Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 5 2006 10.5194/acpd-6-9151-2006 http://www.atmos-chem-phys-discuss.net/6/9151/2006/ http://www.atmos-chem-phys-discuss.net/6/9151/2006/acpd-6-9151-2006.html http://www.atmos-chem-phys-discuss.net/6/9151/2006/acpd-6-9151-2006.pdf 9151 9185 2006-09-25 Modelling the direct effect of aerosols in the solar near-infrared on a planetary scale N. Hatzianastassiou C. Matsoukas A. Fotiadi E. Drakakis P. W. Stackhouse Jr. P. Koepke K.G. Pavlakis D. Hatzidimitriou I. Vardavas Laboratory of Meteorology, Department of Physics, University of Ioannina, 45110 Ioannina, Greece Foundation for Research and Technology-Hellas, Heraklion, Crete, Greece Department of Environment, University of the Aegean, Mytilene, Greece Department of Physics, University of Crete, Crete, Greece Department of Electrical Engineering, Technological Educational Institute of Crete, Crete, Greece Atmospheric Sciences, NASA Langley Research Center, Hampton, Virginia, USA Meteorological Institute, University of Munich, Munich, Germany Department of General Applied Science, Technological Educational Institute of Crete, Greece We used a spectral radiative transfer model to compute the direct radiative effect (DRE) of natural plus anthropogenic aerosols in the solar near-infrared (IR), between 0.85–10 µm, namely, their effect on the outgoing near-IR radiation at the top of atmosphere (TOA, ΔF_TOA), on the atmospheric absorption of near-IR radiation (ΔF_atmab) and on the surface downward and absorbed near-IR radiation (ΔF_surf, and ΔF_surfnet, respectively). The computations were performed on a global scale (over land and ocean) under all-sky conditions, using spectral aerosol optical properties taken from the Global Aerosol Data Set (GADS) supplemented by realistic data for the rest of surface and atmospheric parameters. The computed aerosol DRE, averaged over the 12-year period 1984–1995 for January and July, shows that aerosols produce a planetary cooling by increasing the scattered near-IR radiation back to space (by up to 6 Wm^−2), they warm the atmosphere (by up to 7 Wm^−2) and cool the surface (by up to 12 Wm^−2). However, they can also slightly warm the Earth-atmosphere system or cool the atmosphere (by less than 1 Wm^−2) over limited areas. The magnitude of the near-IR aerosol DRE is smaller than that of the combined ultraviolet (UV) and visible DRE, but it is still energetically important, since it contributes to the total shortwave (SW) DRE by 22–31%. On a global mean basis, the DREs ΔF_TOA, ΔF_atmab, ΔF_surf, and ΔF_surfnet are equal to about 0.48, 0.37, −1.03 and −0.85 Wm^−2, i.e. their magnitude is similar to that of climate forcing associated with increasing concentrations of greenhouse gases. The aerosol induced near-IR surface cooling combined with the atmospheric warming, affects the thermal dynamics of the Earth-atmosphere system, by increasing the atmospheric stability, decreasing thus cloud formation, and precipitation, especially over desertification threatened regions such as the Mediterranean basin. This, together with the fact that the sign of near-IR aerosol DRE is sometimes opposite to that of UV-visible DRE, demonstrates the importance of performing detailed spectral computations to provide estimates of the climatic role of aerosols for the Earth-atmosphere system.