parallel cloud with the same liquid water path. It has been also demonstrated that the shape of the cloud cells in the layer is as important as the horizontal distribution of
Ž .
optical thickness Welch and Zdunkowski, 1981 . Various techniques have been devel- oped for generating inhomogeneous clouds, such as the Bounded Cascade Model
Ž .
Marshak et al., 1994 . However, the generating procedure is rather artificial since it is derived from remotely observed statistical properties of the clouds. It is thus not evident
that the parameters used for describing the inhomogeneity of the internal structure are the most relevant for describing the radiative properties of the layer. Therefore,
additional information is needed about the relationship between the internal cloud structure and the resulting radiative properties.
Our first objective in the EUCREX project is to test the radiative transfer models and the remote sensing retrieval techniques at the scale of the cloud cells. At such a scale, in
situ measurements provide an accurate description of the microphysical properties within the column. The second objective is to expand the analysis to larger scales via a
precise description of the horizontal statistics of the internal cloud structure.
3. The instrumental setup
Ž .
Three aircraft were involved in mission 206. The Merlin-IV M-IV is operated by Meteo-France and it was equipped for in situ measurements. The droplet size distribu-
´ ´
Fig. 1. Overview of the EUCREX-94 field experiment with the aircraft flight tracks during mission 206.
tions were measured with the Fast-FSSP, an improved version of the PMS FSSP-100 Ž
. Brenguier et al., 1998 . The processing and analysis of the M-IV data are discussed in
Ž .
Pawlowska et al. 2000a in this series. The Avion de Recherche Atmospherique et
´
Ž .
Teledetection ARAT is co-operated by the Centre National de la Recherche Scien-
´ ´ ´
Ž .
Ž .
tifique CNRS , Meteo-France, the Institut Geographique National IGN , and the Centre
´ ´ ´
Ž .
National d’Etudes Spatiales CNES . The ARAT was equipped with a multi-directional Ž
. Ž
. radiometer POLDER and a lidar LEANDRE . The DLR-F20 is operated by the
Ž .
Deutsche Forschungsanstalt fur Luft und Raumfahrt e. V. DLR and was equipped with
¨
Ž .
the multidirectional radiometer POLDER and the multi-wavelength radiometer OVID. The processing and analysis of the LEANDRE, and OVID data are presented in this
Ž .
Ž .
EUCREX series of papers by Pelon et al. 2000 and Schuller et al. 2000 , respectively.
¨
Table 1 summarizes the aircraft operations during mission 206 and Fig. 1 shows the common track of the three aircraft. The coordinates of the end points of the leg are
4877W–48842N, respectively for the southern point M and 5857W–49892N for the northern point A. The aircraft have been flying between these two points at various
altitudes, with the M-IV in the cloud layer and the two other aircraft above. Within the various papers of the series, the position of the aircraft will be measured by the distance
from point M along the leg.
Fig. 2. Surface analysis weather map at 12:00 UTC on April 18, 1994.
Ž .
Fig. 3. 12:00 UTC Radiosounding at Brest France , on April 18, 1994.
4. The meteorological situation
The surface weather map at 12:00 UTC on April 18, 1994 is presented in Fig. 2, and Fig. 3 shows the 12:00 UTC radiosounding at Brest. A large zone of high pressure was
centered at 5080N and 2580W, with a ridge extending eastward. The experimental zone was on the southern edge of this ridge, exposed to a very moderate flow blowing from
Ž
y1 y1
. the North–East 5 ms
at the ground, 10 ms at the 500 hPa level . The vertical
structure of the airmass was characterized by a very sharp temperature inversion Ž
. Ž
. DT s 68C located around 850 hPa at 00:00 UTC, and descending to 900 hPa 1050 m
at 12:00 UTC. Below this inversion, a layer of stratocumulus covered the north coast of Brittany, inducing a very small diurnal thermal amplitude of 58C to 68C at night, 88C to
Ž .
98C during the day. These clouds did not extend far off the coast 60 to 80 km and their thickness was about 450 m near the coast, decreasing northward.
5. Discussion
