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JOURNAL OF THE ATMOSPHERIC SCIENCES, 51, 1141-1156, 1994
Solar radiative transfer for wind-sheared cumulus cloud fields
H. W. Barker
Abstract
The Monte Carlo method of photon transport was used to simulate solar radiative transfer for cumulus-like cloud forms
(and cloud fields) possessing structural characteristics similar to those induced by wind shear. Using regular infinite
arrays of finite, slanted-cuboidal clouds (parallelepipeds), it was demonstrated that the magnitude of cloud field albedo
variation as a function of relative solar azimuth angle (up to 40% of albedo) can be larger than the albedo disparities
between plane-parallel clouds and fields of nonsheared finite clouds. In general, cloud field albedo is maximized when
shearing is away from the sun and minimized when shearing is toward the sun. This is explained by changes in effective
cloud fraction presented to the direct solar beam. The albedo of individual clouds, on the other hand, is maximized when
shearing is toward the sun, especially when shearing angle equals solar zenith angle. This is because of both reduced irradiance
onto cloud sides and enhanced effective optical depth of cloud. These results were corroborated by conducting similar experiments
using realistic cloud forms generated by a dynamical/microphysical cloud model. The magnitude of albedo differences between
sheared and corresponding nonsheared broken clouds reached 25% of the albedo. Again, this is due to differing effective cloud
fractions and side illumination.
It was found that the bidirectional reflectance functions (BDRFs) of sheared clouds are sensitive to solar azimuth angle.
Relative differences between BDRFs for clouds sheared toward and away from the sun can be as large as 50% for arrays of
idealized parallelepiped clouds and 25% for more realistic clouds. Differences are minimized when viewing is perpendicular
to the wind shear direction provided clouds are sheared toward or away from the sun. BDRFs for sheared clouds are much more
asymmetric near the zenith than BDRFs for corresponding cubic (nonsheared) clouds. Hence, viewing sheared clouds at a 60-degrees
zenith angle will not necessarily provide least biased estimates of cloud field albedo as is the case for nonsheared clouds.
Finally, it was demonstrated that BDRF differences arising from use of Mie and Henyey-Greenstein phase functions are substantially
smaller than differences associated with varying solar azimuth angle.
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