The mfrsrcldod1min value-added product produces cloud optical depth for overcast liquid clouds using narrowband irradiance measurements from the multifilter rotating shadowband radiometer (MFRSR). If liquid water path is available from the microwave radiometer (MWR), then effective radius is also derived; otherwise, a default effective radius of 8.0 um is assumed.
The mfrsrcldod1min value-added product (VAP) obtains cloud optical depth and effective radius using a retrieval algorithm developed by Min and Harrison (1996). A look-up table of total transmittance at 415 nm is calculated using a radiative transfer model for a range of cloud water path, effective radius, optical depth, and solar zenith angles. The measured transmittance (derived from the MFRSR measurements and the Langley VAP), liquid water path, and solar zenith angle and an assumed surface albedo are input to the algorithm. The optical depth and effective radius are then retrieved by interpolating across the look-up table. The 1-sigma uncertainties in the optical depth and effective radius are also estimated by propagating uncertainty in the input irradiance, liquid water path, and surface albedo.
Quality control (qc) flags have been added to the VAP output. The cloud retrievals in the current version of this VAP are only valid for overcast, optically thick liquid clouds. Cloud fraction from the shortwave flux analysis VAP is used to identify and flag cases with cloud fraction less than 0.9. Retrievals with optical depth < 7 are flagged as indeterminate. Cloud base height is also included in the output files (when available at a given site) so that users can determine whether clouds are likely to be liquid or ice, which depends on the site and season.
The MFRSRCLOD VAP was significantly updated in 2011. Details of the changes and differences with previous versions of the data are available in the VAP report “Changes to MFRSRCLDOD1MIN Datastream”.
The Langley VAP, which is an input to MFRSRCLDOD, was updated in 2010. Details of how that affects the processing of the MFRSRCLDOD will be available in an upcoming technical report.