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A method for retrieving cloud optical depth (τc) using a UCSDdeveloped ground-based sky imager (USI) is presented. The radiance red–blueratio (RRBR) method is motivated from the analysis of simulated images ofvarious τc produced by a radiative transfer model (RTM). Fromthese images the basic parameters affecting the radiance and red–blue ratio(RBR) of a pixel are identified as the solar zenith angle (θ0),τc, solar pixel angle/scattering angle(ϑs), and pixel zenith angle/view angle(ϑz). The effects of these parameters are described andthe functions for radiance, Iλ<mfenced close=")" open="(">τc, θ0, ϑs, ϑz</mfenced>, and RBR<mfenced close=")" open="(">τc, θ0, ϑs, <mspace linebreak="nobreak" width="0.125em"/>ϑz</mfenced> are retrieved from the RTM results. RBR, which is commonly used forcloud detection in sky images, provides non-unique solutions forτc, where RBR increases with τc up to aboutτc = 1 (depending on other parameters) and then decreases.Therefore, the RRBR algorithm uses the measured Iλmeas<mfenced open="(" close=")">ϑs, ϑz</mfenced>, inaddition to RBRmeas<mfenced close=")" open="(">ϑs, ϑz</mfenced>, to obtain a unique solution forτc. The RRBR method is applied to images of liquid waterclouds taken by a USI at the Oklahoma Atmospheric Radiation Measurement (ARM)program site over the course of 220 days and compared against measurementsfrom a microwave radiometer (MWR) and output from the Min et al. (2003)method for overcast skies. τc values ranged from 0 to 80 withvalues over 80, being capped and registered as 80. A τc RMSEof 2.5 between the Min et al. (2003)method and the USI are observed. The MWR and USI  have an RMSE of 2.2, whichis well within the uncertainty of the MWR. The procedure developed hereprovides a foundation to test and develop other cloud detection algorithms

Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth