The role of snow interception in winter-time radiation processes of a coniferous sub-alpine forest

The radiation balance in forests is modified by the presence of a (temporally snow-covered) canopy much more than it is in open areas. The primary effects of the forest canopy are absorption of incoming shortwave radiation, as well as absorption and emission of Ion-wave radiation. In this study, data from a mobile net radiation sensor moving along a 10-m bar in a coniferous sub-alpine stand of central Switzerland were analysed to assess the role of intercepted snow in comparison with other governing factors for winter-time radiation processes in a coniferous sub-alpine forest. The four winters investigated (2003-2007) covered a broad range of weather and snow conditions including several periods with intercepted snow lasting up to 19 days. For cloudless days, the data show that canopy albedo is elevated by the presence of intercepted snow. For transmissivity, the impact of intercepted snow is less distinct. Our analysis showed that the fraction of diffuse radiation and solar elevation are the main factors affecting shortwave transmissivity. Only after compensating for these effects, could intercepted snow be associated with increased transmissivities. Part of this analysis was carried out with the help of a physically based radiative transfer model. The model results suggest that multiple reflections are relevant to understanding the complex interactions between the factors affecting transmissivity. Finally, snow interception load can be derived reasonably well (R(2) = 0.65) from radiation measurements in this sub-alpine forest. Copyright (C) 2008 John Wiley & Sons, Ltd.