Jamie R. Banks

An assessment of the quality of aerosol retrievals over the Red Sea and evaluation of the climatological cloud‐free dust direct radiative effect in the region

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.

The daytime cycle in dust aerosol direct radiative effects observed in the central Sahara during the Fennec campaign in June 2011

The direct clear-sky radiative effect (DRE) of atmospheric mineral dust is diagnosed over the Bordj Badji Mokhtar (BBM) supersite in the central Sahara during the Fennec campaign in June 2011. During this period, thick dust events were observed, with aerosol optical depth values peaking at 3.5. Satellite observations from Meteosat-9 are combined with ground-based radiative flux measurements to obtain estimates of DRE at the surface, top-of-atmosphere (TOA), and within the atmosphere. At TOA, there is a distinct daytime cycle in net DRE. Both shortwave (SW) and longwave (LW) DRE peak around noon and induce a warming of the Earth-atmosphere system. Toward dusk and dawn, the LW DRE reduces while the SW effect can switch sign triggering net radiative cooling. The net TOA DRE mean values range from −9 Wm−2 in the morning to heating of +59 Wm−2 near midday. At the surface, the SW dust impact is larger than at TOA: SW scattering and absorption by dust results in a mean surface radiative cooling of 145Wm−2. The corresponding mean surface heating caused by increased downward LW emission from the dust layer is a factor of 6 smaller. The dust impact on the magnitude and variability of the atmospheric radiative divergence is dominated by the SW cooling of the surface, modified by the smaller SW and LW effects at TOA. Consequently, dust has a mean daytime net radiative warming effect on the atmosphere of 153Wm−2.

Solar Cooking in the Sahel

Solar cookers have the potential to help many of the worlds poorest people, but the availability of sunshine is critical, with clouds or heavy atmospheric dust loads preventing cooking. Using wood for cooking leads to deforestation and air pollution that can cause or exacerbate health problems. For many poor people, obtaining wood is either time-consuming or expensive. Where conflicts have led to displaced people, wood shortages can become acute, leading to often violent clashes between locals and refugees. For many refugee women, this makes collecting wood a high-risk activity. For eight years, Agrometeorological Applications Associates and TchadSolaire (AAA/TS) have been training refugees to manufacture and use solar cookers in northeastern Chad, where there are more than 240,000 refugees. Solar cookers are cheap and simple to make. They are clean and safe, greatly reduce the need for wood, reduce conf licts, reduce the time girls spend collecting wood (thus favoring education), and allow pasteurizatio...