Anisotropy of reflected solar radiation from fields of finite clouds
Date
1987-11
Authors
Alberta, Timothy L., author
Cox, Stephen K., author
Journal Title
Journal ISSN
Volume Title
Abstract
Anisotropy of reflected solar radiation from simulated cloud fields is examined using a laboratory device known as the Cloud Field Optical Simulator (CFOS). Reflectance data from 20 cloud fields, each composed of a different random distribution of simulated clouds covering 30% of the target area, were collected. One hundred sixty-nine detector locations, each representing a different local zenith and azimuth angle, describe a hemispherical solid angle encompassing the cloud field. Radiances measured at these detector locations were numerically integrated, and locations were found where irradiances determined by assuming isotropy underestimated and overestimated the numerically integrated irradiances. Overestimates were found exclusively at large local zenith angles while the detector measured backscattered radiation. Underestimates were found primarily while the detector measured forward scattered radiation . Results of this comparison are presented in the form of frequency distributions, which also show increasing anisotropy with increasing incident zenith angle. Seven different incident zenith angles were analyzed for variations in reflected irradiance arising from different cloud field geometries. Results show irradiance differences as great as 107% at small incident zenith angles. Variations in irradiances were shown to be smaller at larger incident zenith angles. Also indicated are increased irradiance values at larger incident zenith angles. Fourier series analysis was performed on selected portions of the data set to examine the azimuthal dependence of irradiances. It was found that the number of terms required to produce an accurate representation of the intensity field at a specific local zenith angle was highly dependent on the number of azimuthal measurements available. The importance of both amplitude functions involved in the Fourier series calculations were also examined. Results indicate the increased importance of the sine term when assuming azimuthal dependence.
Description
November 1987.
FIRE series no. 4.
FIRE series no. 4.
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Subject
Solar radiation -- Measurement
Meteorological optics
Anisotropy
Cloud physics