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Much of the low Arctic is covered with shrubs that are partially buried by snow in winter and become exposed during melt. This study presents measurements and modeling of shortwave radiation reflection and extinction by a deciduous shrub canopy emerging from a melting snowcover in the mountains of the Yukon Territory, Canada. Shrubs shade most of the snow surface at low solar elevation angles, so only a fraction of the incoming radiation reaches the surface, but there is greater direct shortwave transmission to the surface in gaps between shrubs for higher solar elevations. A simple model is developed to incorporate the changing contributions of sun-lit gaps, shaded gaps, and shrubs to the landscape-averaged (areal) transmission and reflection of shortwave radiation. The areal transmissivity and albedo in this model are lower than in a two-stream approximation that neglects gap shading. A simple shadow parameterization is proposed for calculating shrub tundra snowmelt rates and surface energy balances in hydrological and land-surface models. [ABSTRACT FROM AUTHOR]

At high latitudes, longwave radiation can provide similar, or higher, amounts of energy to snow than shortwave radiation due to the low solar elevation (cosine effect and increased scattering due to long atmospheric path lengths). This effect is magnified in mountains due to shading and longwave emissions from the complex topography. This study examines longwave irradiance at the snow surface in the Wolf Creek Research Basin, Yukon Territory, Canada (60°36′N, 134°57′W) during the springs of 2002 and 2004. Incoming longwave radiation was estimated from standard meteorological measurements by segregating radiation sources into clear sky, clouds and surrounding terrain. A sensitivity study was conducted to detect the atmospheric and topographic conditions under which emission from adjacent terrain significantly increases the longwave irradiance. The total incoming longwave radiation is more sensitive to sky view factor than to the temperature of the emitting terrain surfaces. Brutsaert's equation correctly simulates the clear-sky irradiance for hourly time steps using temperature and humidity. Longwave emissions from clouds, which raised longwave radiation above that from clear skies by 16% on average, were best estimated using daily atmospheric shortwave transmissivity and hourly relative humidity. An independent test of the estimation procedure for a prairie site near Saskatoon, Saskatchewan, Canada, indicated that the calculations are robust in late winter and spring conditions. [ABSTRACT FROM AUTHOR]

Surface energetics and snow ablation were examined during the 1999 snowmelt season in a mountain subarctic tundra valley in the Yukon Territory of Canada. Considerations of melt energetics at small scales were made with respect to the frame of reference of the sloping surface snowpack. During relatively warm and sunny conditions early in melt, snow ablation rates were dramatically higher on the south-facing slope and strongly reduced on the north-facing slope, compared to the valley bottom. Negative spatial covariances developed between maximum snow accumulation and ablation rate during early and middle melt, with the highest ablation rates occurring on slopes with the shallowest snowpacks. Atmospheric conditions were sufficiently well mixed across the valley that reference level air temperatures and humidity among the slopes were close to levels of measurement accuracy. However, under high levels of April insolation, notable differences in incoming solar radiation to varying slopes/aspects caused relatively larger differences in net radiation and surface temperature, which were progressively magnified as shrubs and soil became exposed during snow ablation. Under cloudier conditions later in melt, the south-facing snowpack had mostly ablated, vegetation was exposed at all sites, and ablation rates were virtually identical between the valley bottom and north-facing slope. Driven primarily by initial differences in insolation and snow accumulation, surface energy fluxes changed sign and magnitude over space, not only with insolation, vegetation cover, slope, and aspect, but also with the snow cover state and ground/vegetation exposure. Melt rate was, hence, controlled by both incoming energy and evolving and initial snow states. For these reasons, and because of the slope-based frame of reference necessary to precisely define the snowmelt energy balance, simple aggregate representations of melt in subarctic mountain environments that are based on averaged energy flux, snow state, and flat-plane conceptions may require substantive corrections that should be explored in modeling studies. [ABSTRACT FROM AUTHOR]