Aerosol phase function derived from CIMEL measurements

Abstract

An approach to simulate the signal at the top of the atmosphere (TOA) in the visible and near-infrared regions is based on the exploitation of CIMEL sky radiances in three spectral bands (i.e. 440 nm, 675 nm and 870 nm). An iterative method, developed at the Laboratoire Interdisciplinaire des Sciences de l'Environnement (LISE, Wimereux, France), allows the aerosol phase function to be extracted from these ground-based measurements. Sky radiances are corrected for the multiple scattering based on the use of a radiative transfer tool and the aerosol phase function is derived from the primary scattering approximation. In order to cover the largest range of scattering angles, only the sky radiances acquired at low solar elevations are employed in this retrieval. These extreme geometrical conditions impose to adapt the radiative transfer code and to check its performances. Limits, performances and accuracy of this inverse method are discussed and illustrated both from the radiative transfer computations and from the CIMEL measurements. Moreover, thanks to the fact that the Aerosol Robotic Network (AERONET) also proposes CIMEL derived aerosol phase functions, the latter have been compared with our results. The substantial discrepancies that appear between the two sets are explained by the different objectives used in the two retrieval algorithms.