Ž .
Atmospheric Research 55 2000 3–14 www.elsevier.comrlocateratmos
Introduction to the EUCREX-94 mission 206
Jean-Louis Brenguier
a,
, Yves Fouquart
b
a
Meteo-France, Centre National de Recherches Meteorologiques, GMEIr MNP, 42 aÕ. Coriolis,
´ ´
31057 Toulouse Cedex 01, France
b
Laboratoire d’Optique Atmospherique, UniÕersite des Sciences et Techniques de Lille, Lille, France
´ ´
Received 11 December 1998; accepted 1 March 2000
Abstract
Part of the EUCREX-94 experiment was devoted to the study of the radiative properties of boundary layer clouds in relation with their microphysical and structural properties. Mission 206,
on April 18, is particularly attractive because of a general trend in cloud geometrical thickness within the sampled region, with corresponding values of optical thickness between 4 and 70. The
cloud system has been extensively documented in situ with an instrumented aircraft, while two other aircraft were measuring its radiative properties with radiometers and a lidar. This paper
introduces the scientific objectives of the experiment and describes the instrumental setup. After a presentation of the meteorological situation, the various papers of this series are briefly intro-
duced. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Cloud–radiation interaction; Stratocumulus; Aerosol indirect effect; Climate change
1. Introduction
The climate of the Earth is controlled by a large variety of coupled systems. In the atmospheric system, clouds play a major role in the global energy balance as well as in
the redistribution of energy within the atmosphere. Boundary layer clouds with a high Ž
. Ž
. albedo 30–40 compared to the surface 10 for the ocean give rise to large deficits
in the absorbed solar radiative flux at the top of the atmosphere, while their low altitude Ž
. prevents significant compensation in thermal emission Randall et al., 1984 . Therefore,
small changes in their radiative properties or their geographical extension and lifetime
Corresponding author. Tel.: q33-5-61-07-93-21; fax: q33-5-61-07-96-27. Ž
. E-mail address: jlbmeteo.fr J.-L Brenguier .
0169-8095r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
. PII: S 0 1 6 9 - 8 0 9 5 0 0 0 0 0 5 3 - 3
are likely to significantly influence the climate. Boundary layer clouds are strongly coupled with the thermodynamics of the boundary layer and their diurnal cycle is
particularly sensitive to changes in heat and water vapor fluxes. Various feedback mechanisms involving boundary layer clouds have thus been considered as factors that
Ž could counterbalance the global warming of the climate by green house gases Charlson
et al., 1987; Albrecht, 1989; Arking, 1991; Ackerman et al., 1993; Pincus and Baker, .
Ž .
1994; Boers and Mitchell, 1994; Martin et al., 1997 . In addition, Twomey 1977 suggested that anthropogenic aerosols, which have been released together with green
house gases, are likely to modify the radiative properties of clouds: increasing pollution generally means increasing cloud nucleus concentrations, hence increasing numbers of
cloud drops; this leads to increasing cloud optical thickness and hence, for finite cloud thickness increasing cloud albedo. This additional forcing on the climate system by
Ž .
cloud condensation nuclei CCN is referred hereafter to as the indirect effect of aerosols on climate.
The various interactions between aerosols, cloud dynamics, microphysics and radia- tion cannot be explicitly simulated in a GCM and they must be parameterized. This
series of papers is focused on the parameterization of the cloud microphysicsrradiation interaction. In addition to GCM simulations, such a parameterization is also useful for
the development of techniques for the retrieval of cloud microphysical properties from satellite measurements of their radiative properties.
There is now a consensus for parameterizations based on cloud optical thickness and droplet effective radius, two parameters which determine the optical properties of a
homogeneous cloud volume. In a simulation of the aerosol indirect effect, the first step is to establish relationships between the predicted aerosol properties and the resulting
droplet concentration. In a homogeneous cloud volume, droplet number concentration and effective radius are simply connected via the expression of the liquid water content
Ž
. Ž
Ž . .
LWC in the cloud volume see Eq. 3 in Sec. 2 . In climate models, a boundary layer cloud is generally represented as a horizontally and vertically homogeneous layer. This
Ž .
is referred to hereafter as the vertically uniform plane parallel model VUPPM . In such a model, the droplet effective radius is thus directly derived from the predicted value of
droplet number concentration. However, real clouds are inhomogeneous and LWC is increasing from the cloud base to the cloud top. The droplet effective radius is also
increasing from almost zero below the CCN activation level at the cloud base to a maximum value close to cloud top. Therefore, a relationship must be established
between the predicted droplet number concentration and the equivalent VUPPM effec- tive radius of a real inhomogeneous cloud, a difficulty which is reflected by the variety
of the solutions proposed in the literature.
From the methodological point of view, parameterizations have been developed by first averaging cloud microphysical properties over the scale of a cloud system in order
to define equivalent plane parallel microphysical properties which determine the radia- tive properties of the cloud system. In contrast, the approach proposed in this series of
papers considers the radiative properties of single convective cells in the cloud system that are further averaged in order to get the mean radiative properties of the cloud
system. At the scale of a convective cloud cell, the vertical profiles of the microphysical
Ž .
parameters are often close to adiabatic profiles Slingo et al., 1982 . In this case, simple
relationships exist between the altitude above cloud base and droplet concentration on the one hand, and the droplet effective radius and the extinction coefficient on the other
hand. Such profiles can then be used for deriving the optical properties of the cloud cell as functions of its geometrical thickness and droplet concentration, both parameters
which characterize the morphology of the cloud and the level of pollution of the airmass. The second step then consists in the characterization of the horizontal inhomogeneity of
the cloud layer for the determination of the mean cloud albedo.
Ž .
The EUropean Cloud Radiation EXperiment EUCREX-94 has been designed to test such an approach. The experimental strategy was based on coordinated flights with three
aircraft, one flying in cloud for measurements of the microphysical parameters, the two others flying 2 to 5 km above cloud top for remote sensing measurements of the cloud
Ž .
radiative properties with multidirectional radiometers POLDER , a multi-wavelength Ž
. Ž
. radiometer OVID and a lidar LEANDRE . Additional information about the cloud
radiative properties at a larger scale have been obtained from the space-borne AVHRR radiometer. The case study presented here is mission 206, which was conducted on April
18, 1994. The aerosol background in the boundary layer was significantly affected by pollution from north-western Europe and the droplet number concentration observed in
the stratocumulus was reaching values higher than 400 cm
y3
, quite higher than the values currently measured in pure marine boundary layer clouds. In this introductory
paper, the experimental approach will be discussed in relation to existing theories and parameterization schemes. The instrumental setup will be described and the meteorologi-
cal situation during mission 206 will be presented. The various measurements performed
Ž .
during this case study are presented in separate papers, by Pawlowska et al. 2000a for Ž
. Ž
. in situ measurements, by Schuller et al. 2000 for OVID, by Pelon et al. 2000 for
¨
Ž .
LEANDRE, and by Fouilloux et al. 2000 for AVHRR measurements. Finally, these various observations are summarized and compared in the conclusion paper by
Ž .
Pawlowska et al. 2000b .
2. Parameterization of the cloud microphysicsr r
