A six year satellite-based assessment of the regional variations in aerosol indirect effects 1Earth System Science Center, UAHuntsville, Huntsville, AL, USA
2Department of Atmospheric Science, UAHuntsville, Huntsville, AL, USA
3Cloud-Climate Feedbacks Group, Max Planck Institute for Meteorology, Hamburg, Germany
Received: 29 Sep 2008 – Published in Atmos. Chem. Phys. Discuss.: 05 Dec 2008 – Published: 22 Jun 2009Abstract. Aerosols act as cloud condensation nuclei (CCN) for cloud water droplets,
and changes in aerosol concentrations have significant microphysical impacts
on the corresponding cloud properties. Moderate Resolution Imaging
Spectroradiometer (MODIS) aerosol and cloud properties are combined with
NCEP Reanalysis data for six 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).
Emphasis is placed in examining the relative importance of aerosol
concentration, type, and atmospheric conditions (mainly vertical motion) to
AIE from region to region.
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 (AS), the six-year mean anthropogenic + dust AIE is −0.27 Wm−2 and is greatest during the summer months (<−2.0 Wm−2)
during which aerosol concentrations (from both dust and anthropogenic
sources) are greatest. Comparing AIE as a function of thin (LWP<20 gm−2) vs. thick (LWP≥20 gm−2) clouds under conditions of
large scale ascent or decent at 850 hPa showed that AIE is greatest for
thick clouds during periods of upward vertical motion. In the Bay of Bengal,
AIE is negligible owing to less favorable atmospheric conditions, a lower
concentration of aerosols, and a non-alignment of aerosol and cloud layers.
In the eastern North Atlantic, AIE is weakly positive (+0.1 Wm−2) with
dust aerosol concentration being much greater than the anthropogenic or sea
salt components. However, elevated dust in this region exists above the
maritime cloud layers and does not have a hygroscopic coating, which occurs
in AS, preventing the dust from acting as CCN and limiting AIE. The Western
Atlantic has a large anthropogenic aerosol concentration transported from
the eastern United States producing a modest anthropogenic AIE (−0.46 Wm−2). Anthropogenic AIE is also present off the West African coast
corresponding to aerosols produced from seasonal biomass burning (both
natural and man-made). Interestingly, atmospheric conditions are not
particularly favorable for cloud formation compared to the other regions
during the times where AIE is observed; however, clouds are generally thin
(LWP<20 gm−2) and concentrated very near the surface. Overall, we
conclude that vertical motion, aerosol type, and aerosol layer heights do
make a significant contribution to AIE and that these factors are often more
important than total aerosol concentration alone and that the relative
importance of each differs significantly from region to region.
Citation: Jones, T. A., Christopher, S. A., and Quaas, J.: A six year satellite-based assessment of the regional variations in aerosol indirect effects, Atmos. Chem. Phys., 9, 4091-4114, doi:10.5194/acp-9-4091-2009, 2009.