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[1] A 14-year (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) high resolution European Cloud Climatology has been generated by use of NOAA/ AVHRR data. For selected areas we present spatially averaged monthly means of total cloud cover derived from noon overpasses and compare them with surface SYNOP observations. The climatologies do not reveal a significant trend of cloud cover over the 14-year period. However, both data sets show a clear latitudinal variability and a seasonal dependence which is more pronounced in the satellite than in the SYNOP observations. Mean differences between satellite and SYNOP data range from about À2% to À10% in all seasons except summer when the mean difference is as large as À15.3%. As a special feature we notice the broad minimum of cloud cover during the extreme dry and hot summer in 2003 in Central Europe.

A new aerosol retrieval method called Synergetic Aerosol Retrieval (SYNAER), using simultaneous measurements of the radiometer Along Track Scanning Radiometer (ATSR‐2) and the spectrometer Global Ozone Monitoring Experiment (GOME) in the visible and near‐infrared spectra, was developed. Both instruments are flown onboard the European Remote Sensing (ERS‐2) satellite. SYNAER delivers boundary layer aerosol optical thickness (BLAOT) and aerosol type both over land and over ocean, the latter as BLAOT percentage of six representative components from the Optical Parameters of Aerosols and Clouds (OPAC) data set. The high spatial resolution of ATSR‐2 permits accurate cloud detection. It allows BLAOT calculation over automatically selected dark pixels and surface albedo correction for a set of boundary layer aerosol mixtures. After spatial integration and colocation to GOME pixels, these parameters are used to simulate GOME spectra for the same set of aerosol mixtures. A least squares fit of these spectra to the measured and cloud‐corrected GOME spectrum chooses the aerosol mixture. First validation studies are presented in part 2 of this paper [Holzer‐Popp et al., 2002]. The method will be used for the future sensor pairs Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY)/Advanced ATSR (AATSR) on Envisat and GOME‐2/Advanced Very High Resolution Radiometer (AVHRR) on METOP. Thus, SYNAER holds the potential to extract a long‐term climatological data set.