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, Norway3NOAA/NESDIS/Office of Research and Applications/Climate Research and Applications Division, Washington, D.C.

4NASA Goddard Institute for Space Studies, New York, New York5Laboratoire d'Optique Atmospherique, CNRS, U.S.T. de Lille, Villeneuve d'Ascq, France

6Center for Climate System Research, University of Tokyo, Tokyo, Japan7National Institute for Environmental Studies, Tsukuba, Japan

8Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, Maryland9Biospheric Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland

Comparison with AERONET data

Aerosol optical depth (AOD)

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.

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.

ReferencesMyhre, 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.

Fig 1: Averaged AOD (550 nm) over ocean for the period Nov 96–Jun 97.

The paper can be found at http://folk.uio.no/gunnarmy/manuscript/revised/sat_comp/sat_comp.pdf

Contact: gunnar.myhre@geofysikk.uio.no

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.

Fig 2: Averaged statistics for the five aerosol retrievals over the eight month period, upper) mean, lower) standard deviation divided by the mean

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.

Figure 3: Zonal mean AOD for the five datasets at 550 nm