Vertical distribution of spectral solar irradiance in the cloudless sky: A case study

[1] First airborne measurements of spectral solar down- and upwelling irradiances with a high spectral resolution (2-3 nm) are presented. The data were gathered in cloudless, hazy conditions over sea and land using a new instrument (called albedometer), which is equipped with a unique sensor head leveling technique. The downwelling irradiances and their spectral slope increase with altitude, illustrating the impact of Rayleigh scattering and boundary layer aerosols. The upwelling spectra reveal typical reflection features of the underlying surface (e.g., vegetation step around 700 nm). The measurements are compared with radiative transfer model results based on simultaneous airborne aerosol observations. The downwelling irradiances mostly agree to +/-10% which is within the experimental and modeling uncertainties. The upwelling irradiances are highly sensitive to uncertainties in the spectral surface albedo. Reasonable agreement between measurements and simulations required us to determine the surface albedo from flights at low altitude, rather than adopting literature values.