675 
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 
ABSTRACT : 
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 90- 
100 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. 
1- INTRODUCTION 
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.DATA 
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.