Die Bestimmung der Reflexionsfunktion orographisch strukturierter Landoberflaechen

A new algorithm was developed which made it possible to calculate the BRDF (Bidirectional Reflection Distribution Function) of orographically structured land surfaces from both a digital terrain model and premeasured BRDFs of various homogeneous surface types. The constructed three-dimensional reflection model takes into account the vertical structure of the real surface as well as the anisotropic reflection of subscaled surface types and multiple reflection effects. The model was used to simulate the BRDFs of several test sites in the German alpine regions. Additionally, the BRDF of a corresponding flat surface with the same cover types was calculated. A typical deviation of 25% in the near-infrared region was found between the reflectance afactors of a flat surface and a vertically structured surface. If the vertical structure of the test sites is neglected, in most cases the albedo of the flat surfrace is higher compared with the corresponding albedo of the structured surface. It is concluded that in this spatial scale the assumption of a flat surface can not be a valid approximation for mountainous terrain. The model was validated with a comparison of measured and simulated satellite data in the solar AVHRR channels. The calculated BRDFs are taken as lower boundary of the earth-atmosphere system in a radiative transfer model. An average error of 12% in channel 1 and 11% in channel 2 in the simulated satellite data is caused by errors of the simulated BRDFs. Since the current aerosol loading and water vapor profile are not accurately known, an additional uncertainty in the simulated satellite data with 8% in the visible range and 4.5% in the near-infrared range is considered. In the near-infrared channel of the AVHRR a statistically significant influence of the orographic structure was found in the simulated radiances. It is concluded that the developed reflection model is able to simulate correctly the anisotropic reflection properties of vertical structured natural landscape as it can be found in mountainous terrain. (orig.)SIGLEAvailable from TIB Hannover: RN 437(95-27) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman