Simulation of space measurements of vegetation canopy bidirectional reflectance factors

Abstract

Abstract A method for simulating remote measurements of vegetation canopy bidirectional reflectance factors (BRFs) from satellite‐borne sensors is presented. The atmospheric radiative transfer problem is numerically solved with the lower boundary condition parameterized through vegetation canopy BRFs obtained from a numerical solution of the canopy radiative transfer equation. A horizontally homogeneous cloudless continental atmosphere with both molecular and aerosol loading is assumed. The one‐dimensional turbid medium model of a vegetation canopy complete with specular reflection from the leaf surface and the hot spot effect is used to evaluate canopy BRFs. The magnitude of atmospheric effects in the principal plane at red and near‐infrared wavelengths for two sun positions and increasing aerosol optical depths is evaluated. The net atmospheric effect is positive (negative) at the red (near‐infrared) wavelength due to strong scattering (absorption) in the atmosphere. It appears that the hot spot of a dense vegetation canopy can be detected by satellite‐borne devices at visible wavelengths. The influence of problem parameters on the anisotropy correction factors (g) used to convert broad band radiances to fluxes in radiant energy budget studies is studied. The g‐factors are greater than unity in the forward scattering directions where the actual radiance is less than the mean radiance. The smallest g‐factors are encountered in directions about the retro‐solar direction. These increase with aerosol optical depth depending on the wavelength and solar zenith angle.