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

• We have made spectral directional-reflectance observations of the snow at Dome C– 75°S, 123°E, 3250 m– 350—2400 nm– o 52—87°

• Representative of much of the East Antarctic Plateau

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

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

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

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

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

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

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

Roughness effect at Dome C

• The error increases with solar zenith angle

• The roughness has little effect on near-nadir intensity

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

Effect of clouds at Dome C

• Nights with shallow fog allowed us to observe the reflectance of a cloud over the snow surface

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

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

Observed effect requires rough surface

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

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