VEGETATION
PRELAUNCH PHASE REPORT
February 1998
ANTARCTIC SNOW CHARACTERISTICS
FROM POLDER AND VEGETATION DATA
UTILISATION DES DONNEES POLDER ET VEGETATION
POUR L'ETUDE DE L'ALBEDO DE LA NEIGE
EN ANTARCTIQUE
PI: Michel FILY
Laboratoire de Glaciologie et Géophysique de l'Environnement
Grenoble, FRANCE
Convention N 96/CNES/0394
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PROJECT ABSTRACT
The main objective of this proposal is to use VEGETATION with POLDER data to study the albedo
of snow in Antarctica. Two main factors which can modify the albedo are the roughness and the
grain size, the pollution is negligible in Antarctica. Those factors are changing upon space and time
over the ice sheet, they will be studied on a large scale. This study will help the parameterization of
the albedo in GCM (Global Climate Models) depending on climate parameters such as the snow
accumulation rate, the temperature or the wind.
This proposal is closely linked to 3 other projects which are already accepted and funded:
- Antarctic snow characteristics from POLDER on ADEOS (M. Fily, PI)
POLDER data will provide the necessary information on the BRDF that are necessary to
interpret the directional VEGETATION measurements
- Investigation and modeling of the physical characteristics and properties of snow at the surface of
polar ice sheets (Project Polar Snow accepted by EC, C. Genthon from LGGE is PI)
The surface grain size and the snow albedo are important input and output of the
metamorphism model we use. VEGETATION measurements will be used to get the spatial
and temporal variations of the grain size over the ice sheet.
- Development of a spectro-goniometric system in a cold laboratory for the snow BRDF
measurements (coordinated by B. Schmitt from LGGE)
The BRDF of smooth snow is necessary to be able to get the roughness and the surface
slopes from POLDER measurements. Those measurements will also be used for the
reflectance model validation.
VEGETATION is an adequate instrument because:
- it provides images on a large enough scale to study the complete ice sheet
- the middle infrared channel (MIR) depends strongly on the grain size
- MIR channel will help cloud detection
Many studies are presently under development in our laboratories which are directly related to the
future interpretation of VEGETATON data:
- development of a snow bidirectional reflectance model
- study of the effect of the snow geometry (size and shape) on the reflectance
- study of the effects of topography and roughness on satellite images.
We are associated to many scientific traverses which are planned in the next years on the Antarctic
Ice Sheet. Remote sensing related studies will be done along these traverses for validation, in
particular concerning the roughness and the grain size.
Keywords: Snow, Antarctica, albedo, GCM, POLDER
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1 - PRELAUNCH PHASE REPORT
1.1 - Snow reflectance modeling
1) BRDF:
Presently the discrete ordinate method (Stamnes et al., 1988) and the doubling-adding code (Hansen
and Hovenier, 1971) are available at LGGE. The method of doubling-adding has been chosen,
because it is one of the most flexible to include the polarization effects; it is also well adapted to
thick homogeneous layers, as the snow layers.
In a first step the snow grains were assumed to be spherical with a distribution of sizes, their
radiative characteristics are computed with a Mie code including a truncature procedure for the
forward peak of the phase function. For the BRDF an intercomparison was doned between the results
of two codes (discrete ordinates and doubling-adding). The doubling-adding provides the
polarization of the reflected radiation. The second step consisted in modeling the snow grains by
hexagonal crystals; their radiative characteristics were derived from a code developed at the LOA
and introduced in the adding-doubling code. Comparisons between results obtained for spherical and
hexagonal particles are done.
Ref: Leroux, 1996 (thèse); Leroux et al., 1997
2) Snow roughness (sastrugi):
A model was developed based on Roujean et al. (1992) that takes into account anisotropic
protrusions at the surface of snow. This model was chosen because it is anisotropic and takes into
account the shadowing effects. The results were compared with in situ measurements made at South
Pole and provided by S. Warren from Seattle.
Ref.: Leroux and Fily, 1998
1.2 - Laboratory measurements
1) Directional-hemispherical snow reflectance:
Several laboratory experiments were undertaken at the Centre d'Etudes de la Neige (CEN, C.
Sergent, C. Leroux) on the effect of the grain size geometry on the reflectance. Radiometry
measurements included spectral reflectance and transmittance between 0.4 and 1.6 m under diffuse
illumination of a snow core in an integration sphere. Grain size geometry is obtained from analysis of
digital images of snow grains: many characteristics are obtained from a sample of snow grains such
as the mean convex diameter which seems to be the best parameter to be compared to the effective
radius of snow grain to be included in the model to obtain comparable reflectance.
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Ref.: Sergent et al., 1997
2) BRDF:
A spectro-goniometric system is under development at the LGGE (B. Schmitt) to measure the snow
BRDF between 0.3 m and 3 m. Preliminary studies were done in 94-96, the technical realisation
began in 1996. Actually the experimental device is fully funded (about 1 MF) and the first results
will be available in 1998. We want this experiment to be excellent and a lot of care is taken at each
step of its development. This is a key element of the program because we will be able to get a
complete set of BRDF for smooth snow surfaces at a precision never obtained so far.
1.3 - In situ measurements
1) Alps:
Measurements were made in 1995 with the polarized radiometer from LOA at 3 spectral bands
(Visible , Near Infrared and Middle Infrared) at many different incidence angles. Simultaneous
measurements of the snow characteristics are available. Comparisons with model results were done.
The main result is that the grain shape is very important at 1.6 m. It will improve our ability to
measure the snow grain size.
Ref.: Leroux et al, 1998 a; Leroux et al., 1997
Measurements were obtained in 97 and 98 from a field radiometer on different sites near Grenoble.
The reflectance is measured at different viewing angles in four channels including a 1.6 m channel.
Snow samples are collected for the size measurements in laboratory. More than 10 set of data are
available and the interpretation is under progress.
2) Antarctica:
Snow grain size:
The grain size is the most important snow characteristic for albedo in Antarctica. Therefore its
measurement is an important element of our program. This work is also done in the framework of the
european Polar Project directed by C. Genthon from LGGE.
Thanks to French Polar expedition and to many collaborations with other countries we were able to
collect a lot of snow samples from Antarctica (Fig. 1). The sampling technique is easy: a small
amount of snow is kept in a little flask of isooctane to prevent snow from metamorphism. Then the
samples are analysed at Grenoble with an image analysis system. This technique provides an
homogeneous, objective measurement of the snow grain size at very different sites. The main
problem is often to keep the samples frozen from Antarctica to Grenoble.
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So far samples were acquired at the following dates and locations:
- 95-96 samples were acquired between Dumont d'Urville and Dôme C and at Vostok
- 96-97:
- Italian traverse: Terra-Nova Bay -Talos Dôme
- Norwegian ang German traverses in Queen Maud Land
- French expedition at Dôme C
- Spanish expedition in the Antarctic peninsula.
A total of about 300 snow samples were collected and about 10 000 digital images of snow grains
were acquired in a cold lab at Grenoble. All the images were stored on CD-ROM and distributed to
collaborators. New algorithms were implemented to get a robust size parameter and the results will
be published soon.
This is the first large scale experiment to get comparable (objective) grain sizes in Antarctica.
New samples are being acquired in 97-98 by English and Belgium expeditions.
Roughness measurements:
Roughness is the second important surface parameter for snow albedo.
Roughness measurements were also done along traverses: the technique depends of the available
time and personnel. The most simple is photos, other techniques include stereo-photo, topographic
technique, differential GPS.
The best set of data was acquired so far between Dumont d'Urville and DômeC at very different sites
where the roughness is small and where there are high sastrugi. Ground photos and also aerial photos
were taken. From aerial photos the sastrugi direction is available: a first set of photos were acquired
in 96-97, a second one in 97-98.
Ref.: Mondet et al., 1997
1.4 - Satellite data interpretation
For the hereafter studies we acquired satellite data from different sensors. The selected period was
November 96 because POLDER data were available at this time and the selected area is Terre Adelie
because of available ground truth. The satellite available data are POLDER (every day), ATSR
(about 20 images), AVHRR (about 20 images).
Cloud discrimination
Clouds are similar to snow, in that they are composed of ice and/or water particles in the air. The
main differences are that, for clouds, the density is lower and the grains are generally smaller. The
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spectral signature of clouds is similar to that of snow but, because the grains are smaller, their
reflectance is generally higher at the longest wavelengths of the solar spectrum. If a middle infrared
channel is available, as the 1.6 m VEGETATION channel, it is easier to discriminate clouds from
snow except in the case of thin cirrus clouds.
As the cloud discrimination is an important issue we are using different methods:
- 1.6 m cloud higher reflectance with ATSR
- textural analysis or visual inspection of images: ATSR and AVHRR. In fact this is possible by
comparing a serie of images of the same area. We also have tested a textural analysis based on the
comparison of satellite data in the solar spectrum with an illuminated Digital Elevation Model
available from altimetry: AVHRR, ATSR and POLDER
- another way of discriminating clouds is to use the polarization of the signal as shown by Goloub et
al. (1992) from POLDER measurements, it works fine for water clouds
- use of the O2 channels of POLDER to determine an 'oxygen pressure' between the target and the
satellite: this seems quite effective on the ice sheet.
In conclusion for Vegetation the best methods will be to use the 1.6 m channel and a textural
analysis method based on the real topoghraphy.
Snow reflectance:
For cloud free areas the satellite data are used for:
- intercomparison between satellites
- determination of BRDF from POLDER data
- comparison between model and measurements at different wavelengths and different geometry
(POLDER, ATSR, AVHRR)
- study of the effect of sastrugi (roughness) (mainmy POLDER)
- study of the grain size: mainly at 1.6 m (ATSR) but also at shorter wavelengths (POLDER,
AVHRR)
This work is in progress.
References directly linked to the project:
Fily M., C. Leroux, J. Lenoble, and C. Sergent, 1998, Terrestrial snow studies from remote sensing
in the solar spectrum and the thermal infrared, Solar System Ices, Kluwer Academic Publishers, B.
Schmitt et al. (eds.), 421-441.
Fily M., B. Bourdelles, J.P. Dedieu and C. Sergent, 1996, Comparison of in situ and Landsat
Thematic Mapper derived snow grain characteristics in the Alps, Remote Sensing of Environment,
59: 452-460.
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Sergent C., C. Leroux, E. Pougatch, and Florence Guirado, 1997, Hemispherical-directional
reflectance measurements of natural snows in the 0.9-1.45 m spectral range. Comparison with
adding-doubling modelling, Annals of Glaciology, in press.
Leroux C., J. Lenoble, G. Brogniez, J.W. Hovenier, and J.F. De Haan, 1997, A model for the
bidirectional polarized reflectance of snow, J. Quant. Spectro. Radiat. Transfer, in press.
Leroux C., J.L. Deuze, P. Goloub, C. Sergent, and M. Fily, 1998 a), Ground measurements of the
polarized bidirectional reflectance of snow in the middle infrared spectral domain. Comparisons with
model results. Submitted to J. Geophys. Res.
Leroux C. and M. Fily, 1998 b), Modelling the effect of sastrugi on the snow reflectance. Submitted
to J. Geophys. Res.
Mondet J., C. Leroux, G. Vinay, F. Candaudap, E. Le Meur, and M. Fily, 1997, Images of Antarctic
sastrugi at different scales from ground to satellite. Proceedings of the EARSeL Workshop 'Remote
sensing of land ice and snow, University of Freiburg, Germany, 17-18 April 1997.
2 - DATA REQUIREMENTS
It is obviously impossible to get data over Antarctica all the year because of the polar night. Based on
a maximum solar zenith angle of 75 at noon, the useful period extends from 2 months at South Pole
to 6 months at 65 latitude. Due to the large number of tracks on the polar areas it will therefore be
possible to acquire VEGETATION data everywhere in Antarctica.
We propose to get 3 sets of data per year: 2 sets in November and February will cover most part of
the ice sheet at the beginning and the end of summer. Those 2 sets will be important to know the
temporal variability of the albedo. A third set will be acquired at the end of December to get a
complete coverage of the ice sheet including the South Pole.
A set of data is a clear sky complete coverage of the areas where illumination is sufficient above the
Antarctic ice sheet (South of 65 S). Many images of each area must be obtained at different time in
a short interval in order to get different solar illumination (azimuth mainly) for the same surface.
VGT-P products on CD-Rom are the most adequate for us.
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