Relations between geological characteristics and satellite-derived infrared and microwave emissivities over deserts in northern Africa and the Arabian Peninsula

Infrared and microwave emissivities have been previously calculated over the globe from satellite observations to estimate the longwave radiative budget of the planet in global circulation models (GCM) or to facilitate the assimilation of surface-sensitive satellite observations over land in numerical weather prediction (NWP) models. This study examines the relationship between these emissivities and the soil lithology over the deserts in northern Africa and the Arabian Peninsula at large scales. For this purpose, a lithology map of the area has been carefully derived from combining several geological maps and in situ lithostratigraphic columns. Infrared and microwave emissivities show large spatial variability, much more than often assumed in GCM or in NWP models, that relate well to the surface properties, especially the lithology. Changes in the lithology explain similar to 40% and up to 50% of the emissivities variability at the considered frequencies in the infrared and in the microwaves. In particular, siliceous and carbonate outcrops have specific signatures that can be identify in these wavelength ranges, with a surprisingly good sensitivity of the microwaves to the presence of carbonates. This study clearly indicates that geological information is necessary for emissivity modeling, not only in the infrared but also in the microwaves, for GCM and NWP applications. The lithology maps created from the geological maps for this study is available under request. Our study also suggests that the combination of infrared and microwave emissivities has some potential for lithology mapping at large scales, especially for land surface models.