A sensitivity study of daytime net radiation during snowmelt to forest canopy and atmospheric conditions

摘要

This study investigates the dependence of net radiation at snow surfaces under forest canopies on the overlying canopy density. The daily sum of positive values of net radiation is used as an index of the snowmelt rate. Canopy cover is represented in terms of shortwave transmissivity and sky-view factor. The cases studied are a spruce forest in the Wolf Creek basin, Yukon Territory, Canada, and a pine forest near Fraser, Colorado. Of particular interest are the atmospheric conditions that favor an offset between shortwave energy attenuation and longwave irradiance enhancement by the canopy, such that net radiation does not decrease with increasing forest density. Such an offset is favored in dry climates and at high altitudes, where atmospheric emissivities are low, and in early spring when snow albedos are high and solar elevations are low. For low snow albedos, a steady decrease in snowmelt energy with increasing canopy cover is found, up to a forest density close to the actual densities of mature spruce forests. Snowmelt rates for high albedos are either insensitive or increase with increasing canopy cover. At both sites, foliage area indices close to 2 are associated with a minimum in net radiation, independent of snow albedo or cloud cover. However, these results are more uncertain for open forests because solar heating of trees may invalidate the longwave assumptions, increasing the longwave irradiance.