Implementation and Evaluation of a Generalized Radiative Transfer Scheme within Canopy in the Soil–Vegetation–Atmosphere Transfer (SVAT) Model

The process of radiative transfer over vegetated areas has a profound impact on energy, water and carbon balances over the terrestrial surface. In this paper, a generalized radiative transfer scheme (GRTS) within canopy is implemented in the Simplified Simple Biosphere land surface model (SSiB). The main concept and structure of GRTS and its coupling methodology to a land model are presented. Different from the two-stream method, the GRTS takes into account the effects of complex canopy morphology and inhomogeneous optical properties of leaves on radiative transfer process within the canopy. In the offline SSiB/GRTS simulation for the period of 2001–2012, the non-uniform leaf angle distribution within canopy layers is considered in SSiB/GRTS in the areas of evergreen broadleaf trees. Compared with the SSiB/two stream method, SSiB/GRTS produces lower canopy reflectance and higher transmittance, which leads to more realistic albedo simulation. The canopy absorbed radiation flux in SSiB/GRTS simulation is lower than that in SSiB/two stream method simulation throughout the year in the areas of evergreen broadleaf trees. The largest difference of −18.4W/m² occurs in the Amazon region in the autumn. The ground absorbed radiation flux increases in the SSiB/GRTS simulation, especially in the spring and autumn. The largest difference in the ground absorbed radiation flux between SSiB/GRTS simulation and SSiB/two stream method simulation is 25.45 W/m². In the boreal winter season, compared with the two-stream method in the SSiB, the GRTS gives higher surface albedo in the areas with high snow cover fraction over leaf.