Detailed Modelling of Shortwave Radiation Transfer through Forest Canopy using High Resolution Hemispherical Images

Abstract:

Solar radiation is an important driver for snow hydrological processes in forested environments and is a primary requirement in land surface and snowmelt models. The size, location and distribution of gaps in a forest canopy are particularly important controls on the shortwave radiation transmission to the snow surface. Hemispherical photography is an efficient means of characterizing canopy structure relative to a given position on the ground. It can provide the basis for calculating temporally variable sub-canopy incoming shortwave radiation at great detail. Here we present a fully integrated modelling tool for this very purpose, which calculates diffuse and direct shortwave radiation at a point based on sky-view fraction and canopy transmissivity in the path of the solar disc. Performance of the tool is demonstrated by comparison of simulated sub-canopy radiation to various data sets, including spatially distributed pyranometer measurements from a cable car system. We then integrate the radiation transfer modelling tool into the forest snow model FSM2. Inclusion of accurate shortwave irradiance patterns resulted in much improved snow simulations of the coupled model system. These results demonstrate the benefit of incorporating detailed canopy structure information into snow models for reproducing snowmelt dynamics in discontinuous forested environments.