Longwave radiative forcing of Indian Ocean tropospheric aerosol

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Abstract
A spectrally resolved discrete‐ordinates radiative transfer model is used to calculate the change in downwelling surface and top‐of‐the‐atmosphere (TOA) outgoing longwave (3.9–500 μm) radiative fluxes induced by tropospheric aerosols of the type observed over the Indian Ocean during the Indian Ocean Experiment (INDOEX). Both external and internal aerosol mixtures were considered. Throughout the longwave, the aerosol volume extinction depends more strongly on relative humidity than in most of the shortwave (0.28–3.9 μm), implying that particle growth factors and realistic relative humidity profiles must be taken into account when modeling the longwave radiative effects of aerosols. A typical boundary layer aerosol loading, with a 500‐nm optical depth of 0.3, will increase the downwelling longwave flux at the surface by 7.7 W m−2 over the clean air case while decreasing the outgoing longwave radiation by 1.3 W m−2. A more vertically extended aerosol loading, exhibiting a high opacity plume between 2 and 3 km above the surface and having a typical 500‐nm optical depth of 0.7, will increase the downwelling longwave flux at the surface by 11.2 W m−2 over the clean air case while decreasing the outgoing longwave radiation by 2.7 W m−2. For a vertically extended aerosol profile, approximately 30% of the TOA radiative forcing comes from sea salt and approximately 60% of the forcing comes from the combination of sea salt and dust. The remaining forcing is from anthropogenic constituents. These results are for the external mixture. For an internal mixture, TOA longwave forcings can be up to a factor of two larger. Therefore, to complete our understanding of this region's longwave aerosol radiative properties, more detailed information is needed about aerosol mixing states. These longwave radiative effects partially offset the large shortwave aerosol radiative forcing and should be included in regional and global climate modeling simulations.