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

Webster, Clare, Rutter, Nick and Jonas, Tobias (2017) Improving representation of canopy temperatures for modeling subcanopy incoming longwave radiation to the snow surface. Journal of Geophysical Research - Atmospheres, 122. pp. 9154-9172. ISSN 2169-8996

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Abstract

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.

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