Scholarly Communications Project

A Numerical Study of the Sensitivity of Cloudy-Scene Bidirectional Reflectivity Distribution Functions to Variations in Cloud Parameters


Pierre V. Villeneuve

Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of

Doctor of Phylosophy


J. Robert Mahan, Chair
Curtis H. Stern
Elaine P. Scott
Douglas J. Nelson
James B. Campbell

June 28, 1996
Blacksburg, Virginia


The goal of this research has been to characterize the sensitivity of the earth's shortwave bidirectional reflectivity distribution function (BRDF) to variations in cloud parameters. The BRDF is a remote sensing tool used to predict the flux reflected from a given earth scene from a satellite-based measurement of the reflected intensity. The BRDF is necessary in order to account for the anisotropic nature of the shortwave radiation field. A shortwave atmospheric radiation Monte-Carlo ray-trace model has been developed as part of this research to predict the earth-reflected radiation field at the top of the atmosphere. This model was developed while paying special attention to clouds including realistic three-dimensional cloud fields characterized by fundamental physical properties. This model was used to predict the BRDF for various cloud fields where a single cloud parameter was varied as part of the sensitivity analysis. The results show that the shortwave BRDF is very sensitive to changes in cloud vertical thickness and mean cloud size. This sensitivity is also strongly dependent on the direction from which the scene is observed. In a related analysis, a study was done of the error associated with using a BRDF from one scene to retrieve fluxes from a second scene. The model was also used to predict images of cloud fields for comparison with experimental data from the Rutherford Appleton Laboratories satellite-based Along Track Scanning Radiometer (ATSR). Finally the output from the radiation model was integrated with the end-to-end radiative electrothermal model of a practical earth radiation budget instrument. This integrated model was used to predict the instrument response to scanning a realistic partly-cloudy earth scene.

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