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Atmospheric Research 55 2000 47–64 www.elsevier.comrlocateratmos
Optical and microphysical parameters of dense stratocumulus clouds during mission 206 of
EUCREX ’94 as retrieved from measurements made with the airborne lidar LEANDRE 1
J. Pelon
a,
, C. Flamant
a
, V. Trouillet
a
, P.H. Flamant
b
a
SerÕice d’Aeronomie du CNRS, 4 Place Jussieu, B 102, F-75252 Paris, France
´
b
Laboratoire de Meteorologie Dynamique, France
´ ´
Received 4 December 1999; accepted 1 March 2000
Abstract
Cloud parameters derived from measurements performed with the airborne backscatter lidar LEANDRE 1 during mission 206 of the EUCREX ’94 campaign are reported. A new method has
been developed to retrieve the extinction coefficient at the top of the dense stratocumulus deck under scrutiny during this mission. The largest extinction values are found to be related to the
highest cloud top altitude revealing the small-scale structure of vertical motions within the
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stratocumulus field. Cloud optical depth COD is estimated from extinction retrievals, as well as cloud top and cloud base altitude using nadir and zenith lidar observations, respectively.
Lidar-derived CODs are compared with CODs deduced from radiometric measurements made onboard the French research aircraft Avion de Recherche Atmospherique et de Teledetection
´ ´ ´ ´
Ž .
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ARATrF27 . A fair agreement is obtained within 20 for COD’s larger than 10. Our results show the potential of lidar measurements to analyze cloud properties at optical depths larger than
5. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Lidar; Stratocumulus; Cloud microphysics; Cloud optical properties
1. Introduction
Stratocumulus clouds are frequently observed in mid- and high latitudes to extend over large oceanic areas. They significantly reduce the solar flux reaching the surface,
Corresponding author. Tel.: q33-1-44-27-37-79. Ž
. E-mail address: jacques.pelonaero.jussieu.fr J. Pelon .
0169-8095r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
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and increase the earth–atmosphere albedo. As they weakly modify the upward longwave Ž
fluxes, their occurrence results in a strong modification of the radiation budget net .
cooling . Marine and continental stratocumulus are known to have different microphysi- Ž
. cal properties Martin et al., 1994 and, in turn, different impact on the shortwave
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radiation budget at the global scale Kiehl, 1994 . Measuring modifications in their characteristics is thus a critical issue with respect to climate change. This point has been
emphasized by the potential impact of aerosol on cloud nucleation, the so-called indirect Ž
. radiative forcing of aerosol Twomey, 1977; Boucher and Lohman, 1995 . The retrieval
of stratocumulus optical and microphysical properties at the global scale is, thus, an important step towards the understanding of their radiative impact. To this respect, the
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most significant parameters are the cloud structure cloud fraction, top and base heights , the effective radius and the liquid water content which control cloud optical depth
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. COD
Zhang et al., 1995 . In situ measurements provide accurate measurements of such parameters. However, they are only obtained over selected areas during field
campaigns. Remote sensing is the only way to derive quantitative information about the occurrence of clouds and their properties at scales large enough to be used as inputs in
general circulation models. Retrieval of cloud parameters using such techniques as
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aimed at in EUCREX ’94 Raschke et al., 1997
is thus, of importance. Passive measurements have been widely used for such a purpose. However, it has been shown
that the retrieval of cloud properties using standard techniques may be biased due to the Ž
. presence of overlying semi-transparent clouds Doutriaux et al., 1998 and to spatial
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cloud inhomogeneity Fouquart et al., 1990; Cahalan et al., 1994 . Active remote sensing by lidar appears to be a promising tool for complementary
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cloud parameter analysis at the global scale Doutriaux et al., 1998 . However, stratocu- mulus clouds being optically dense, they, most of the time, preclude lidar observations
over their whole vertical extent. Thus, the COD cannot be determined using standard Ž
inversion procedures as used for semi-transparent clouds Carnuth and Reiter, 1986; .
Flamant et al., 1996 . Up to now, airborne backscatter lidar have been used to analyze microphysical properties of moderately dense stratocumulus cloud at the meso-scale in
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complement to passive observations Spinhirne et al., 1989 . The objectives of the EUCREX ’94 campaign were focused on the retrieval of optical
and microphysical properties of both high and low cloud by means of remote sensing. The French airborne lidar LEANDRE 1 was one of the components of the experimental
setup. Lidar observations were made from one of the French research aircraft, the Avion Ž
. de Recherche Atmospherique et de Teledetection ARATrF27 . Results from flights
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Ž dedicated to the study of cirrus clouds have been presented elsewhere Sauvage et al.,
. 1999; Chepfer et al., 1999 .
The scope of this paper is to present the procedure developed to improve lidar signal Ž
y3
analysis in dense clouds typically characterized by water contents larger than 0.3 g m .
and optical depth larger than 10 and present cloud parameters obtained by lidar on the Ž
. case study of mission 206 18 April 1994 of EUCREX ’94 selected for the analysis of
Ž stratocumulus properties using remote sensing and in situ techniques Brenguier and
. Fouquart, 2000 . The description of the flight pattern is first given in Section 2.
Meteorological conditions observed during the flight are summarized in the same section. The analysis of airborne lidar data is presented and discussed in Section 3.
Lidar-derived CODs are discussed in Section 4 in connection with those derived from visible fluxes measured simultaneously by passive radiometers onboard the ARAT.
Liquid water content derived from the lidar data analysis in convective cells is used to Ž
. derive an estimate of droplet concentration and effective radius at cloud top Section 5 .
The cloud parameters retrieved from the lidar data will be further compared to other estimates from in situ and remote sensing instruments in the conclusion paper of the
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series Pawlowska et al., 2000b .
2. Airborne measurements
