ARM Develops Innovative Technique for Measuring Optical Depth of Broken-Cloud Skies


An innovative technique for measuring cloud optical depth (the measure of extinction of incoming solar radiation between different layers in the atmosphere) under conditions with broken clouds. The new technique, which will provide improved measurements of a fundamental property needed to calculate the amount of solar radiation entering and leaving the earth’s atmosphere, has been successfully tested using both comparisons to satellite, aircraft, and surface data from the ARM Southern Great Plains facility and model simulations. Current techniques for measuring cloud optical depth work well for completely overcast skies, but are less accurate for broken-cloud skies. Optical depth measurements under such conditions must take into account a combination of factors such as the amount of direct sunlight shining through the spaces between the clouds, the sunlight scattered both through and from the sides of clouds, the fraction of the sky covered by clouds, the 3-D shape of the clouds, their location relative to both the sun and the location of ground-based instruments that measure the radiation, and the average optical depth of the clouds. The new technique, which takes advantage of the reflective qualities of vegetation on the Earth s surface, measures radiation at the base of clouds from two sources, 1) the direct solar radiation incident on the top that is transmitted through the cloud, and 2) the diffuse solar radiation that is reflected from the ground surface to the cloud and then back from the cloud to the ground.