Four-stream Radiative Transfer Parameterization Scheme in a Land Surface Process Model

摘要

Accurate estimates of albedos are required in climate modeling.Accurate and simple schemes for radiative transfer within canopy are required for these estimates,but severe limitations exist.This paper developed a four-stream solar radiative transfer model and coupled it with a land surface process model.The radiative model uses a four-stream approximation method as in the atmosphere to obtain analytic solutions of the basic equation of canopy radiative transfer.As an analytical model,the four-stream radiative transfer model can be easily applied efficiently to improve the parameterization of land surface radiation in climate models. Our four-stream solar radiative transfer model is based on a two-stream short wave radiative transfer model.It can simulate short wave solar radiative transfer within canopy according to the relevant theory in the atmosphere.Each parameter of the basic radiative transfer equation of canopy has special geometry and optical characters of leaves or canopy.The upward or downward radiative fluxes are related to the diffuse phase function,the G-function,leaf reflectivity and transmission,leaf area index,and the solar angle of the incident beam. The four-stream simulation is compared with that of the two-stream model.The four-stream model is proved successful through its consistent modeling of canopy albedo at any solar incident angle. In order to compare and find differences between the results predicted by the four-and two-stream models, a number of numerical experiments are performed through examining the effects of different leaf area indices, leaf angle distributions,optical properties of leaves,and ground surface conditions on the canopy albedo. Parallel experiments show that the canopy albedos predicted by the two models differ significantly when the leaf angle distribution is spherical and vertical.The results also show that the difference is particularly great for different incident solar beams. One additional experiment is carried out to evaluate the simulations of the BATS land surface model coupled with the two- and four-stream radiative transfer models.Station observations in 1998 are used for comparison.The results indicate that the simulation of BATS coupled with the four-stream model is the best because the surface absorbed solar radiation from the four-stream model is the closest to the observation.