Processes of Solar Radiation Transfer Through a Sub-Arctic Shrub Canopy
Much of the low Arctic is covered with shrub tundra. Snowmelt energy exchange under shrub canopies is strongly influenced by the transmission of shortwave radiation through the canopy and the reflectance from snow under the canopy. Tundra shrub canopies are distinctive in that they are partially bur...
| Main Authors: | , , , |
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| Other Authors: | |
| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.526.8653 http://www.easternsnow.org/proceedings/2005/bewley.pdf |
| Summary: | Much of the low Arctic is covered with shrub tundra. Snowmelt energy exchange under shrub canopies is strongly influenced by the transmission of shortwave radiation through the canopy and the reflectance from snow under the canopy. Tundra shrub canopies are distinctive in that they are partially buried in winter and become exposed during snowmelt as snow depth decreases. The objectives of this study were to measure and model shortwave radiation reflection and extinction by one such dynamic heterogeneous deciduous shrub canopy over a melting snowcover. Observations were made in tundra shrub canopies of varying height in the mountains of the Yukon Territory, Canada. Measurements were taken over the course of spring snowmelt, during which solar elevation was low to moderate and the fractional area of exposed shrub overlying snow increased three-fold. Shrubs shaded most of the snow surface at low solar elevation angles, therefore a large component of shortwave radiation never reached the snow surface but was extinguished by the shrub canopy. At higher solar angles there was greater direct shortwave transmission to the snow surface between shrubs. A simple model was developed to incorporate this changing shade factor contribution to the overall shortwave transmissivity of shrub canopies. The model simulates both the landscape-averaged (areal) transmission and reflection of shortwave radiation, and results indicate that areal transmissivity and reflection can be up to 0.70 and 0.61 lower, respectively, relative to that simulated by a two-stream approximation in which no shadows are cast. A simple parameterization of this shaded landscape fraction is then proposed with which to integrate into hydrological and land surface models, for the purpose of simulating more accurately the melt rate and atmospheric fluxes in other areas of shrub tundra. |
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