The influence of tree temperatures on potential snowmelt energy in a discontinuous coniferous forest

Abstract:

The heating of tree trunks and canopy elements is known to contribute to snow surface longwave irradiance; however, mapping the snowmelt energy contribution from individual trees in a discontinuous forested environment remains unexplored. A measurement campaign at the Marmot Creek Research Basin, Alberta, Canada documented the meteorological forcing and resulting tree temperatures on shaded and sun-exposed sides of a forest clearing. Physically based model estimates of trunk and canopy temperatures of individual trees were compared to measurements. Two trunk temperature models were evaluated: a PDE-based heat transfer model and a computationally efficient energy balance model including internal heat storage effects. The models simulated surface temperatures on eight azimuthal trunk sides. Both models performed favorably (RMSE and biases better than 1.7°C and ±0.4°C) with the PDE model that simulated a bark layer best capturing diurnal temperature spikes. On the sun-exposed edge, simulated daily maximum trunk temperatures exceeded air temperature values by 10°C - 15°C and were highest later in the day on the southwestern side; temperatures on the northern side exceeded daytime air temperatures by only 1.5°C. Measurements and models agreed that the trunk surfaces returned to or cooled below ambient air temperature values near sunset; however, the PDE model tracked internal heat storage from daytime surface heating well into the night. On the shaded side of the clearing, daytime trunk temperatures exceeded air temperatures by 3°C to 6°C and the deviations were highly sensitive to the timing and duration of intermittent insolation. Canopy temperatures modeled with an energy balance approach were within the range of thermocouple measurements and exceeded daytime air temperatures by 5°C to 8°C and 1°C to 3°C on the sun-exposed and shaded clearing edges, respectively. Radiative transfer modeling was used to map the energy contribution of trees to the snow surface. Trees provided substantial, but localized, longwave irradiance with higher simulated values on the southern sides of trees and on the sun-exposed clearing edge. The results suggest that snow energy balance models of forested areas with small clearings could be improved by including treatment of gap edges and differences between sunlit and shaded edges.