SOLAR RADIATIVE-TRANSFER FOR WIND-SHEARED CUMULUS CLOUD FIELDS

The Monte Carlo method of photon transport was used to simulate solar radiative transfer for cumulus-like cloud forms (and cloud fields) pos sessing structural characteristics similar to those induced by wind sh ear. Using regular infinite arrays of finite, slanted-cuboidal clouds (parallelepipeds), it was demonstrated that the magnitude of cloud fie ld albedo variation as a function of relative solar azimuth angle (up to 40% of albedo) can be larger than the albedo disparities between pl ane-parallel clouds and fields of nonsheared finite clouds. In general , cloud field albedo is maximized when shearing is away from the sun a nd minimized when shearing is toward the sun. This is explained by cha nges in effective cloud fraction presented to the direct solar beam. T he albedo of individual clouds, on the other hand, is maximized when s hearing is toward the sun, especially when shearing angle equals solar zenith angle. This is because of both reduced irradiance onto cloud s ides 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 magnitud e of albedo differences between sheared and corresponding nonsheared b roken clouds reached 25% of the albedo. Again, this is due to differin g effective cloud fractions and side illumination. It was found that t he bidirectional reflectance functions (BDRFs) of sheared clouds are s ensitive to solar azimuth angle. Relative differences between BDRFs fo r clouds sheared toward and away from the sun can be as large as 50% f or arrays of idealized parallelepiped clouds and 25% for more realisti c clouds. Differences are minimized when viewing is perpendicular to t he wind shear direction provided clouds are sheared toward or away fro m 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 necessari ly provide least biased estimates of cloud field albedo as is the case for nonsheared clouds. Finally, it was demonstrated that BDRF differe nces arising from use of Mie and Henyey-Greenstein phase functions are substantially smaller than differences associated with varying solar azimuth angle.