Improving representation of canopy temperatures for modeling subcanopy incoming longwave radiation to the snow surface

A comprehensive analysis of canopy temperatures was conducted around a small and large gap at a forested alpine site in the Swiss Alps during the 2015 and 2016 snowmelt seasons (March-April). Canopy temperatures within the small gap were within 2-3°C of measured reference air temperature. Vertical and horizontal variations in canopy temperatures were greatest around the large gap, varying up to 18°C above measured reference air temperature during clear-sky days. Night-time canopy temperatures around the study site were up to 3°C cooler than reference air temperature. These measurements were used to develop a simple parametrization for correcting reference air temperature for elevated canopy temperatures during 1) night-time conditions (sub-canopy shortwave radiation is 0 Wm-2) and 2) periods of increased sub-canopy shortwave radiation > 400Wm-2 representing penetration of shortwave radiation through the canopy. Sub-canopy shortwave and longwave radiation collected at a single point in the sub-canopy over a 24-hour clear sky period was used to calculate a night-time bulk offset of 3°C for scenario 1 and develop a multiple linear regression model for scenario 2 using reference air temperature and sub-canopy shortwave radiation to predict canopy temperature with an RMSE of 0.7°C. Outside of these two scenarios, reference air temperature was used to predict sub-canopy incoming longwave radiation. Modelling at 20 radiometer locations throughout two snowmelt seasons using these parametrizations reduced the mean bias and RMSE to below 10 Wm-2 at all locations.