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The Radiative Properties of Uniform and Broken Stratus: An Observational and Modelling Study Utilizing the Independent Column Approximation for Solar Radiative Transfer

Clothiaux, E.E., The Pennsylvania State University; Barker, H.W., Atmospheric Environment Service of Canada; Kato, S., Hampton University; Dong, X., Analytical Service and Materials, Inc. Ackerman, T.P., The Pennsylvania State University; Liljegren, J.C., Ames Laboratory
Ninth Atmospheric Radiation Measurement (ARM) Science Team Meeting

The Millimeter-Wave Cloud Radar (MMCR) has operated continuously at the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains (SGP) site since November 11, 1996. As yet, much of the early data has not been calibrated correctly and insect contamination in the boundary layer is a problem during all warm seasons. Nevertheless, these data can be used to identify periods of isolated, low-altitude stratus. All such cloud cases from November 1996 through June 1998 are identified and liquid water paths for these clouds are estimated from Microwave Radiometer (MWR) retrievals and normal incidence pyranometer measurements of direct solar beam transmittance assuming values for cloud droplet effective diameter. The resulting liquid water paths, together with fixed droplet effective diameters, are incorporated into a two-stream radiative transfer model in order to compute surface irradiance. Time series of estimated irradiance are obtained by applying the model to each retrieved 'column' independently, hence, neglecting the instantaneous effects of horizontal energy transport. Then, the computed irradiances are compared to surface pyranometer measurements in order to 1) assess the accuracy and range of applicability of the independent column approximation for computing surface irradiance, 2) determine the relative radiative importance of cloud macrophysical and microphysical structures and 3) to elucidate whether biases exist in the cloud liquid water path retrievals.

Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).