Canopy structure and air temperature inversions impact simulation of sub-canopy longwave radiation in snow-covered boreal forests

Longwave radiation is often the dominant source of energy for snowmelt in forests. Measurements at forest sites of varying density in Sweden and Finland show that downwelling longwave radiation is enhanced under forest canopies, even for sparse canopies and particularly for clear skies. Canopy density must be estimated accurately to predict this enhancement. Linear regression with above-canopy longwave radiation and air temperature as predictors of sub-canopy radiation gives good predictions of sub-canopy longwave radiation with weightings for transmission through canopy gaps that are close to measured sky view fractions. Air temperature serves here as a proxy for effective canopy radiative temperature. Adding above-canopy shortwave radiation as a predictor gives little improvement in the predictions, suggesting that daytime heating of trunks above the air temperature (“hot trees”) has limited influence on longwave radiation under these continuous canopies. The influence of canopy temperatures falling below the above-canopy air temperature (“cold trees”) on calm, clear nights, however, is apparent. Decoupling of canopy and above-canopy air temperatures in an energy balance model of the type used in large-scale land surface modeling allows this cooling.