Instrumentation and Technique for Deducing Cloud Optical Depth

Abstract

The feasibility of using a photodiode radiometer to infer optical depth of thin clouds from solar intensity measurements is examined. Data were collected by a photodiode radiometer which measured incident radiation at angular fields of view of 2, 5, 10, 20 and 28°. Values of normalized annular radiance and transmittance were calculated from the observations and compared to similar calculations from a Monte Carlo radiative transfer model. The Monte Carlo results were for cloud optical depths of 1 through 6 over a spectral bandpass of 0.3 to 2.8 ?m. Eight case studies including high, middle and low clouds were examined. Experimental values of cloud optical depth were determined by comparing plots of transmittance versus field of view with the model calculated curves and from the average of the five optical depths calculated for each field of view. Analysis of the case study results indicates that the photodiode radiometer can be used effectively to determine the optical depth of thin clouds.