Development and Evaluation of a Simple Algorithm to Find Cloud Optical Depth with Emphasis on Thin Ice Clouds

An algorithm is presented here for determining cloud optical depth, tau , using data from shortwave broadband irradiances, focusing on the case of optically thin clouds. This method is empirical and consists of a one-line equation. This method is applied to cirrus clouds observed at the Atmospheric Radiation Measurement Program Climate Research Facility (ACRF) at Darwin, Australia, during the Tropical Warm Pool International Cloud Experiment (TWP- ICE) campaign and cirrus clouds observed at the ACRF Southern Great Plains (SGP) site. These cases were chosen because independent verification of cloud optical depth retrievals was possible. For the TWP-ICE case, the calculated optical depths agree to within 1 unit with tau calculated from a vertical profile of ice particle size distributions obtained from an aircraft sounding. For the SGP case, the results from the algorithm correspond reasonably well with tau values obtained from an average over other retrieval methods, some of which have been subject to independent verification. The medians of the two time series are 0.79 and 0.81, for the empirical and averaged values, respectively. Because such close agreement is likely to be fortuitous and therefore not truly represent the performance of our method, tau values derived from our method were compared to values obtained from lidar data. Over a three year period, the difference in median values between the two methods is about 0.6, with the lidar optical depths being larger. This tool may be applied wherever measurements of the direct, diffuse, and total components of the shortwave broadband flux are available at 1- to 5-minute resolution. Because these measurements are made across the world, it then becomes possible to estimate optical depth for both liquid water and ice clouds at many locations.