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