Global aerosols studied by NILU

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Aerosols scatter and absorb solar radiation and thereby influence the radiation in the atmosphere and the global climate. To understand the climate system and the ongoing climate change it is important to know the distribution of aerosols in the atmosphere. Satellites provide an extremely useful tool to study this distribution, especially since it is very inhomogeneous with respect to geographical location.

NILU has been comparing a measure of the amount of aerosols, aerosol optical depth –AOD, derived over global oceans with five different retrieval algorithms applied to four satellite instruments flown onboard three satellite platforms.

The study comprises data for an eight months period from November 1996 through June 1997. 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.

In this study, 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 period studied.

Global aerosols studied by NILU
Global aerosols studied by NILU

NILU plans similar analyses of other datasets from a series of newer instruments. This work is a part of NILU’s participation in the EU funded project DAEDALUS and the project AEROZCLIM funded by the Research Council of Norway. The work is done in collaboration with the providers of the satellite data in Europe, the US and Japan.

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 the satellite 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.

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