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This contribution to the EUCREX mission 206 series aims at evaluating the performances of three methods considered for the retrieval of cloud microphysical properties (optical depth, droplet effective radius and liquid water path (LWP)) from Advanced Very High Resolution Radiometer (AVHRR) satellite images. These procedures, namely a set of empirical parameterizations, a physically based inversion code, and a neuronal approach, are tested with the mission 206 case study. The influence of horizontal averaging on the retrieved cloud properties is also investigated to assess the limitations of the comparison between satellite observations and aircraft measurements performed at a smaller scale.

Postlaunch calibration of the Meteosat visible (VIS) channel has been performed with Meteosat-1, Meteosat-2, and Meteosat-4. The radiance at the satellite radiometer aperture has been determined by independent means and has been related to the simultaneous satellite radiometer count. This technique has been named vicarious calibration. Airborne radiance measurements over homogeneous targets (two land surfaces, clear ocean, and stratiform clouds) have been performed. The derived calibration factors are scene dependent because of the nonrectangular shape of the Meteosat VIS channel spectral responsivity curve. The latter and, hence, the scene dependence, slightly differs between the three satellites. Therefore, the calibration factors also differ within ±15%. Average values of the calibration factors are about 11 W m−2 sr−1 per count for land surfaces and clouds and about 1.5 W m−2 sr−1 per count for ocean.

The albedo of four vegetated surfaces was investigated to derive its variability with differing distributions of the irradiance. The results are based on measured values of the spectral biconical reflectance factor, which are combined with calculated spectral irradiances for low and high atmospheric turbidity. The solar zenith angle is varied from 0° to 80°. The derived spectral albedos are then integrated with respect to wavelength in order to achieve the albedo. It is found that the variability of the albedo with respect to the atmospheric turbidity is less than 0.01 in nearly all cases. The variability of the albedo with respect to the solar elevation angle, however, is larger than 0.02 in many cases. For solar elevation angles from 20° to 60°, the variability of the albedo of the four surfaces can be represented by a mean curve which fits the individual variabilities with an accuracy of 0.015.

The average variability of the reflected radiation field due to differing distributions of the irradiation in case of a savannah was given in a previous paper. In this paper, equivalent results are given for three more surfaces: bog, pasture land, and coniferous forest. Because the results are rather similar, mean values for vegetated surfaces can be derived. They indicate a change of the reflected radiance by ± 1% per degree change of the solar zenith angle and per 10% change of the optical depth of the atmosphere.