A six year satellite-based assessment of the regional variations in aerosol indirect effects

Abstract. Since aerosols act as cloud condensation nuclei (CCN) for cloud water droplets, changes in aerosol concentrations having significant impacts on the corresponding cloud properties. An increase in aerosol concentration leads to an increase in CCN, with an associated decrease in cloud droplet size for a given cloud liquid water content. Smaller droplet sizes may then lead to a reduction in precipitation efficiency and an increase in cloud lifetimes, which induces more reflection of solar radiation back into space, cooling the atmosphere below the cloud layer. In reality, this relationship is much more complex and is interrelated between aerosol, cloud, and atmospheric conditions present at any one time. MODIS aerosol and cloud properties are combined with NCEP Reanalysis data for eight different regions around the globe between March 2000 and December 2005 to study the effects of different aerosol, cloud, and atmospheric conditions on the aerosol indirect effect (AIE). The first AIE for both anthropogenic and dust aerosols is calculated so that the importance of each can be compared. The unique aspect of this research is that it combines multiple satellite data sets over a six year period to provide a comprehensive analysis of indirect effects for different aerosol regimes around the globe. Results show that in most regions, AIE has a distinct seasonal cycle, though the cycle varies in significance and period from region to region. In the Arabian Sea, the six-year mean anthropogenic + dust AIE is −0.4 Wm−2 and is greatest during the summer months (<−2.0 Wm−2) during which dust aerosol concentration is greatest, significant concentrations of anthropogenic aerosols are present, and upward vertical motion is also present providing a favorable environment for cloud formation. In the Bay of Bengal, AIE was negligible owing to less favorable atmospheric conditions and a lower concentration of aerosols. In the eastern North Atlantic, AIE was also small (<0.1 Wm−2) and in this region dust aerosol concentration is much greater than the anthropogenic or sea salt components. However, elevated dust in this region may also absorb solar radiation and warm the atmosphere, stabilizing the atmosphere as evidenced by weak vertical motion during the summer (0.02 Pa s−1) when AOT is greatest. Lower average cloud fraction compared to other regions allows the absorbing effect to offset the cooling effect associated with increasing CCN. The western Atlantic and Pacific oceans have large anthropogenic aerosol concentrations transported from the United States and China respectively and produce modest anthropogenic AIE (0.7, 0.9 Wm−2) in these regions as expected. Anthropogenic AIE was also present off the West African coast corresponding to aerosols produced from seasonal biomass burning. Interestingly, atmospheric conditions were not particularly favorable for cloud formation compared to the other regions during the times where AIE was observed. Overall, we are able to conclude that aerosol type, atmospheric conditions and their relative vertical distributions are a key factors as to whether or not significant AIE occurs and simple correlations between AOT and cloud properties are insufficient to explain the AIE.