Correction of the ground-based measurements of radiance by the surface contribution for a better estimate of the aerosol radiative forcing

摘要

A 'measure' of the surface effect on downwelling radiance in the solar principal plane (SPP), K, is built by analysing simulations by a radiative transfer code. K describes the solar angle dependence of the ratio of radiance measured at a viewing angle θ_v=40° over radiance measured at θ_v=70°, both in SPP. The surface effect is described by a surface albedo value, used as an input parameter of the Successive Orders of Scattering code [Deuze et al, 1989]. K is mainly sensitive to the surface albedo but also to the Angstroem exponent α and to the real part of the refractive index m_R. α is derived from independent measurements of spectral aerosol optical thickness and m_R is estimated in a first step of the method from radiance measurements made at θ_(Sun)=70° The method is applied on data acquired at the AERONET site of Sede Boker, Israel, in 2000. Results give a high reflective surface with small spectral dependence. An event of increase of vegetation cover (which we suppose is caused by rain falls) is observed as the surface albedo at 675 nm decreases while it remains constant at 870 nm. Aerosol single scattering albedo derived at 675 nm from radiance measurements varies little around 0.90 only if surface albedo change is considered.