IR Radiation from Trees to a Ski Run: A Case Study

Accurately calculating the surface radiation budget of a groomed ski run is crucial when determining snow surface temperature and other snow-related variables, knowledge of which is important for ski racing. Downwelling longwave radiation can compose a large part of the surface radiation budget in mountainous terrain. At a location on a ski run, a portion of the downwelling longwave radiation comes from the sky and a portion comes from tall evergreen trees. Infrared photographs taken during daytime at a ski run on Whistler Mountain, British Columbia, Canada, for a clear-sky day in February 2012 show that trees can enhance the downwelling longwave radiation at the center of the ski run considerably, with a maximum estimated enhancement of 75.6 +/- 16.8 W m (-2) for trees in direct sunlight. The average needle and trunk brightness temperatures from the IR photographs were correlated with measured meteorological data. Regressions were found to allow estimation of longwave radiation from trees using nearby routine meteorological data. Absolute errors in tree longwave radiation estimations using the derived trunk and needle temperatures did not exceed 4 W m (2). The effect of the intervening air upon longwave radiative transfer between trees and the point of interest on the ski run was found to be small for these very short pathlengths of 50 m or less. These results can be used to improve calculations of the surface radiation budget of a groomed ski run under clear skies.