Direct radiative forcing of Saharan dust in the Mediterranean from measurements at Lampedusa Island and MISR space‐borne observations

Abstract

Ground‐based observations were carried out at the island of Lampedusa, in the central Mediterranean, simultaneously to measurements of the Multiangle Imaging Spectro‐Radiometer (MISR) instrument aboard the Terra satellite, in 2 days of July 2002 (14 and 16) when Saharan dust was present. Ground‐based measurements include ultraviolet spectral irradiance; visible global and diffuse irradiances at several bands; aerosol optical depth in the ultraviolet, visible, and near infrared; aerosol backscattering; and depolarization profiles. The aerosol optical depth at 500 nm was very similar on both days (0.23). Large, irregular particles were present in the atmospheric layers between 2 and 6 km on 14 July and between 1 and 5 km on 16 July. According to backward trajectories, the airmasses arriving at Lampedusa spent a large fraction of the previous days over central Sahara on 14 July, while passing over northern Africa on 16 July. The different trajectories influence the value of the Ångström exponent, depolarization, and backscattering produced by aerosols. A significant increase of the aerosol optical depth in the ultraviolet is observed, and is attributed to steep variations of the particles' refractive index. A radiative transfer model is used to reconstruct the observed radiative fluxes at the surface and the radiance at the satellite altitude, and to estimate the direct aerosol radiative forcing in the 290–800 nm and in the visible spectral range. The best agreement between measured and modeled radiative fluxes is found for values of the single scattering albedo of 0.96–0.97 (14 July) and 0.88–0.89 (16 July) and of the asymmetry parameter of 0.79–0.80 (14 July) and 0.81–0.82 (16 July), indicating that significantly different optical properties pertain to dust particles of different origin. The instantaneous direct radiative forcing per unit optical depth over the 400–700 nm spectral interval is −(39–79) W m−2 at the surface and −(5–20) W m−2 at the top of the atmosphere. Although desert dust is the dominant aerosol type on both days, large differences, as large as a factor of 2 in the forcing efficiency at the surface and a factor of 3 at the top of the atmosphere, are found. Calculations were also performed to study the effect of nonspherical aerosol shapes by using different scattering phase functions, and comparing modeled and measured reflectivities at the nine observation angles of MISR. The different phase functions match the observed wavelength dependence of the aerosol optical depth and produce similar reflectance patterns for a scattering angle <150°. The effect of nonspherical particles becomes important mainly at scattering angles in the 150–180° interval. This effect should be taken into account in the derivation of radiation fluxes from satellite radiance measurements.