Diurnal variation in Sahara desert sand emissivity during the dry season from IASI observations

• Published in: Journal of geophysical research. Atmospheres Vol. 119; no. 3; pp. 1626 - 1638
• Main Authors: Masiello, Guido, Serio, Carmine, Venafra, Sara, DeFeis, Italia, Borbas, Eva E.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 16-02-2014
• Subjects: Atmospheric sciences; Atmospheric temperature; Atmospherics; Boundary layers; Deserts; Diurnal variations; Dry season; Emissions; Emissivity; Geophysics; infrared; Meteorological satellites; Meteorology; remote sensing; Retrieval; Sand; Sand & gravel; satellites; Surface chemistry; Surface temperature; Temperature inversions; Water vapor; Africa, Sahara Desert
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1002/jgrd.50863

The problem of diurnal variation in surface emissivity over the Sahara Desert during non‐raining days is studied and assessed with observations from the Infrared Atmospheric Sounding Interferometer (IASI). The analysis has been performed over a Sahara Desert dune target area during July 2010. Spinning Enhanced Visible and Infrared Imager observations from the European geostationary platform Meteosat‐9 (Meteorological Satellite 9) have been also used to characterize the target area. Although the amplitude of this daily cycle has been shown to be very small, we argue that suitable nighttime meteorological conditions and the strong contrast of the reststrahlen absorption bands of quartz (8–14 μm) can amplify its effect over the surface spectral emissivity. The retrieval of atmospheric parameters show that, at nighttime, an atmospheric temperature inversion occurs close to the surface yielding a thin boundary layer which acts like a lid, keeping normal convective overturning of the atmosphere from penetrating through the inversion. This mechanism traps water vapor close to the land and drives the direct adsorption of water vapor at the surface during the night. The diurnal variation in emissivity at 8.7 μm has been found to be as large as 0.03 with high values at night and low values during the day. At 10.8 μm and 12 μm, the variation has the same sign as that at 8.7 μm, but with a smaller amplitude, 0.019 and 0.014, respectively. The impact of these diurnal variations on the retrieval of surface temperature and atmospheric parameters has been analyzed.
Key Points
Diurnal variation in desert emissivity during the dry season assessed with IASI
Assessment of the meteorological conditions which favors the phenomenon
Impact of emissivity variation on atmospheric parameters retrieval