Several fundamental physical properties of snow modulate energy exchanges between the surface and the atmosphere. The most important properties are the surface reflectance (albedo), the thermal insulating properties of snow, and the ability to change state (latent heat). Physical properties of a snowpack such as crystal structure, density, and liquid water content are also important for transfers of heat and water. These basic properties also determine the mechanical state of the snow cover, which is important for over-snow transportation and avalanche potential. The following paragraphs adapted from the EOS Science Plan (Goodison et al., 1999) outline the importance of these properties for the climate system:
The surface reflectance of incoming solar radiation is important for the surface energy balance (Wiscombe and Warren, 1981). Typical albedo values for non-melting snow-covered surfaces are high (~80-90%) except in the case of forests (see Table 1.1). The higher albedo for snow causes rapid shifts in surface reflectivity in autumn and spring in high latitudes. However, the overall climatic
1.2 Importance of snow in the climate system Table 1.1 Typical ranges for surface albedo.
Fresh, dry snow 0.80-0.95
Wet snow 0.50-0.70
Melting ice/snow 0.25-0.80
Snow-covered forest 0.25-0.40
Snow-free vegetation/soil 0.10-0.30
Water (high solar elevation) 0.05-0.10
significance is spatially and temporally modulated by cloud cover (planetary albedo is determined principally by cloud cover), and by the small amount of total solar radiation received in high latitudes during winter months. The high reflectivity of snow generates positive feedbacks to surface air temperature through the so-called "snow-albedo feedback," e.g. an initial warming results in a retreat in snow cover, lower albedo, higher absorbed solar energy, and warmer air temperatures. Grois-man et al. (1994a,b) observed that snow cover exhibited the greatest influence on the earth radiative balance in the spring (April to May) period when incoming solar radiation was greatest over snow-covered areas.
The thermal properties of snow also have important climatic consequences. Snow on the ground typically has a density in the range of 100-500 kg m-3 and the significant air fraction means snow is very effective at cutting off (or de-coupling) heat and moisture transfers from the ground surface to the overlying atmosphere. The thermal conductivity (a measure of the ability to conduct heat) of fresh snow is ~0.1 W m-1 K-1 which is 10-20 times lower than values for ice or wet soil. Snow also has important influences on heat flow through ice (e.g. river, lake, or sea ice). The flux of heat through thin ice continues to be substantial until it attains a thickness in excess of 30-40 cm. However, even a small amount of snow on top of the ice will dramatically reduce the heat flux and slow down the rate of ice growth. The insulating effect of snow also has major implications for the hydrological cycle. In non-permafrost regions, the insulating effect of snow is such that only near-surface ground freezes and deep water drainage is uninterrupted (Lynch-Stieglitz, 1994).
The large amount of energy required to melt ice (the latent heat of fusion, 3.34 x 105 J kg-1 at 0 °C) means that snow retards warming during the melt period. However, the strong static stability of the atmosphere over areas of extensive snow tends to confine the immediate cooling effect to a relatively shallow layer, so that associated atmospheric anomalies are usually short-lived and local to regional in scale (Cohen and Rind, 1991; Cohen, 1994). In some areas of the world such as Eurasia, the cooling associated with a heavy snowpack and moist spring soils is known to play a role in modulating the summer monsoon circulation (e.g.
Vernekar et al., 1995). Gutzler and Preston (1997) presented evidence for a similar snow-summer circulation feedback over the southwestern United States.
Snow-climate feedbacks such as the snow-albedo feedback operate over a wide range of spatial and temporal scales and the feedback mechanisms involved are often complex and incompletely understood. A major thrust of recent snow modeling work (see Chapter 4) is the correct representation of important snow processes in climate models in order to properly simulate the response of the climate system to external forcing such as increased greenhouse gases. A review of earlier efforts to model snow-climate interactions and feedbacks was provided by Groisman (in Jones et al., 2001).
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