and vegetational zones, as well as on the basis of depth, duration and density (Formozov, 1946; Rikhter, 1954; Epenshade and Schytt, 1956; Bilello, 1957; Benson, 1969). A classification for land that takes account of the stratigraphic and textural attributes of a snowpack is proposed by Sturm et al. (1995). The classification has a number of categories of seasonal snow cover: tundra, taiga, maritime, alpine, prairie and ephemeral. There is also a special class for highly variable mountain snow cover. While several of the names follow earlier schemes based on climatic/vegetation zones, these designations do not require such geographical associations; only the relevant physical characteristics are considered (see Table 2 in Sturm et al., 1995). The classes were identified on the basis of a

Figure 2.16 Maximum snow depth (mm) over Eurasia compiled from Russian sources (courtesy of H. Ye, California State University, Los Angeles, CA).

statistical analysis of field measurements and climatic data for Alaska. Five of the six classes are found in Alaska. Tundra and taiga snow dominate most of the Arctic. Tundra snow is a thin, wind-blown snow, with maximum depths of about 750 mm, and a bulk density of typically 380 kg m-3. It consists of a basal layer of depth hoar, overlain by multiple wind slabs. Surface sastrugi are common. Depth hoar represents a low-density (low cohesion or poorly bonded) layer formed by sublimation associated with a strong temperature gradient in the snow. Sastrugi refer to drifts of wind-packed snow aligned along the prevailing wind direction. Taiga snow is thin to moderately deep (300-1200 mm) with a lower density than tundra snow (260 kg m-3). It is found in forest climates where wind, initial snow density and air temperature are all low.

There are a number of terrestrial snow depth data sets for land areas although these have limited spatial coverage and resolution. High Arctic coverage is poor, especially over the Canadian sector. Figure 2.16 represents an attempt to map the mean annual maximum snow depth for Eurasia, based on Russian data. Depending on the region, maximum snow depth tends to occur between February and early April. North of 60° N, maximum snow depths range from 300 to 800 mm. Assuming an average density value of about 300 kg m-3 across Eurasia yields maximum snow water equivalents of about 100 to 270 mm. On a local scale the snowpack thickness varies considerably in relation to terrain. Surveys carried out over seven years in a small basin near Resolute Bay, NWT (Woo et al., 1983), show that gullies and valleys have the largest accumulations, and exposed hilltops the least. This basic pattern tends to be maintained from year to year. Amounts on slopes are variable according to exposure to the prevailing wind and locally concave/convex surfaces. The snow melt season on the tundra is typically brief, lasting only about two weeks (Weller et al., 1972). When bare ground begins to appear, local heat advection from the warmer exposed surface accelerates the melt of the shrinking snow patches.

Depth data were collected by Russian scientists during airborne landings at many locations across the ice covered Arctic Ocean, annually each spring between 1959 and 1988 under the Sever ("North") program (Romanov, 1995). These records provide average snow thickness on the predominant sea ice type in the landing area, in sastrugi

Figure 2.17 Mean annual cycle of snow depth (mm) and ±1 standard deviation over the central Arctic Ocean based on NP data (from Colony et al., 1998, by permission of Cambridge University Press).

behind pressure ridges, and an average for the windward and lee side of hummocks (Colony et al., 1998). More consistent spatial information has been derived from the snow survey lines at the Russian NP drifting stations during 1954-91. These represent measurements from snow lines of one kilometer length as well as at snow stakes installed at the meteorological station sites. Measurements were made daily at the stakes and once to three times monthly along the snow lines at 10-m intervals. Snow density along each line was sampled at every 100 m. Snow lines were chosen to be over flat ice.

The mean seasonal cycles in mean snow height (essentially equivalent to snow depth and hence treated as such here), snow density and the ±1 standard deviations based on grouping all available snow line data are reproduced in Figure 2.17 and Figure 2.18. The distribution of snow depths in April is given in Figure 2.19. Snow depth is greatest in May, with a mean of about 350 mm. The onset of melt is observed as a rapid decline in depth between June and July to a minimum of about 50 mm in August. The standard deviation of snow depth is fairly large and naturally increases with mean snow depth. This represents both spatial and year-to-year variability. During April, snow depths range from near zero to over 1000 mm, pointing to drifting and wind scour. As the snow lines are over flat ice, the standard deviation in snow depth is lower than would be obtained for regional averages that include rough ice. Taking the mean snow density and depth, the average water equivalent during April and May is about 100-110 mm. Locally, the snow depth also depends on the ice age; when a lead forms in the ice in winter, new ice growth takes place. The snow then accumulates on the surface of the new ice from snowfall and drifting.

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Figure 2.18 Mean annual cycle of snow density (kg m-3) and ±1 standard deviation over the central Arctic Ocean based on NP data (from Colony et al, 1998, by permission of Cambridge University Press).

Figure 2.19 Frequency distribution of snow depth (mm) over the central Arctic Ocean for April from NP data (from Colony et al., 1998, by permission of Cambridge University Press).

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