On the differences of ultraviolet and visible irradiance calculations in the Mediterranean basin due to model- and satellite-derived climatologies of aerosol optical properties

Abstract

The variability of aerosol optical properties over the Mediterranean basin, in the ultraviolet (UV) and visible (VIS) spectral regions, is examined, using ground-based data from eight stations of the AErosol RObotic NETwork (AERONET), climatological values from the AeroCom (Aerosol Comparisons between Observations and Models) project and data from the MODerate resolution Imaging Spectroradiometer (MODIS) on-board the Terra and Aqua satellites. The aerosol optical properties from the three datasets are used as input data in a radiative transfer model, in order to estimate the differences in irradiance at the ground under cloud-free skies. The MODIS aerosol optical depth climatology shows better agreement with AERONET data. The highest difference in the monthly average values is equal to 0.09 at 550 nm, while the differences between the AERONET and the AeroCom climatologies reach 0.25 and 0.15 in the UV and VIS wavelengths, respectively. As a result, the AERONET modelled VIS and UV irradiances are closer to MODIS, with the absolute differences in average values reaching 15 (6%) and 2 W m–2 (6%), respectively, while absolute differences with AeroCom irradiances can reach up to 30 (12%) and 4.5 W m–2 (12%). The differences are higher in FORTH Crete, Nes Ziona, Sede Boker and Blida, which are influenced by desert dust aerosols. The separate effect on irradiances due to differences in aerosol optical depth and single scattering albedo from the three datasets has been also examined; according to results, the effect of aerosol optical depth is dominant. At this stage, the use of MODIS aerosol climatology would be recommended for calculations of UV and VIS irradiance in the Mediterranean basin. However, the AeroCom climatology, with some further improvements in the dust cycle, could be a valuable tool in future studies in this area, due to the provision of natural and manmade aerosol optical properties with higher spectral resolution.