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JOURNAL OF GEOPHYSICAL RESEARCH,
VOL. 112,
D19202,
doi:10.1029/2007JD008541,
2007
An explanation for the effect of clouds over snow on the top-of-atmosphere bidirectional reflectance
Stephen R. Hudson
Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
Stephen G. Warren
Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
Abstract
It has been a long-standing puzzle why clouds, which should interact with solar radiation similarly to a thin layer of snow,
have such a dramatic effect on the reflectance observed by satellites over snow-covered regions. The presence of a cloud over
the snow strongly enhances the anisotropy of the scene, so that a cloud-over-snow scene appears darker than clear sky over
snow when observed near nadir, but much brighter when observed at large zenith angles in the forward reflected direction.
By contrast, when a plane-parallel cloud is placed above a plane-parallel snow surface in a model, it slightly decreases the
anisotropy of the system because of the cloud's smaller particles. Using a parameterization for the directional reflectance
from East Antarctic snow, developed from extensive near-surface observations from a tower, we show that the unexpected effect
of clouds over snow in this region is due to the non-plane-parallel nature of the snow surface, not to unexpected features
of the clouds. The snow surface roughness reduces the anisotropy of the reflected sunlight compared to that from a plane-parallel
snow surface. Clouds, by hiding this roughness with a surface that is very smooth in units of optical depth, increase the
anisotropy by bringing the system closer to the plane-parallel case. We use the surface parameterization to accurately model
reflectance observations made from the tower over a ground fog and from the top of the atmosphere over cloud-covered snow
by the MISR satellite instrument.
Received 10
February
2007;
accepted 10
July
2007;
published 10
October
2007.
Keywords: BRDF;
snow;
Antarctica.
Index Terms: 0758 Cryosphere: Remote sensing; 0736 Cryosphere: Snow (1827, 1863); 0321 Atmospheric Composition and Structure: Cloud/radiation interaction; 9310 Geographic Location: Antarctica (4207).
Read Full Article (file size: 733299 bytes) Cited by
Citation: Hudson, S. R., and S. G. Warren
(2007),
An explanation for the effect of clouds over snow on the top-of-atmosphere bidirectional reflectance,
J. Geophys. Res.,
112,
D19202,
doi:10.1029/2007JD008541.
Copyright 2007 by the American Geophysical Union.
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