Bidirectional Reflectivity in AVHRR Channel 3: Application to a Region in Northern Africa

This article is devoted to the retrieval of bidirectional reflectivity in AVHRR Channel 3 (3.75 μm). In the mid-infrared spectral region, the ground radiance is a combination of a radiance emitted by the surface and a reflected radiance due to sun irradiation. Using combined day/night AVHRR data, the emitted part of the radiance is estimated allowing for the determination of the reflected part. Emphasis is placed on the accuracy achievable on the measurement of the reflectivity in AVHRR Channel 3. A detailed sensitivity analysis of the method is performed, taking into account the different sources of error. The analysis is applied to series of images covering a region of Northern Africa. Atmospheric corrections are performed using the MODTRAN code with atmospheric data extracted from the AVISO (Météo France) data base. Processing a large number of orbits gives access to a wide range of satellite view angles. Furthermore, use of morning NOAA 12 and noon NOAA 14 overpasses leads to essentially two Sun illumination directions (principal and perpendicular planes). For several selected areas, the angular reflectivity is represented along with absolute error. Reflectivity exhibits a large angular variation demonstrating a non-Lambertian behavior at the scale of the measurements. The levels of reflectivity are different from one area to another. A simple semiempirical model is used to represent the angular reflectivity. Emissivity in Channel 3, retrieved from reflectivity by means of an angular form factor, may exhibit strong variations for large angles (>18%). If angular variation of reflectivity is not taken into account, error on emissivity can be up to 15% for the areas studied in this work.