Enhanced two-layer radiative transfer scheme for a land surface model with a discontinuous upper canopy

This paper describes a radiative transfer scheme in which vegetation canopies are represented by two layers: a more or less discontinuous upper canopy of trees or shrubs and a continuous lower canopy overlying the soil surface. The model employs a modified geometric-optic approach to estimate the probability that incident radiation will intersect one or more trees, and turbid media radiative transfer to estimate attenuation within the trees and in the underlying herbaceous layer. The primary difference between this and earlier models is that the structure of the model makes it suitable across a range of vegetation types, from woodland and forest to savanna and grassland. An empirical method for separation of green leaf and non-green interception and absorption is also presented. The scheme was developed for inclusion in a land surface model requiring short time-step (e.g. 10–30 min) resolution of the interception and absorption of solar energy by the land surface for use in computations of surface energy balance, evapotranspiration and photosynthesis. The model is applied with good results to a series of sites in Niger, West Africa, where detailed radiative transfer and canopy structural measurements were made during the HAPEX-Sahel experiment.