Spatial and Temporal Variabilities of Solar and Longwave Radiation Fluxes below a Coniferous Forest in the French Alps

At high altitudes and latitudes, snow has a large influence on hydrological processes. Large fractions of these regions are covered by forests, which have a strong influence on snow accumulation and melting processes. Trees absorb a large part of the incoming shortwave radiation and this heat load is mostly dissipated as longwave radiation. Trees shelter the snow surface from wind, so sub-canopy snowmelt depends mainly on the radiative fluxes: vegetation attenuates the transmission of shortwave radiation but enhances longwave irradiance to the surface. 13 pyranometers and 11 pyrgeometers were deployed on the snow surface below a coniferous forest at the CEN-MeteoFrance Col de Porte station in the French Alps (1325m asl) during the winters 2016-17 and 2017-18 in order to investigate spatial and temporal variabilities of solar and infrared irradiances in different meteorological conditions. Sky view factors measured with hemispherical photographs at each radiometer location ranged from 1.5 to 3.5. In clear sky conditions, the attenuation of solar radiation by the canopy reached 96% and its spatial variability exceeded 100 W.m-2. Longwave irradiance varied by 30 W.m-2 from dense canopy to gap areas. In overcast conditions, the spatial variabilities of solar and infrared irradiances were reduced and remained closely related to the sky view factor. Comparing the measurements at different radiometer locations, we investigated the dependence of surface net radiation on the overlying canopy density. Of particular interest were the atmospheric conditions that favor an offset between shortwave energy attenuation and longwave irradiance enhancement by the canopy, such that net radiation does not decrease with increasing forest density (situations of “radiation paradox”). It was found that cloud effects on the shortwave transmissivity and longwave emissivity factors of the canopy have a strong impact on the subcanopy radiation fluxes: canopy largely counteracts the effects of clouds on the incoming radiation fluxes. As a result, variations in net surface radiation due to forest cover appear to depend largely on meteorological conditions: “radiative paradox” conditions were more frequent during the winter of 2017 than in 2018, which was cloudier and colder. As a result, variations in surface net surface radiation by canopy cover appear to be largely dependent on weather conditions: “radiative paradox” conditions were more prevalent during the winter of 2017 than in 2018, which was cloudier and colder.