Transcript The Powerpoint used for my talk on the effect of surface roughness and clouds on the BRDF 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
• 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
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 macroscale 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 backwardreflected intensities caused by the presence of a
cloud over snow seems to be caused by the cloud
hiding the rough surface