Modelling the direct effect of aerosols in the solar near-infrared on a planetary scale

N. Hatzianastassiou (1, 2) , C. Matsoukas (2, 3) , A. Fotiadi (2, 4) , E. Drakakis (2, 5) , P. W. Stackhouse Jr. (6) , P. Koepke (7) , K.G. Pavlakis (2, 8) , D. Hatzidimitriou (2, 4) , I. Vardavas (2, 4)

1 Laboratory of Meteorology
2 Foundation for Research and Technology-Hellas
3 Department of Environment [Mytilene]
4 Department of Physics
5 Department of Electrical Engineering
6 Atmospheric Sciences Division [Hampton]
7 Meteorological Institute [Hamburg]
8 Department of General Applied Science

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, ?F_TOA), on the atmospheric absorption of near-IR radiation (?F_atmab) and on the surface downward and absorbed near-IR radiation (?F_surf, and ?F_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 ?F_TOA, ?F_atmab, ?F_surf, and ?F_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.

Domaines: Océan, Atmosphère

N. Hatzianastassiou, C. Matsoukas, A. Fotiadi, E. Drakakis, P. W. Stackhouse Jr., et al.. Modelling the direct effect of aerosols in the solar near-infrared on a planetary scale. Atmospheric Chemistry and Physics Discussions, 2006, 6 (5), pp.9151-9185. ⟨hal-00302135⟩