Intercomparison of satellite retrieved aerosol optical depth over ocean
Gunnar Myhre1,2 Frode Stordal1,2 Mona Johnsrud1 Alexander Ignatov3 Michael I. Mishchenko4 Igor V. Geogdzhayev4 Didier Tanré5
Jean-Luc Deuzé5 Philippe Goloub5 Teruyuki Nakajima6 Akiko Higurashi7 Omar Torres8 Brent N. Holben9
1Norwegian Institute for Air Research (NILU), Kjeller, Norway
2Department of Geophysics, University of Oslo, Oslo, Norway
of Research and Applications/Climate Research and Applications Division, Washington, D.C.
4NASA Goddard Institute for Space Studies, New York, New York
5Laboratoire d'Optique Atmospherique, CNRS, U.S.T. de Lille, Villeneuve d'Ascq, France
6Center for Climate System Research, University of Tokyo, Tokyo, Japan
7National Institute for Environmental Studies, Tsukuba, Japan
8Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, Maryland
9Biospheric Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland
3NOAA/NESDIS/Office
Introdution
•For an eight month period we compare aerosol optical depth (AOD) derived over
global oceans with five different retrieval algorithms applied to four satellite
instruments flown onboard three satellite platforms.
•The Advanced Very High Resolution Radiometer (AVHRR) was flown onboard
NOAA-14, the Ocean Color and Temperature Scanner (OCTS) and the
POLarization and Directionality of the Earth's Reflectances (POLDER) onboard
ADEOS, and the Total Ozone Mapping Spectrometer (TOMS)onboard the Earth
Probe satellites.
•The aerosol data are presented on the same format and converted to the same
wavelength in the comparison and can therefore be a useful tool in validation of
global aerosol models, in particular models that can be driven with
meteorological data for the November 1996 to June 1997 period studied here.
Aerosol optical depth (AOD)
Figure 3: Zonal mean AOD for the five datasets at 550 nm
Comparison with AERONET data
Fig 1: Averaged AOD (550 nm) over ocean for the period Nov 96–Jun 97.
Upper part of Figure 2 shows the
average AOD of the five retrievals
for the eight month period. To
determine in which regions the
aerosol retrievals are most
uncertain, a useful quantity is the
ratio of the standard deviation to
the mean of the five retrievals. A
global distribution of this quantity
is shown in the lower part of
Figure 2. Large hemispheric
differences are seen. The ratio is
particularly large close to 60°
approaching the border beyond
which the aerosol retrievals at solar
wavelengths cannot be used. In this
region few measurements are
available and a further complicating
factor is the high amount of clouds
in this region. Around 30° high
values can also be seen in the
figure, in a region with generally
very low AOD.
Fig 2: Averaged statistics for the five aerosol retrievals over the eight month
period, upper) mean, lower) standard deviation divided by the mean
Fig 4:AOD from AERONET as well as the five satellite retrievals. Data represents
monthly mean values. The satellite data are for 550 nm, while the AERONET data
are mean values of AOD at 440 and 670 nm (500 and 670 nm at Ascension Island,
Male and Lanai). Note the different AOD scales for the various stations.
Summary
 Satellite data for aerosols provides extremely useful information and new
knowledge and are an important tool for validation global aerosol models
 In satellite retrieval of aerosols several unknown quantities exist and assumptions
are needed in the retrievals. In addition there are other uncertainties
 We find large uncertainties in the global mean AOD. There is at least a factor of
two difference between the AOD from thesatellite retrievals.
 We find the largest uncertainties in the southern hemisphere and the smallest
differences mostly near the continents in the northern hemisphere.
 We have not attempted to judge the quality of the various datasets. In fact what our
study shows is that there is in general no obvious difference in their quality. There
is no single data set which stands out as very different from the others.
 The largest relative differences are probably caused by differences in cloud
screening.
References
Myhre, G., F. Stordal, M. Johnsrud, A. Ignatov, M. I. Mishchenko, I. V. Geogdzhayev, D. Tanré,
J.L. Deuzé, P. Goloub, T. Nakajima, A. Higurashi, O. Torres, and B. N. Holben, 2002,
Intercomparison of satellite retrieved aerosol optical depth over ocean, Accepted J. Atmos. Sci.
The paper can be found at
http://folk.uio.no/gunnarmy/manuscript/revised/sat_comp/sat_comp.pdf
Contact: gunnar.myhre@geofysikk.uio.no