Improved understanding of aerosol processes using satellite observations of aerosol optical properties
Abstract
Atmospheric aerosols are the largest remaining uncertainty in the Earth’s radiative budget and it
is important that we improve our knowledge of aerosol processes if we are to understand current
radiative forcing and accurately project changes in future climate. Aerosols affect the radiation
balance directly through the absorption and scattering of incoming solar radiation and indirectly
through the modification of cloud microphysical properties. Understanding aerosol forcing remains
challenging due to the short atmospheric residence time of aerosols resulting in large spatial
and temporal heterogeneity in aerosol loading and chemical composition. Satellite retrievals are
becoming increasingly important to improving our knowledge of aerosol forcing. They provide
regular global data at finer spatial and temporal resolution than available through sparse groundbased
point measurements or localised aircraft campaigns, but cannot unambiguously determine
aerosol speciation, relying heavily on a priori assumptions. In this thesis I use data from two
satellite instruments: the Along Track Scanning Radiometer 2 (ATSR-2) and the Spinning Enhanced
Visible and InfraRed Imager (SEVIRI) interpreted using the Oxford-RAL Aerosol and
Cloud (ORAC) retrieval scheme in three pieces of interrelated work.
First I use satellite observations of aerosol optical depth a and cloud particle effective radius
re from the ATSR-2 instrument in 1997 to investigate the Twomey indirect effect (IE, -δ ln re /δ
ln τa) in regions of continental outflow. I generally find a negative correlation between τa and
re with the strongest inverse relationships downwind of Africa. North America and eastern Asian
continental outflow exhibits a strong seasonal dependence, as expected. Global values for IE range
from 0.10 to 0.16, consistent with theoretical predictions. Downwind of Africa, I find that the IE
is unphysically high but robust (r = −0.85) during JJA associated with high aerosol loading, and
attribute this tentatively to the Twomey hypothesis accounting only for a limited number of physical
properties of aerosols.
Second, I test the response of the Oxford-RAL Aerosol and Cloud (ORAC) retrieval algorithm
for MSG SEVIRI to changes in the aerosol properties used in the dust aerosol model, using data
from the Dust Outflow and Deposition to the Ocean (DODO) flight campaign in August 2006. I
find that using the observed DODO free tropospheric aerosol size distribution and refractive index
compared with the dust aerosol properties from the Optical Properties of Aerosol and Cloud
(OPAC) package, increases simulated top of the atmosphere radiance at 0.55 μm assuming a fixed
aerosol optical depth of 0.5, by 10–15%, reaching a maximum difference at low solar zenith angles.
This difference is sensitive to changes in AOD, increasing by ~2–4% between AOD of
0.4–0.6. I test the sensitivity of the retrieval to the vertical distribution of the aerosol and find that
this is unimportant in determining simulated radiance at 0.55 μm. I also test the ability of the
ORAC retrieval when used to produce the GlobAerosol dataset to correctly identify continental
aerosol outflow from the African continent and I find that it poorly constrains aerosol speciation.
I develop spatially and temporally resolved prior distributions of aerosols to inform the retrieval
which incorporates five aerosol models: desert dust, maritime, biomass burning, urban and continental.
I use a Saharan Dust Index and the GEOS-Chem chemistry transport model to describe
dust and biomass burning aerosol outflow, and compare AOD using my speciation against the
GlobAerosol retrieval during January and July 2006. I find AOD discrepancies of 0.2–1 over regions
of biomass burning outflow, where AOD from my aerosol speciation and the GlobAerosol
speciation can differ by as much as 50 - 70 %.
Finally I use satellite observations of aerosol optical depth and cloud fraction from the MSG
SEVIRI instrument to investigate the semi-direct effect of Saharan dust aerosol on marine stratocumulus
cloud cover over the Atlantic during July 2006. I first use these data to study the spatial
autocorrelation of aerosol optical depth and find that it is correlated over a lag of 0.1◦ (approximately
10 km at low latitudes), beyond which it rapidly decorrelates. I find a 15 % higher cloud
fraction in regions with high dust loading (AOD > 0.5), compared with scenes with a lower dust
loading (AOD < 0.5), which for high dust scenes increases with local static stability. I attribute
this tentatively to aerosol solar shielding enhancing longwave cloud top radiative cooling which
drives marine stratocumulus convection.