A three-dimensional model for calculating reflection functions of inhomogeneous and orographically structured natural landscapes

A three-dimensional reflection model has been developed for calculating the reflection functions of inhomogeneous and orographically structured natural land surfaces. The site of an area whose reflection properties are to be modeled can be freely chosen; the task is limited, however, by its allocated CPU time. The surface structure of such an area is taken from a digital terrain model. The known anisotropic reflection properties of various flat surface types, for example, coniferous forest, hardwood forest, grassland, and agricultural cover types, are taken into account. The effects of multiple reflection in the subscale orographic structure of an area are calculated up to the second order of reflection. For the test sites, a typical difference of 25% in the reflection factors of a vertically structured surface and a corresponding flat surface was found, with the higher values for a flat surface. The model is validated by comparing simulated radiances with measured radiances in the solar channels of the AVHRR onboard NOAA-11 and NOAA-12 for several test sites in the German alpine region. The simulated radiances are calculated with an angular-dependent radiative transfer model with the modeled reflection functions as the lower boundary layer of the coupled Earth-atmosphere system. The results show good agreement between measured and simulated satellite data, especially in Channel 2, if the vertical structure of the surface is considered. A statistically significant influence of the orographic structure was found in the simulated radiances as compared with radiances calculated for a corresponding flat surface with the same cover types. It is concluded that the developed reflection model is able to simulate correctly the anisotropic reflection properties of vertically structured natural landscape as it can be found in mountainous terrain.