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In the decade before the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) facility was established in 1992, early modern climate models exhibited large differences in the climate response [...]

The ultraviolet multi-filter rotating shadow-band radiometer (UV-MFRSR) is a seven-channel radiometer with narrowband filters centered between wavelengths 300 and 368 nm. Four of the middle wavelengths in this device are near those used in the Dobson spectrometer to retrieve ozone column abundance. In this paper measurements from Mauna Loa Observatory (MLO) were used first to calibrate the instrument using the Langley plot method and subsequently to derive column ozone and aerosol optical depths. The ozone derived from the UV-MFRSR was compared to the ozone measured by a Dobson spectrophotometer that operates daily at the MLO, resulting in column values within about 1 DU on average for 43 d in 2018. The aerosol optical depth (AOD) retrievals are more challenging. Generally, the AOD increases with wavelength between 305 and 332 nm, not what is expected given the typical AOD wavelength dependence at visible wavelengths. An example of this behavior is discussed, and research by others is cited that indicates similar behavior at these wavelengths, at least for the low-aerosol-optical-depth conditions encountered at high-altitude sites. [ABSTRACT FROM AUTHOR]

The ultraviolet multi-filter rotating shadowband radiometer (UV-MFRSR) is a seven-channel radiometer with narrowband filters centered between wavelengths 300 and 368 nm. Four of the middle wavelengths in this device are near those used in the Dobson spectrometer to retrieve ozone column abundance. In this paper measurements from Mauna Loa Observatory (MLO) were used, first, to calibrate the instrument using the Langley plot method, and, subsequently, to derive column ozone and aerosol optical depths. The ozone derived from the UV-MFRSR was compared to the ozone measured by a Dobson spectrophotometer that operates daily at the MLO resulting in column values within about 1 DU on average for 43 days in 2018. The aerosol optical depth (AOD) retrievals are more challenging. Generally, the AOD increases with wavelength between 305 and 332 nm; not what is expected given the typical AOD wavelength dependence at visible wavelengths. An example of this behavior is discussed, and research by others is cited that indicates similar behavior at these wavelengths, at least for the low aerosol optical depth conditions encountered at high altitude sites. [ABSTRACT FROM AUTHOR]

Shadow-band radiometers in general, and especially the Multi-Filter Rotating Shadow-band Radiometer (MFRSR), are widely used for atmospheric optical depth measurements. The major programs running MFRSR networks in the United States include the Department of Energy Atmospheric Radiation Measurement (ARM) Program, U.S. Department of Agriculture UV-B Monitoring and Research Program, National Oceanic and Atmospheric Administration Surface Radiation (SURFRAD) Network, and NASA Solar Irradiance Research Network (SIRN). We discuss a number of technical issues specific to shadowband radiometers and their impact on the optical depth measurements. These problems include instrument tilt and misalignment, as well as some data processing artifacts. Techniques for data evaluation and automatic detection of some of these problems are described. OCIS codes: 120.4640, 120.5240, 120.5630, 120.0280, 010.1100, 010.1120.

Absolute calibration of spectral shortwave radiometers is usually performed with National Institute of Standards and Technology (NIST) or NIST-traceable incandescent lamps. We compare 18 irradiance standards from NIST and three commercial vendors using the same spectrometer to assess their agreement with our working standard. The NIST procedure is followed for the 1000-W FEL lamps from NIST, Optronics, and EG&G. A modified calibration procedure developed by Li-Cor is followed for their 200-W tungsten-halogen lamps. Results are reproducible from one day to the next to approximately 0.1% using the same spectrometer. Measurements taken four months apart using two similar but different spectrometers were reproducible to 0.5%. The comparisons suggest that even NIST standards may disagree with each other beyond their stated accuracy. Some of the 1000-W commercial lamps agreed with the NIST lamps to within their stated accuracy, but not all. Surprisingly, the lowest-cost lamps from Li-Cor agreed much better with the NIST lamps than their stated accuracy of 4%, typically within 2%. An analysis of errors leads us to conclude that we can transfer the calibration from a standard lamp to a secondary standard lamp with approximately 1% added uncertainty. A field spectrometer was calibrated with a secondary standard, producing a responsivity for the spectrometer that was within 5% of the responsivity obtained by Langley calibration using routine field measurements.

The multi-filter rotating shadowband radiometer (MFRSR) uses a silicon photodiode sensor to measure shortwave global and diffuse horizontal irradiance from which direct normal irradiance is calculated. Silicon sensors are rugged, stable, and have a fast time response. However, silicon sensors are both thermally and spectrally sensitive. In addition they, as do all pyranometric sensors, have an imperfect cosine response, especially at high solar-incidence angles. In the MFRSR two of these sources of error are minimized: the cosine response of the MFRSR is measured, and the acquired data are corrected accordingly; an automatic heater maintains the MFRSR detector at a constant temperature near 40 [degrees] C. This paper demonstrates that there is substantial agreement between first-class thermopile instruments and the MFRSR silicon sensor with the elimination of these two sources of error. Furthermore, this paper describes corrections, based on sky conditions, to lower the remaining errors. The data base for deriving and testing these corrections was collected in Albany, New York, during 1993. After correction equations are applied, the root-mean-square differences for 5 min averages of the global horizontal, diffuse horizontal, and direct normal irradiance are 8.8, 9.1, and 16.3 W/[m.sup.2], respectively. The differences in time response or time keeping between silicon and thermopile instruments may explain much of the remaining root-mean-square differences.

Optical depth retrieval by means of Langley regression is complicated by cloud transits and other time-varying interferences. An algorithm is described that objectively selects data points from a continuous time series and performs the required regression. The performance of this algorithm is compared by a double-blind test with an analysis done subjectively. The limits to accuracy imposed by time-averaged data are discussed, and an additional iterative postprocessing algorithm is described that improves the accuracy of optical depth inferences made from data with time-averaging periods longer than 5 min. Such routine algorithms are required to provide intercomparable retrievals of optical depths from widely varying historical data sets and to support large networks of instruments such as the multifilter rotating shadow-band radiometer.

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 [[micro]meter]) 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 that 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.

A new model that synthesizes sky luminance angular distribution from routine irradiance measurements for all insolation conditions was recently presented by the authors. The model produces continuous sky luminance distribution patterns representative of specific insolation conditions from overcast to clear through partly cloudy. In this article we address the discontinuous aspects of skylight, created by 'one of a kind' cloud patterns. We show that these patterns, which are superimposed on the continuous luminance distributions, have predictable physical characteristics that can be parameterized as a function of insolation conditions. We present a method for incorporating these random but predictable luminance effects as an option in the continuous skylight distribution model.

This paper reports on a third generation rotating shadowband spectroradiometer (RSS) used to measure global and diffuse horizontal plus direct normal irradiances and transmissions at 1002 wavelengths between 360 and 1070 nm. The prism-dispersed spectral data are from the ARM Southern Great Plains site in north central Oklahoma (36.605 N, 97.486 W) and cover dates between August 2009 and February 2014. The refurbished RSS isolates the detector in a vacuum chamber with pressures near 10-7 torr. This prevents the deposition of outgassed vapors from the interior of the spectrometer shell on the cooled detector that affected the operation of the first commercial RSS. Methods for (1) ensuring the correct wavelength registration of the data and (2) deriving extraterrestrial responses over the entire spectrum, including throughout strong water vapor and oxygen bands, are described. The resulting data produced are archived as ARM data records and include cloud-screened aerosol optical depths as well as spectral irradiances and solar transmissions for all three solar components. [ABSTRACT FROM AUTHOR]