A clumped-foliage canopy radiative transfer model for a global dynamic terrestrial ecosystem model. I: Theory

Abstract

This study develops a simple but physically based canopy radiative transfer scheme for photosynthesis, radiative fluxes and surface albedo estimates in dynamic global vegetation models (DGVMs), and particularly for the Ent Dynamic Global Terrestrial Ecosystem Model (Ent DGTEM). The Ent DGTEM can represent vegetation in mixed as opposed to homogeneous canopies. With active growth and competition, it must predict radiative transfer for dynamically changing vegetation structure, and requires computational speed for coupling with atmospheric general circulation models (GCMs). The canopy radiative transfer scheme accounts for both vertical and horizontal heterogeneity of plant canopies by combining the simple two-stream scheme with a well-described actual vertical foliage profile, an analytically derived foliage clumping factor from geometric optical theory, and, for needleleaf trees, an empirical needle-to-shoot-clumping factor. In addition, the model accounts for the effect of trunks, which is significant in bare canopies. This model provides better radiation estimates (light profiles, albedo) than the two-stream scheme currently being used in most GCMs to describe light interactions with vegetation canopies. This scheme has the same computational cost as the current typical scheme being used in GCMs, but promises to provide better canopy radiative transfer estimates for DGVMs, particularly those that model heterogeneous vegetation canopies.