The direct effect of aerosols on solar radiation over the broader Mediterranean basin

Abstract. For the first time, the direct radiative effect (DRE) of aerosols on solar radiation is computed over the entire Mediterranean basin, one of the most climatically sensitive world regions, using a deterministic spectral radiation transfer model (RTM). The DRE effects on the outgoing shortwave radiation at the top of atmosphere (TOA), DRE_TOA, on the absorption of solar radiation in the atmospheric column, DRE_atm, and on the downward and absorbed surface solar radiation (SSR), DRE_surf and DRE_netsurf, respectively, are computed separately. The model uses input data for the period 2000–2007 for various surface and atmospheric parameters, taken from satellite (International Satellite Cloud Climatology Project, ISCCP-D2), Global Reanalysis projects (National Centers for Environmental Prediction – National Center for Atmospheric Research, NCEP/NCAR), and other global databases. The spectral aerosol optical properties (aerosol optical depth, AOD, asymmetry parameter, g_aer and single scattering albedo, ω_aer), are taken from the MODerate resolution Imaging Spectroradiometer (MODIS) of NASA (National Aeronautics and Space Administration) and they are supplemented by the Global Aerosol Data Set (GADS). The model SSR fluxes have been successfully validated against measurements from 80 surface stations of the Global Energy Balance Archive (GEBA) covering the period 2000–2007.

A planetary cooling is found above the Mediterranean on an annual basis (regional mean DRE_TOA = −2.4 W m⁻²). Although a planetary cooling is found over most of the region, of up to −7 W m⁻², large positive DRE_TOA values (up to +25 W m⁻²) are found over North Africa, indicating a strong planetary warming, and a weaker warming over the Alps (+0.5 W m⁻²). Aerosols are found to increase the absorption of solar radiation in the atmospheric column over the region (DRE_atm = +11.1 W m⁻²) and to decrease SSR (DRE_surf = −16.5 W m⁻² and DRE_netsurf−13.5 W m⁻²) inducing thus significant atmospheric warming and surface radiative cooling. The calculated seasonal and monthly DREs are even larger, reaching −25.4 W m⁻² (for DRE_surf). Within the range of observed natural or anthropogenic variability of aerosol optical properties, AOD seems to be the main responsible parameter for modifications of regional aerosol radiative effects, which are found to be quasi-linearly dependent on AOD, ω_aer and g_aer.