Modeling microwave emissions of erg surfaces in the Sahara desert

Sand seas (ergs) of the Sahara are the most dynamic parts of the desert. Aeolian erosion, transportation, and deposition continue to reshape the surface of the ergs. The large-scale features (dunes) of these bedforms reflect the characteristics of the sand and the long-term wind. Radiometric emissions from the ergs have strong dependence on the surface geometry. We model the erg surface as composed of tilted rough facets. Each facet is characterized by a tilt distribution dependent upon the surface roughness of the facet. The radiometric temperature (T_b) of ergs is then the weighted sum of the T_b from all the facets. We use dual-polarization T_b measurements at 19 and 37 GHz from the Special Sensor Microwave Imager aboard the Defense Meteorological Satellite Program and the Tropical Rainfall Measuring Mission Microwave Imager to analyze the radiometric response of erg surfaces and compare them to the model results. The azimuth angle (φ) modulation of T_b is caused by the surface geometrical characteristics. It is found that longitudinal and transverse dune fields are differentiable based on their polarization difference (ΔT_b) φ-modulation, which reflects type and orientation of dune facets. ΔT_b measurements at 19 and 37 GHz provide consistent results. The magnitude of ΔT_b at 37 GHz is lower than at 19 GHz due to higher attenuation. The analysis of ΔT_b over dry sand provides a unique insight into radiometric emission over ergs.