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A comparison of theoretical and observed radiances from non-precipitating cumulus clouds




DeMaria, Mark, author
McKee, Thomas B., author
Department of Atmospheric Science, Colorado State University, publisher

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Relative radiances from a theoretical model for the scattering of solar radiation by finite clouds were compared to observed radiances from the sides of non-precipitating cumulus clouds. The sides of 5 cumulus clouds ranging from 1-3 km in depth with width to depth ratios of about 1.5 were scanned with an aircraft mounted radiometer sensitive in the spectral range of 0.65 to 0.95 μm. Photographs of each cloud were taken just before each scan with a camera optically aligned with the radiometer. The theoretical model uses the Monte Carlo method of radiative transfer for finite clouds ·in the shape of rectangular parallelepipeds. The radiometer field of view intercepted a circular spot with a diameter of about 1/20 of the cloud height. On this scale the radiometer scans were strongly affected by small scale cloud features so that the model radiances which are averages over a smooth cloud face did not compare well. A linear regression between the model and actual relative radiances resulted in a correlation coefficient (r) of only 0.47. When each scan was averaged, the smaller scale cloud features became less important. A linear regression for scan averages resulted in a correlation coefficient of 0.64. On this scale, the model calculations were verified by the observations except for a few observations where smaller scale cloud features and model cloud corners were important. Elimination of the data points related to cloud features not contained in the model resulted in a correlation coefficient of 0.86. When several successive scans were averaged together, the correlation coefficient increased to 0.91. On this scale, the effects of the smaller scale cloud features and model cloud corners became noticeably less important. The variability in relative radiance from the sides of the clouds was also studied. An upper limit for the relative radiance variations in the observed clouds was estimated from film density to be by a factor of between four and five. Variations of radiance were less for the smaller clouds. The radiance variations of the clouds gradually decreased as the diameter of the area viewed increased from 10 m to 800 m. Most radiance variations were resolved on the scale of 100 m with the radiometer.


Includes bibliographical references (pages 82-83).
June 1979.

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Atmospheric radiation -- Measurement
Radiative transfer -- Measurement


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