The multifilter rotating shadow-band radiometer is a ground-based instrument that uses independent interference-filter-photodiode detectors and the automated rotating shadow-band technique to make spectrally resolved measurements at seven wavelength passbands (chosen at the time of manufacture between 350 nm and 1.7 µm) of direct-normal, total-horizontal, and diffuse-horizontal irradiances. This instrument achieves an accuracy in direct-normal spectral irradiance comparable with that of tracking radiometers, and it is more accurate than conventional instruments for the determination of the diffuse and total-horizontal spectral irradiances because the angular acceptance function of the instrument closely approximates the ideal cosine response, and because the measured direct-normal component can be corrected for the remaining angular acceptance error. The three irradiance components are measured with the same detector for a given wavelength. Together with the automated shadow-band technique, this guarantees hat the calibration coefficients are identical for each, thus reducing errors when one compares them (as opposed to measurements made with independent instruments). One can use the direct-normal component observations for Langley analysis to obtain depths and to provide an ongoing calibration against the solar constant by extrapolation to zero air mass. Thus the long-term stability of all three measured components can be tied to the solar constant by an analysis of the routinely collected data.

We describe a family of inversion methods to infer the optical depth, τ, of warm clouds from surface measurements of spectral irradiance. Our most complex retrieval also uses the total liquid water path measured by a microwave radiometer to obtain the effective radius, re, of the cloud droplets. We apply these retrievals to data from the Atmospheric Radiation Measurement (ARM) Program, and compare our results to those produced by the GOES satellite for episodes where total overcast was observed. Our surface‐based estimates of τ agree with those from GOES when the optical depths are <10, but are consistently larger by as much as a factor of 2 when optical depths are greater. We show that the uncertainties associated with the surface‐based retrievals are less than those done from a satellite, and argue from the time series of the observations and the statistics of the measurements that the disagreement is not merely a consequence of the larger spatial average sampled by the satellite.

Abstract. The U.S. Department of Energy funded the development of the multifilter rotating shadowband radiometer (MFRSR) as part of the Atmospheric Radiation Measurement (ARM) program. This seven-channel radiometer began operation at the first ARM site in 1992 and at the Department of Energy Quantitative Links (QL) sites in the fall of 1991; three of the QL sites continue to operate, although this program was discontinued after 1995. This paper describes the use of the MFRSR in acquiring aerosol optical depth data, including the in-field calibration procedure and a partial validation of this process. Multiyear measurements of aerosol optical depth from three of the sites indicate similar phasing of seasonal and interannual changes, but with notable differences in the magnitude of the aerosol optical depth. Published papers that use these aerosol data are highlighted, and public access to these and future data sets for scientific studies are explained.

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