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Spectral Bidirectional Reflectance of Antarctic Snow

Spectral Bidirectional Reflectance of Antarctic Snow. Surface Roughness and Clouds. Stephen R. Hudson. Coauthors: Stephen G. Warren, Richard E. Brandt, Thomas C. Grenfell, and Delphine Six. Background — Observations.

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Spectral Bidirectional Reflectance of Antarctic Snow

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  1. Spectral Bidirectional Reflectance of Antarctic Snow Surface Roughness and Clouds Stephen R. Hudson Coauthors: Stephen G. Warren, Richard E. Brandt, Thomas C. Grenfell, and Delphine Six

  2. Background — Observations • We have made spectral directional-reflectance observations of the snow at Dome C • 75°S, 123°E, 3250 m • l 350—2400 nm • qo 52—87° • Representative of much of the East Antarctic Plateau

  3. Background — Observations The observations were made with a 15° conical field of view from 32 m above the surface to capture the effects of the natural snow-surface roughness

  4. Background — Parameterization • Using these observations we developed parameterizations for the anisotropic reflectance factor of Antarctic snow for most wavelengths, solar zenith angles, and viewing angles • They provide a realistic surface boundary for Antarctic RT modeling

  5. Background — Advertisement • Details about the observations and parameterizations are in the extended abstract and in press in JGR • Today I will discuss the importance of surface roughness and how it relates to the effect of clouds on TOA-BRDF

  6. What does surface roughness do? • Looking towards the sun you see shaded faces • Looking away from the sun you see faces tilted towards the sun

  7. Is the roughness effect important? • At South Pole, Warren et al. (1998) found intensities near the forward reflectance peak were about 25% greater when the solar azimuth was perpendicular to the sastrugi than when it was parallel to them • In the perpendicular case they also observed a smaller increase in backscattered intensity • There was little effect on near-nadir intensity • Leroux and Fily (1998) obtained similar results with a modeling study, but the magnitude of their effect was larger due to the idealized geometry of the sastrugi in their model

  8. Roughness effect at Dome C • Used DISORT to model the surface reflectance with a variety of phase functions (Mie, HG, Yang and Xie) • Placed the snow under a clear, summertime-average, Dome-C atmosphere

  9. Roughness effect at Dome C • Rough aggregate grains produce the best match between the model and observations, but the model produces significant error consistent with macro-scale roughness effects for all of the phase functions

  10. Roughness effect at Dome C • The error increases with solar zenith angle • The roughness has little effect on near-nadir intensity

  11. Effect of clouds on BRDF over snow • The presence of a cloud over a snow surface has been observed to enhance forward reflectance into large viewing angles while reducing reflectance into other angles, including nadir (Welch & Wielicki 1989, Landsat; Wilson & Di Girolamo 2004, MISR; Kato & Loeb 2005, CERES) • This observation is unexpected because the cloud particles are smaller, and are therefore likely to be more isotropically-scattering, than the snow grains • We believe much of this effect is caused by clouds hiding the surface roughness, not by differences in the single-scattering properties of snow and cloud particles

  12. Effect of clouds at Dome C • Nights with shallow fog allowed us to observe the reflectance of a cloud over the snow surface

  13. Observation of fog at Dome C • The difference caused by fog at Dome C is similar to the error in the plane-parallel modeling results

  14. Modeling fog at Dome C • Using DISORT to model the upwelling intensity above a thin cloud over a surface with the observed BRDF gives results very similar to the foggy observation

  15. Observed effect requires rough surface • When the same cloud is placed over a modeled (flat) snow surface it does not produce the correct effect

  16. Summary • Snow-surface roughness significantly affects the BRDF of snow • Macroscale roughness should be considered along with microscale snow properties in modeling and observational studies of snow BRDF • The strong enhancement of forward-reflected intensities and the reduction of backward-reflected intensities caused by the presence of a cloud over snow seems to be caused by the cloud hiding the rough surface

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