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The Use of Vegetation For Estimating Broken-Cloud Optical Properties From Surface MeasurementsIt is known that because of complex three-dimensional (3D) radiative effects of broken clouds, the retrieval of cloud optical properties from upward measurements based on a one-dimensional (1D) inversion technique almost surely fails. To remove radiative effects of 3D cloud structure, we have developed a new technique that retrieves cloud optical thickness for broken clouds above green vegetation from simultaneous surface measurements in the VIS and Near Infrared (NIR) spectral regions. The theoretical basis of the method is the very different spectral behavior of cloud liquid water drops and green vegetation. For example, cloud optical properties, and hence cloud reflectivities, change little between 650 and 860 nm, while the vegetated surface albedo changes from 0.05 to 0.5 between the same two wavelengths. This spectral contrast in surface albedo suggests using ground measurements at both wavelengths not independently, but as an algebraic combination (a spectral index). For a spectral band in the NIR region, the green vegetation acts as a powerful reflector that "illuminates" horizontally inhomogeneous clouds from below. This provides the extra information needed to largely remove the 3D radiative effects, especially in the case of broken clouds; this in turn allows the retrieval of cloud optical depth using traditional 1D radiative transfer theory. This approach is similar to the so-called Green's function problem for radiative transfer where a laser beam illuminates clouds and the resulting "spot-size" of the reflected light around the beam characterizes cloud properties. We generalize Green's function theory to surf ace-cloud interaction and develop new spectral indices from which broken-cloud optical depth can be retrieved.
Document ID
20000083034
Acquisition Source
Goddard Space Flight Center
Document Type
Conference Paper
Authors
Marshak, A.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Knyazikhin, Y.
(Boston Univ. Boston, MA United States)
Davis, A. B.
(Los Alamos National Lab. NM United States)
Wiscombe, W.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Date Acquired
August 19, 2013
Publication Date
January 1, 2000
Subject Category
Earth Resources And Remote Sensing
Meeting Information
Meeting: International Radiation Symposium
Location: Saint Petersburg
Country: Russia
Start Date: July 24, 2000
End Date: July 29, 2000
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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