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Bidirectional Reflectance of Snow
The instruments used to measure bidirectional reflectance from the top of the tower.
Bidirectional reflectance measurements were made from the top of the 32-meter tower during
both the second and third field seasons. The measurements were made using a
spectroradiometer manufactured by Analytical Spectral Devices (ASD). The ASD records the
radiance every nanometer from 350—2500 nm. In the photo above, the ASD is inside
the lower box, with a signal cable attached to the laptop computer in the upper box. A
fiber optic cable channels the incoming light to the ASD, and is mounted to a goniometer
that allowed us to position the fiber to measure spectra of reflected sunlight coming from
any nadir angle over a 255° range of azimuths containing undisturbed snow.|
Bidirectional reflectance measurements were made for 31 distinct solar zenith angles (differing by 1 degree or more) from 52 degrees (noon) to 87 degrees (midnight). Counting repeated solar zenith angles, we have 98 observations of the pattern. These measurements are being used to determine the anisotropic reflectance factor of the snow and, along with spectral albedo measurements, the bidirectional reflectance distribution function (BRDF) of the snow.
The BRDF will be used as a lower boundary condition in models that will accurately simulate radiative transfer through the atmosphere above the Antarctic Plateau. These models will be be used to determine the top-of-atmosphere (TOA) BRDF, and to investigate the effect of clouds on the TOA BRDF. To assist in the investigation of the effect of clouds, several observations of the surface bidirectional reflectance were made with thin fog layers between the surface and the top of the tower.
Studies of the Solar Radiation Budget
The observed BRDFs will also be used as the lower boundary condition in radiative transfer
models to investigate the effect of clouds and absorbing gases on the solar radiation budget
of the Antarctic Plateau. The models will be used, along with cloud observations, to
estimate surface and TOA shortwave cloud radiative forcing. Such estimates are necessary
because satellite estimates of cloud radiative forcing in Antarctica are highly uncertain
because satellite cloud identification over Antarctica is extremely unreliable.|
The models will also be used to determine the effect of gaseous absorption of solar radiation by the Antarctic atmosphere. This absorption may be more significant over Antarctica than over most parts of the planet because in summer there is a large amount of sunlight traveling both up and down due to the high albedo of the Antarctic surface. It is also possible that the relative importance of certain absorbing gases may be different in Antarctica than elsewhere because of the low water vapor concentrations and variable ozone concentrations.
Absorption Spectrum of Ice
The instruments used for measuring transmission of sunlight into the snow.
|Using the ASD on the surface, with the fiber optic cable fitted in a probe that could be inserted to various depths in the snow, the spectral transmission of sunlight into the snow was measured. The scattering coefficient, which is independent of wavelength in this region, can be determined by looking at these measurements at a wavelength at which the absorption coefficient is known. Using this scattering coefficient and the measurements at wavelengths where the absorption coefficient is uncertain (λ<600 nm), the absorption coefficient can be determined. This work is being carried out to resolve a discrepancy in published absorption spectra of ice.|
Antarctica as a Satellite Calibration Target
|Our collaborators at LGGE are developing a method to track the calibration drift of the Vegetation sensor on the SPOT4 satellite. The region around Dome C is useful for this because it is a spatially-homogeneous surface with little temporal variability in reflectance characteristics, and its high latitude location allows for frequent overpasses by polar-orbiting satellites such as SPOT4. For more on this part of the project, see the third link below.|
Additional materials with more information:
Steve Hudson's thesis proposal contains more details about the plans for the research involving the surface and TOA BRDFs and cloud radiative forcing.
The 2003/04 field report for IPEV, prepared by Delphine Six, describes the activities of the second field season.
This LGGE site describes the satellite calibration research.
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