Microwave backscatter modeling of erg surfaces in the Sahara desert

Abstract

Abstract-The Sahara desert includes large expanses of sand dunes called ergs. These dunes are formed and constantly reshaped by prevailing winds. Previous study shows that Saharan ergs exhibit significant radar backscatter ( ) modulation with azimuth angle ( ). We use measurements observed at various incidence angles and from the NASA Scatterometer (NSCAT), the SeaWinds scatterometer, the ERS scatterometer (ESCAT), and the Tropical Rainfall Measuring Mission's Precipitation Radar to model the response from sand dunes. Observations reveal a characteristic relationship between the backscatter modulation and the dune type, i.e., the number and orientation of the dune slopes. Sand dunes are modeled as a composite of tilted rough facets, which are characterized by a probability distribution of tilt with a mean value, and small ripples on the facet surface. The small ripples are modeled as cosinusoidal surface waves that contribute to the return signal at Bragg angles only. Longitudinal and transverse dunes are modeled with rough facets having Gaussian tilt distributions. The model results in a response similar to NSCAT and ESCAT observations over areas of known dune types in the Sahara. The response is high at look angles equal to the mean tilts of the rough facets and is lower elsewhere. This analysis provides a unique insight into scattering by large-scale sand bedforms.

Seven Years of Advanced Synthetic Aperture Radar (ASAR) Global Monitoring (GM) of Surface Soil Moisture over Africa

“…The strong bias can be explained by the azimuthal effects that occur due to the spatial orientation of topographic features within the sensor footprint. A similar behavior is observable also over mountainous areas and has been demonstrated in the case of scatterometer acquisitions [39,40]. Usage of only ascending or descending orbit could overcome the problem; this would, however, further reduce the temporal resolution of the product.…”

Radar Signatures of Sahelian Surfaces in Mali Using ENVISAT-ASAR Data

“…These differences are expected to be large over arid regions showing well-orientated structures such as sand dunes [7], [30]. The two azimuthal angles of the ASAR instrument are about 100…”

“…On one hand, owing to its high temporal repetitivity (four to five days in theory) associated to a spatial resolution of about 50 km, the ERS WSC has provided useful data over the Sahel during a ten-year period [2]- [6]. Since 2001, QuickScat scatterometer observations took over from those delivered by ERS [7], although at a higher microwave frequency. Studies of these data have shown that Sahelian surfaces exhibit marked seasonal and interannual signatures [2].…”

Global Soil Moisture Patterns Observed by Space Borne Microwave Radiometers and Scatterometers

“…They explained this by the presence of sastrugi, which are streamlined features formed on the snow surface in the wind direction. Over land, areas of strong azimuthal anisotrophy can be found in sand deserts, where the prevailing winds form the shape and microrelief of the sand dunes (Stephen and Long 2005;Bartalis et al 2006).…”