www.elsevier.comrlocateratmos
Introduction to the EUCREX-94 mission 206
Jean-Louis Brenguier
a,), Yves Fouquart
ba
Meteo-France, Centre National de Recherches Meteorologiques, GMEI´ ´ rMNP, 42 aÕ. Coriolis,
31057 Toulouse Cedex 01, France
b
Laboratoire d’Optique Atmospherique, Uni´ Õersite 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.q2000 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
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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
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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.
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E-mail address: jlb@meteo.fr J.-L Brenguier .
0169-8095r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved. Ž .
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
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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
ŽLWC in the cloud volume see Eq. 3 in Sec. 2 . In climate models, a boundary layer. Ž Ž . .
cloud is generally represented as a horizontally and vertically homogeneous layer. This
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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
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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.
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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
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radiative properties with multidirectional radiometers POLDER , a multi-wavelength
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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 cmy3, 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
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during this case study are presented in separate papers, by Pawlowska et al. 2000a for
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in situ measurements, by Schuller et al. 2000 for OVID, by Pelon et al. 2000 for
¨
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LEANDRE, and by Fouilloux et al. 2000 for AVHRR measurements. Finally, these various observations are summarized and compared in the conclusion paper by
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Pawlowska et al. 2000b .
2. Parameterization of the cloud microphysicsrrrrrradiation interaction
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The complex interaction between cloud droplets and radiation in the short-wave SW range can be reduced to a set of three parameters: the extinction coefficient sext, the single scattering albedo v, and the asymmetry factor g. Extinction is expressed as
` Ž . 2 Ž . Ž .
sextsH0Qext x pr n r d r, where r is the droplet radius and n r their size distribu-Ž .
tion, lis the radiation wavelength and xs2prrlis the size parameter. Qext x is the
Ž .
Mie efficiency factor van de Hulst, 1957 , that is generally replaced by its mean value over the range of x values corresponding to cloud droplets and short wave radiation, a value close to 2. The above formula is thus simplified as:
sexts2prs2N,
Ž .
1` Ž .
where NsH0n r d r is the total droplet concentration and r is the mean surface radiuss
Theoretical calculations have shown that the single scattering albedo and the asym-metry factor can be parameterized as linear functions of the effective radius of the droplet size distribution, resrv3rrs2, where rv is the mean volume radius of the
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distribution Hansen and Travis, 1974; Twomey and Cocks, 1989 . With this definition of the effective radius, the above expression for the extinction becomes:
3 w
where rw is the liquid water density.
Therefore, the first step for parameterizing radiative properties of inhomogeneous clouds consists in the definition of the spatial distribution of w and r . The emissivity ofe
Ž .
a cloud in the long-wave IR range is proportional to the vertical integral of w, also referred to as the liquid water path W. The conservation of W in any parameterization is thus a constraint in the choice of the vertical profile of w. The simplest solution is to assume that w and r are constant both in the horizontal and in the vertical, withe
Ž .
wsWrH where H is the cloud geometrical thickness VUPPM . More sophisticated
parameterizations have been proposed to study the effects of horizontal inhomogeneity and vertical stratification of cloud microphysics on the radiative properties. Our
method-Ž
ology relies upon the independent pixel approximation Cahalan et al., 1994a; Davis et
.
al., 1997a,b , which states that the radiative properties of a cloud column are fully determined by its microphysical properties and are independent from properties of adjacent columns, as long as the size of the cloud column is larger than the geometrical cloud thickness.
2.1. Vertical stratification
Ž . Ž . Ž
Stephens 1978a,b and Stephens et al. 1978 present, in a series of three papers PI,
.
PII, and PIII, respectively , an extensive study of the radiative properties of extended water clouds. In PI, theoretical calculations are performed with various types of clouds in order to derive their SW heating rate and IR cooling rate. In PII, analytical approximations are validated against the detailed calculations, and in PIII, predictions are compared to in situ measurements. The clouds are supposed to be horizontally homogeneous and various assumptions are tested for the vertical profiles of w and r :e
constant w and r , constant r with w increasing with height above cloud base, ande e
profile of w is included. In PI, Sec. 4d, it is also stated that whereas the absorption is relatively insensitive to changes of cloud microstructure, the cloud albedo is more strongly dependent on drop-size distribution. This series of papers thus suggest that it is crucial for the study of the aerosol indirect effect to consider in radiative transfer calculations the vertical profiles of w and r , and the relationship between N and r .e e Detailed measurements of the thermodynamics, cloud physics, and radiation fields
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performed during the Joint Air–Sea Interaction experiment JASIN are reported by
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Slingo et al. 1982 who precisely document the difference between the horizontal and the vertical variabilities of the microphysics in stratocumulus. The observed vertical profiles of LWC are close to the adiabatic reference while the total droplet concentration is almost constant with altitude. Therefore, the observed droplet spectra show an
Ž .
increase of the mean volume radius with altitude above cloud base according to Eq. 3 . In contrast, the horizontal variability, mainly close to the cloud top, is related to entrainment of dry air and mixing that results in diluted droplet concentrations while the droplet radii remain close to their adiabatic values at that level.
These observations and data from previous experiments are used in Slingo and
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Schrecker 1982 for validating the following parameterizations of bulk radiative properties as functions of w and r .e
sextsw a
Ž
qbrre.
Ž .
41yvscqd re
Ž .
5gseqfr ,e
Ž .
6where a, b, c, d, e, and f are wavelength dependent. Simulations with constant w and
re are performed to test the minimum number of spectral bands needed for the calculation of realistic profiles of the heating rate. The second series of simulations is made with a fixed r and w increasing according to the adiabatic profile. Typical valuese
Ž .
of r , reported in the literature between 4.21 and 16.6e mm , are selected to test the dependence of cloud SW properties on drop size distribution. They are considered as representative of the whole cloud depth. For LWC profiles, three typical types of clouds are selected with w values at cloud top from 0.41 to 1.11 g my3
. The predicted variations of the cloud absorption and cloud albedo as functions of the effective radius provide information about the indirect effect. However, the assumption of constant
Ž .
effective radius throughout the cloud depth with no explicit reference to Eq. 3 leads to unrealistic results. For example, the assumption of an effective radius of 4.21mm for a sub-tropical type cloud with a LWC value at the top of 1.11 g my3, corresponds to an unrealistic total droplet concentration larger than 3500 cmy3. The third set of
simula-tions is performed with various vertical profiles of w and r . They show that the heatinge rate profiles mainly depend on the vertical distribution of LWC while the assumption on
r does not seem to be crucial. However, the indirect effect refers specifically to thee
increase of the droplet concentration that could result from changes in the properties of
Ž .
Ž1989 or restricted through Eq. 3 to realistic values of the droplet concentration as in. Ž .
Ž . Ž .
Slingo 1990 and in Jones et al. 1994 .
2.2. Horizontal inhomogeneity
The various approaches described in the previous section have considered simple idealized models. Actual clouds are more complicated because of the spatial variability of the microphysical fields. The adiabatic model provides a simple and unique descrip-tion of the vertical profiles but it is actually restricted to the convective cores in a cloud layer. The effects of entrainment and mixing with dry air and of the precipitations lead to an infinite variety of spatial distributions of the LWC and droplet sizes, that is of the in-cloud extinction. The optical properties of a cloud layer are dependent upon the statistical distributions of these parameters but also upon the scale of the inhomo-geneities, especially at scales comparable to the mean free path of the photons in the cloud layer.
Two steps are particularly crucial for the study of the effects of inhomogeneities on cloud radiative properties. The first is to develop numerical models able to simulate the radiative transfer in an inhomogeneous medium. The second is to generate for such models cloud simulations with realistic distributions of the internal properties. Monte
Ž .
Carlo techniques Cashwell and Everett, 1959 have provided an efficient solution to the first step, but the second, namely, the characterization of the microphysical variability in actual clouds, is still a puzzle.
Ž .
Cahalan et al. 1994a,b have shown that a mosaic of plane parallel clouds with various values of optical thickness has a lower albedo than the homogeneous plane
Table 1
Description of the legs flown by the three aircraft, between points M and A, during mission 206
Ž .
Aircraft Time Height m Comments
Merlin 9:30–9:51 850 M–A horizontal
9:57–10:18 400–1200 A–M zigzag
10:21–10:41 700–1100 M–A horizontal near cloud top 10:46–11:06 700–1000 A–M horizontal near cloud top
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
ŽMarshak 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-
´ ´
tions were measured with the Fast-FSSP, an improved version of the PMS FSSP-100
ŽBrenguier 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.
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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
ŽDTs68C located around 850 hPa at 00:00 UTC, and descending to 900 hPa 1050 m. Ž .
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
The cloud layer observed during mission 206 is particularly suited for the objectives of the project. The satellite image of the layer at the meso-scale is presented in Fig. 1 of
Ž .
Fouilloux et al. 2000 . As a whole, the layer looks homogeneous but a detailed analysis shows large fluctuations of optical thickness due to convective cells embedded within
Ž Ž ..
the layer see the POLDER images in Pawlowska et al. 2000b . In addition, there is a continuous trend, with deep convective cells at the southern end of the leg to thin and broken clouds at the northern end. Such a large variability is appropriate for accurate validations of remote sensing instruments and parameterization schemes over a broad range of realizations.
Up to now, most of the emphasis in the studies of the aerosol’s indirect effect has been put on the Twomey effect, that is changes in cloud albedo with droplet concentra-tion. However, there is a second indirect effect related to changes in precipitation efficiency with changes in droplet concentration. This could result in changes of the
Ž .
cloud extent and lifetime and significantly intensify the indirect effect Albrecht, 1989 . The evaluation of the second indirect effect thus requires detailed experimental informa-tion on the interacinforma-tion between the cloud structures, the internal microphysics and the radiative properties, such as the one described in this series of papers.
Ž .
Pawlowska et al. 2000a describe in situ measurements with the M-IV. The vertical profile of the microphysics and its space and time variations between points M and A are documented from ascents and descents of the aircraft through the cloud layer. Horizontal statistics of these parameters are then derived from constant level legs.
Ž .
In Fouilloux et al. 2000 , the AVHRR image recorded at 08:25 UTC is analyzed. Various methods for the retrieval of cloud properties from the measured reflectances in the visible and near infra-red are tested. Despite the time lag between this image and the
Ž .
aircraft experiment 1.5 h , the results will be used for comparison with in situ
Ž . Ž
measurements M-IV and with close remote sensing measurements DLR-F20 and
.
Ž .
In Schuller et al. 2000 , multi-wavelength measurements of cloud radiances with the
¨
OVID radiometer, on board the DLR-F20 are analyzed. A method is developed for the retrieval of cloud optical thickness and droplet effective radius. These parameters are calculated along the legs flown by the DLR-F20. The comparison with the values derived from POLDER is particularly interesting as these two instruments are based on different principles.
Ž .
In Pelon et al. 2000 , lidar measurements performed on board the ARAT are analyzed. A method is developed for the retrieval of cloud top height, scattering extinction coefficient at cloud top, cloud optical thickness, effective radius and droplet concentration. The good resolution in the retrieval of cloud top altitude is particularly useful for the interpretation of the radiometric measurements.
Ž .
The conclusion paper Pawlowska et al., 2000b combines all these approaches. The adiabatic parameterization of cloud optical thickness is tested at the scale of the cloud cells by comparing estimates derived from in situ measurements of the cloud micro-physics with estimates derived from remote sensing measurements. The analysis is then extended to the scale of the cloud system by comparing the frequency distributions of optical thickness and effective radius, as retrieved at different scales with the remote sensing instruments on board the DLR-F20, the ARAT and the AVHRR satellite.
Acknowledgements
The authors are grateful to the Meteo-France Merlin-IV, ARAT, and DLR-F20 teams
´ ´
for their efficient contribution to the data collection. This work has been supported by INSU-PATOM under grant 94r08 and by the European Union, Environment and Climate Division, under grant ENV4-CT95-0117.
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