while it overestimates significantly the optical thickness when the layer is thin and broken. Finally, it is shown that the effective optical thickness over the whole sampled cloud is smaller
than the adiabatic prediction based on the mean geometrical thickness of the cloud layer. The high sensitivity of the optical thickness on cloud geometrical thickness suggests that the effect of
aerosol and droplet concentration on precipitation efficiency, and therefore on cloud extent and lifetime, is likely to be more significant than the Twomey effect. q 2000 Elsevier Science B.V.
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Keywords: Cloud–radiation interaction; Stratocumulus; Aerosol indirect effect; Climate change
1. Introduction
Part of the EUCREX-94 experiment was dedicated to the study of the indirect effect of aerosols on boundary layer stratocumulus clouds. In order to document and parame-
terize the relationships between cloud microphysics and cloud radiative properties, a strategy based on simultaneous measurements of both cloud properties has been
Ž .
designed Brenguier and Fouquart, 2000 . The various papers in this special issue have described the instruments and the data processing techniques. This conclusion paper
aims to combine all these approaches in order to test parameterization schemes of cloud optical thickness, document the natural variability of this parameter, and evaluate its
effects on the mean cloud radiative properties.
Ž .
Fig. 1 in the work of Fouilloux et al. 2000 , hereafter referred to as FG, reveals that the stratocumulus layer sampled by the instrumented aircraft along the leg M–A is not
homogeneous horizontally. The cloud layer is thick and continuous at the southern end Ž .
Ž . M , while it is thin and broken at the northern end A . This trend in the direction
Ž .
perpendicular to the mean flow from the North-East has been documented in more details by in situ measurements. At the beginning of the M-IV flight, from 09:30 to
10:30, i.e. 1–2 h after the AVHRR image shown in FG, clouds as thick as 400 m were observed close to M and the layer was continuous as shown by the statistics of LWC in
Ž .
Ž Fig. 7 of the work of Pawlowska et al. 2000 , hereafter referred to as PBB only 10 of
. Ž
. the samples were in clear air . Towards A, the layer was thinner less than 200 m and
Ž .
broken 35 of the samples were in clear air . This geographical trend evolved with Ž
. Ž
. time and at the end of the flight 12:30 UTC , the thickest 300 m cells were observed
Ž .
at the middle of the leg while at both ends the cloud layer was thinner Fig. 4 in PBB . In this conclusion paper, the first step will be to document spatial and time variations of
the cloud radiative properties as measured remotely. The second step will be to compare the remotely measured optical thickness to the one derived from in situ measurements of
droplet concentration and cloud geometrical thickness, in order to test the adiabatic model discussed in PBB versus the parameterization based on a vertically uniform plane
Ž .
parallel model VUPPM . Independent estimates of cloud geometrical thickness with POLDER and the lidar LEANDRE will be used to strengthen the validation. The third
step will be to provide detailed information about the statistics of optical thickness, which are particularly needed for the parameterization of the effects of horizontal cloud
inhomogeneities on mean cloud albedo.
2. Spatial and time variations of optical thickness
