Tropospheric aerosol imposes one of the least understood impacts on the Earth's climate. The limited lifetime (commonly on the order of days) of aerosol and uncertain strengths of aerosol sources and aerosol removal processes, including interactions with clouds and effects on heterogeneous chemistry, make it very difficult to define characteristic aerosol properties. Yet aerosol properties (and aerosol induced changes to cloud properties) are needed at high accuracy on global and seasonal scales to create confidence in model derived predictions on the aerosol climatic impact and the climate change attributed to anthropogenic aerosol.

The characterization of aerosol concentration and aerosol properties in these models is highly parameterized and frequently far from reality. Some of the aerosol information in these models due to their need for global coverage is based on satellite data. These data by themselves carry significant uncertainties. It is our goal to demonstrate uncertainties and to reveal misrepresentation as part of a collaborative intercomparison.

A central piece for this intercomparison is a statistics provided by the AErosol RObotic NET work. AERONET is a network of automatic sun/sky radiometers distributed throughout the world, whose data are centrally monitored, maintained and archived at NASA-Goddard. Probably the most meaningful aerosol property (also from a visual [reduced visibility] point of view) is the mid-visible aerosol optical depth. Here, aerosol optical depths at selected AERONET sites are compared to representations in models and to satellite retrievals near those sites. A statistical approach was selected, because local (AERONET-) measurements only represent a sample within regions of either the footprint of a satellite pixel or of areas represented by a grid-point in global circulation models. Aside from spatial inconsistencies there are also usually differences in time. Most model simulations or satellite data-sets relate (back) to years for which AERONET data were not available. Monthly and seasonal time-averages were chosen, also in part as global aerosol satellite products and aerosol properties in global models are commonly presented for these averages.

First, AERONET data, which provide the basis for the aerosol optical depth intercomparison are introduced. Special attention is given to uncertainties regarding the statistics. Then, comparisons to and among currently available operational satellite data are discussed. Finally, comparisons to and among five global models are presented. Since all models distinguish among sulfate, carbon, dust and sea-salt, more details on the model-behavior could be deduced from comparisons on a component level.

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