Abstract
Ground-based and satellite observations are used in conjunction with the Rapid Radiative Transfer Model (RRTM) to assess climatological aerosol loading and the associated cloud-free aerosol direct radiative effect (DRE) over the Red Sea. Aerosol optical depth (AOD) retrievals from the Moderate Resolution Imaging Spectroradiometer and Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instruments are first evaluated via comparison with ship-based observations. Correlations are typically better than 0.9 with very small root-mean-square and bias differences. Calculations of the DRE along the ship cruises using RRTM also show good agreement with colocated estimates from the Geostationary Earth Radiation Budget instrument if the aerosol asymmetry parameter is adjusted to account for the presence of large particles. A monthly climatology of AOD over the Red Sea is then created from 5 years of SEVIRI retrievals. This shows enhanced aerosol loading and a distinct north to south gradient across the basin in the summer relative to the winter months. The climatology is used with RRTM to estimate the DRE at the top and bottom of the atmosphere and the atmospheric absorption due to dust aerosol. These climatological estimates indicate that although longwave effects can reach tens of W m−2, shortwave cooling typically dominates the net radiative effect over the Sea, being particularly pronounced in the summer, reaching 120 W m−2 at the surface. The spatial gradient in summertime AOD is reflected in the radiative effect at the surface and in associated differential heating by aerosol within the atmosphere above the Sea. This asymmetric effect is expected to exert a significant influence on the regional atmospheric and oceanic circulation.
Highlights
Quantifying the impact of aerosols on the Earth’s climate has been recognized as one of the key challenges facing climate scientists today [Intergovernmental Panel on Climate Change, 2007, 2013]
In the first part of this paper we introduce the ship-based measurements and use them to evaluate the performance of colocated satellite retrievals from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) and the Moderate Resolution Imaging Spectroradiometer (MODIS)
The performance of the SEVIRI retrieval algorithm over the Red Sea shown earlier coupled with previous comparisons under a variety of conditions [e.g., Brindley and Ignatov, 2006; De Paepe et al, 2008; Haywood et al, 2011] gives confidence that, with the caveats discussed in section 3, the retrievals can be used to develop a longer term climatology of aerosol over the Sea
Summary
Quantifying the impact of aerosols on the Earth’s climate has been recognized as one of the key challenges facing climate scientists today [Intergovernmental Panel on Climate Change, 2007, 2013]. These data represent the first set of detailed aerosol measurements from the Red Sea since earlier scientific cruises within the Sea sampled conditions for, at most, up to a fortnight [Tomasi and Prodi, 1982; Shifrin et al, 1985] They provide a unique opportunity to assess the performance of satellite retrieval algorithms in this region across a range of different atmospheric aerosol loadings.
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