Lee Harrison

Automated multifilter rotating shadow-band radiometer: an instrument for optical depth and radiation measurements

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.

Objective Algorithms for the Retrieval of Optical Depths from Ground-Based Measurements

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.

Cloud properties derived from surface MFRSR measurements and comparison with GOES results at the ARM SGP Site

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.

Comparison of aerosol optical depth from four solar radiometers during the fall 1997 ARM intensive observation period

In the Fall of 1997 the Atmospheric Radiation Measurement (ARM) program conducted an Intensive Observation Period (IOP) to study aerosols. Five sun-tracking radiometers were present to measure the total column aerosol optical depth. This comparison performed on the Southern Great Plains (SGP) demonstrates the capabilities and limitations of modern tracking sunphotometers at a location typical of where aerosol measurements are required. The key result was agreement in aerosol optical depth measured by 4 of the 5 instruments within 0.015 (rms). The key to this level of agreement was meticulous care in the calibrations of the instruments.

Multiyear measurements of aerosol optical depth in the Atmospheric Radiation Measurement and Quantitative Links programs

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.

Cosine response characteristics of some radiometric and photometric sensors

Abstract Global and diffuse irradiance and illuminance are measured with instruments that are assumed to have true cosine responses. From one refereed paper, some institutional reports, and by word-of-mouth, it is generally known that no instrument is perfect in this regard. This paper reports on measurements of cosine responses for several commercial instruments and on the cosine response of a multi-filter rotating shadowband radiometer. The measurements were made with an automated cosine response test bench using the same protocol for each instrument. The cosine bench measures with variable angular resolution as fine as 0.25°. The automated rotation is in one plane. A manual rotation allows measurements at other azimuths.

Comparison of spectral direct and diffuse solar irradiance measurements and calculations for cloud-free conditions

Ground-based spectral measurements of direct and diffuse solar irradiance from the Rotating Shadowband Spectroradiometer, taken in cloud-free conditions in Oklahoma in the fall of 1997, are compared over the spectral range 10000–28500 cm−1 to corresponding calculations by an accurate multiple-scattering radiative transfer model. For each case analyzed, the aerosol optical depths used in the calculation were determined by fitting an Angstrom relation based on the ratio of the direct-beam measurements to a direct-beam calculation with no aerosols present. Also used in the calculation was a spectrally-independent aerosol single-scattering albedo chosen to provide agreement with the diffuse measurements. The spectral agreement between the measurements and calculations for the direct and diffuse irradiances is very good, providing strong evidence that in this spectral range there are no unmodeled molecular absorbers of significance to the atmospheric energy balance. Especially notable is the correspondence between the observations and calculations for a case characterized by a large amount of water vapor in the direct-beam path, directly contradicting the suggestion that water vapor absorbs more shortwave radiation than is represented in radiative transfer models.

The rotating shadowband spectroradiometer (RSS) at SGP

The RSS provides continuous spectral measurements of total-horizontal, diffuse-horizontal, and direct-normal irradiance from 360 to 1100 nm using the automated shadowband technique. We show instrument performance, calibration method and accuracies, case data from the first-generation instrument operated at the Southern Great Plains (SGP) site, and comparisons with the improved design now starting operation.

Comparison of spectral irradiance standards used to calibrate shortwave radiometers and spectroradiometers

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.

Joint statistics of photon path length and cloud optical depth: Case studies

We show the joint statistics of photon path length and cloud optical depth for cloudy sky cases observed at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site between September and December 1997. The photon path lengths are retrieved from moderate resolution oxygen A-band observations taken by a rotating shadow band spectroradiometer (RSS). For high optical depth cloud cases, two different populations in the scattergram of the path length versus cloud optical depth are apparent. One population is a result of single-layer cloud cases that exhibit a small variation of path length enhancement over a large optical depth range, together with a strong correlation between the radiation field and the cloud liquid water path, while the second population is attributed to multiple-layer cloud cases with large variability of enhanced photon path lengths. When the optical depth is less than 5, the population of cases appears to bifurcate as the solar air mass increases, with the lower branch exhibiting pressure-weighted path lengths shorter than the direct beam path lengths at these larger solar zenith angles. Using information from a millimeter-wave cloud radar, together with lidar and balloon-borne sonde data to further analyze these cases demonstrates that this bifurcation is caused by the altitude of the scattering; thin clouds aloft produce the lower branch and low-level aerosols produce the upper branch.