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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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Atmos. Meas. Tech., 9, 4151-4165, 2016
https://doi.org/10.5194/amt-9-4151-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
30 Aug 2016
Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth
Felipe A. Mejia, Ben Kurtz, Keenan Murray, Laura M. Hinkelman, Manajit Sengupta, Yu Xie, and Jan Kleissl Center for Renewable Resources and Integration, Department of Mechanical and Aerospace Engineering, University of California, San Diego 9500 Gilman Dr., La Jolla, CA 92093, USA
Abstract. A method for retrieving cloud optical depth (τc) 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 τc 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 (θ0), τc, solar pixel angle/scattering angle (ϑs), and pixel zenith angle/view angle (ϑz). The effects of these parameters are described and the functions for radiance, Iλτc, θ0, ϑs, ϑz, and RBRτc, θ0, ϑs, ϑz are retrieved from the RTM results. RBR, which is commonly used for cloud detection in sky images, provides non-unique solutions for τc, where RBR increases with τc up to about τc = 1 (depending on other parameters) and then decreases. Therefore, the RRBR algorithm uses the measured Iλmeasϑs, ϑz, in addition to RBRmeasϑs, ϑz, to obtain a unique solution for τc. 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. τc values ranged from 0 to 80 with values over 80, being capped and registered as 80. A τc 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.

Citation: Mejia, F. A., Kurtz, B., Murray, K., Hinkelman, L. M., Sengupta, M., Xie, Y., and Kleissl, J.: Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth, Atmos. Meas. Tech., 9, 4151-4165, https://doi.org/10.5194/amt-9-4151-2016, 2016.
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Short summary
A method for retrieving cloud optical depth using a sky imager is presented. The method is applied to images taken at the Atmospheric Radiation Measurement site and validated against measurements from a microwave radiometer (MWR), output from the Min method for overcast skies, and τc  retrieved by Beer's law from direct normal irradiance (DNI) measurements.
A method for retrieving cloud optical depth using a sky imager is presented. The method is...
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