A TOPOGRAPHIC SOLAR RADIATION TRANSFER MODEL FOR GEOSCIENCE APPLICATIONS IN HIGH-MOUNTAINS

Complex radiative transfer interactions between the atmosphere and topography make it difficult to use satellite imagery for geoscience applications in high-mountains. Multi-scale topographic effects on the irradiant and radiant flux produce the “topographic effect” in satellite imagery. Anisotropic-reflectance correction is required to account for multi-scale topographic influences, solar and viewing geometry and the bi-directional reflectance distribution function (BRDF). In general, empirical and semi-empirical models to not account for many factors which are required to explain sensor response. Consequently, a topographic solar radiation transfer model was developed to account for the scene spectral variance in satellite imagery. The model was used to study the influence of topography on radiation parameters and sensor response in order to evaluate the efficacy of operational anisotropic-reflectance correction of satellite imagery.

A SPOT near-infrared satellite image and a digital elevation model (DEM) were used as boundary conditions to model radiation transfer processes over the Nanga Parbat massif in Northern Pakistan. The extreme relief and complex topography make this an ideal location for studying multi-scale topographic effects. Computer simulations indicate that the model can be used to explain the majority of the scene spectral variance using simple parameterizations to account for the topography and the surface BRDF. Simulation results indicate that the diffuse-irradiance component is essential in modeling efforts. Furthermore, the BRDF image was found to be suitable for mapping applications. These results indicate that the model works very well in complex terrain. Model accuracy, however, is a function of DEM accuracy.