A method for retrieving cloud optical depth (tau(c)) using a UCSD developed ground-based sky imager (USI) is presented. The radiance red-blue ratio (RRBR) method is motivated from the analysis of simulated images of various tau(c) produced by a radiative transfer model (RTM). From these images the basic parameters affecting the radiance and red-blue ratio (RBR) of a pixel are identified as the solar zenith angle (theta(0)), tau(c), solar pixel angle)scattering angle (upsilon(s)), and pixel zenith angle)view angle (upsilon(z)). The effects of these parameters are described and the functions for radiance, I-lambda (tau(c), theta(0), upsilon(s), upsilon(z)), and RBR (tau(c), theta(0), upsilon(s), upsilon(z)) are retrieved from the RTM results. RBR, which is commonly used for cloud detection in sky images, provides non-unique solutions for tau(c), where RBR increases with tau(c) up to about tau(c) = 1 (depending on other parameters) and then decreases. Therefore, the RRBR algorithm uses the measured I-lambda(meas) (upsilon(s), upsilon(z)), in addition to RBRmeas (upsilon(s), upsilon(z)), to obtain a unique solution for tau(c). The RRBR method is applied to images of liquid water clouds taken by a USI at the Oklahoma Atmospheric Radiation Measurement (ARM) program site over the course of 220 days and compared against measurements from a microwave radiometer (MWR) and output from the Min et al. (2003) method for overcast skies. tau(c) values ranged from 0 to 80 with values over 80, being capped and registered as 80. A tau(c) RMSE of 2.5 between the Min et al. (2003) method and the USI are observed. The MWR and USI have an RMSE of 2.2, which is well within the uncertainty of the MWR. The procedure developed here provides a foundation to test and develop other cloud detection algorithms.
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