A fast, accurate algorithm to account for non-Lambertian surface effects on TOA radiance

Surface bidirectional reflectance distribution function (BRDF) influences b oth the radiance just above the surface and that emerging from the top of t he atmosphere (TOA). In this study we propose a new, fast, and accurate alg orithm CASBIR (correction for anisotropic surface bidirectional reflection) to account for such influences on TOA radiance. This new algorithm is base d on four-stream theory that separates the radiation field into direct and diffuse components in both upwelling and downwelling directions. Such a sep aration is important because the direct component accounts for a substantia l portion of incident radiation under a clear sky, and the BRDF effect is s trongest in the reflection of the incident direct radiation. The model is v alidated by comparison with a full-scale, vector radiative transfer model f or the atmosphere-surface system [Ahmad and Fraser, 1982] for wavelengths f rom UV to near-IR over three typical but very different surface types. The result demonstrates that CASBIR is accurate for all solar and viewing zenit h and azimuth angles considered, with overall relative difference of less t han 0.7%. Application of this algorithm includes both accounting for non-La mbertian surface scattering on the emergent radiation above TOA and develop ing a more effective approach for surface BRDF retrieval from satellite-mea sured radiance. Comparison with the result from the Lambertian model indica tes that surface BRDF influence on TOA radiance is both angle and wavelengt h dependent. It increases as solar zenith angle decreases or wavelength inc reases and becomes strongest in the view directions where the surface refle ction is most anisotropic (such as in the hot spot or Sun glint regions).