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Mesures physiques et signatures en télédétection

Polarization Properties of Snow and Clouds as Observed by the
Airborne POLDER Instrument
P Goloub G) ) M Herman^ 1 ), JL Deuz^ 1 ), R Frouin^ 2 )
(1) Laboratoire d "Optique Atmosphérique, Université de Lille, Villeneuve d'Ascq, France
(2) Scripps Institution of Oceanography, University of California, San Diego, La Jolla, USA
Aircraft photopolarimetric observation of cloud and snow fields made by the POLDER
instrument during the EUCREX and RACER campaigns are presented. Over clouds, the
polarized component of the reflectance at the wavelength of 450 nm and scettering angles of 90100
90100 degrees is sensitive to the molecular optical thickness between the cloud top and aircraft
altitudes and, therefore, may be used for cloud altimetry . Liquid water clouds and snow
exhibit similar spectral and bidirectional characteristics, but distinct polarization signatures. In
the rainbow region (scattering angles of about 140 degrees), water droplets strongly polarized
incident sunlight while snow crystals do not, making it possible to distinguish the two types of
targets by scanning these angular polarization.
Keywords : Polarization , Cloud altimetry, Snow-Cloud discrimination.
The purpose of this paper is to describe and contrast the optical (spectral, bidirectional and
polarization) properties of cloud and snow fields as observed by the POLarization and
Directionaly of the Earth Reflectance (POLDER) instruments (Deschamps et al, 1994). In the
first part we focus on clouds, investigating the possibility of using polarization measurements
to estimate the cloud top of altitude. Data acquired at various altitudes above clouds during
the 1991 European Cloud Radiation Experiment (EUCREX) campaign are used in this study. In
the second part we show, using data acquired over snow in clear sky condition and over clouds
above snow/ice during the 1991-1992 Research Antarctica Costal Ecosystem Rates (RACER)
campaign, that clouds and snow may have different polarization signatures, and we discuss the
implications for discriminating the two types of targets.
2.1 Experimental campaigns
Measurements made by the POLDER instrument during the abovementioned campaigns ,
namely EUCREX and RACER are analysed during. During EUCREX, the POLDER instrument
was flown aboard the DLR Falcon aircraft over the Alpes. During RACER, the POLDER
instrument was flown aboard the British Antarctic Survey (BAS) Twin Otter aircraft over the
Antarctic Peninsula. While the first campaign was dedicated to cloud studies, the second one
was designed for ocean color remote sensing. Yet observations over clouds and snow were
made during this campaign, allowing a comparison of the optical properties of the two target
in a environment where they coexist.
2.2 Flight conditions
On one occasion during the EUCREX campaign, the same extended cloud was overflown at
different altitudes by the Falcon aircraft. During the RACER campaign Twin Otter aircraft was
at a constant level of 3.5 km.