Piotr W. Kiedron

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.

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.

O2‐O2 absorption band identification based on optical depth spectra of the visible and near‐infrared

The first implementation of the rotating shadowband spectroradiometer allows the routine collection of direct normal solar spectral irradiance data at 512 wavelengths. These data are used on clear days to calculate spectra in optical depth. A persistent absorption band near 477 nm in the first optical depth spectra collected led to the eventual identification of six bands in the 400 to 1080 nm range resulting from absorption by O2 collision pairs. All of these O2-O2 bands were discovered earlier using atmospheric long pathlength or laboratory high pressure techniques, however, it is notable that this methods sensitivity permitted their detection at one air mass. These data suggest that there are no unexplained narrowband absorption features in the visible/near-infrared, that excess optical depth measured near 1000 nm may now have an explanation, and that the linearity of the O2-O2 absorption may be exploited to investigate mean pathlengths in the atmosphere.

A robust retrieval of water vapor column in dry Arctic conditions using the rotating shadowband spectroradiometer

A method to retrieve water vapor column using the 940-nm water vapor absorption band in dry Arctic conditions is presented. The retrievals with this method are stable with respect to uncertainties in instrument radiometric calibration, air pressure, solar source function, and aerosols. The water vapor column was retrieved with this method using spectra obtained with the rotating shadowband spectroradiometer (RSS) that was deployed during an intensive observation period near Barrow, Alaska, in March 1999. A line-by-line radiative transfer model was used to compute water vapor transmittance. The retrievals with this method are compared with retrievals obtained from three independent measurements with microwave radiometers. All four measurements show the same pattern of temporal variations. The RSS results agree most closely with retrievals obtained with the millimeter-wave imaging radiometer (MIR) at its 183 GHz±7 double-side band channel. Their correlation over a period of 7 days when water vapor column varied between 0.75 mm and 3.6 mm (according to RSS) is 0.968 with MIR readings 0.12 mm higher on average.

A differential technique to retrieve column water vapor using sun radiometry

Techniques for retrieving column water vapor from Sun radiometer measurements involving the 940-nm water vapor absorption band have been around for the better part of a century. Arguably, the best method to use for this retrieval is the modified Langley technique. However, to apply this method one must obtain the instrument response at the top of the atmosphere using modified Langley plots on clear days with a very stable water vapor column. Using subsequent measurements in this filter, ratioed to the top-of-the-atmosphere response allows one to determine the transmission in the 940-nm water band. In this paper, we present an approach that does not require an absolute knowledge of the extraterrestrial instrument response. The method discussed here relies, instead, on relative measurements of a calibration lamp and the extraterrestrial spectral irradiance within and just outside the 940-nm absorption band. We execute these retrievals for the rotating shadowband spectroradiometer (RSS) on 3 days during the Department of Energys Atmospheric Radiation Measurement programs 1997 Water Vapor Intensive Observation Period. We compare the results to those retrieved from a colocated multifilter rotating shadowband radiometer (MFRSR) that uses an empirical calibration and from a colocated microwave radiometer. Since our optical method of retrieving column water vapor from RSS measurements does not depend on a calibration performed against another water vapor measurement, it contributes an independent estimate in the search for absolute accuracy. The major contributors to the uncertainty of this retrieval are the water vapor band strength calculations, the difference in aerosol extinction in and near the water vapor band, the relative spectral irradiance output of the calibration lamp and the Sun at the nonabsorbing and band-centered wavelengths, and the stability of the spectral response of the instrument, which will be discussed in detail.

United States Department of Agriculture reference ultraviolet spectroradiometer: current performance and operational experience at Table Mountain, Colorado

At present the United States Department of Agriculture (USDA) Reference Spectroradiometric Network consists of three sites: Table Mountain, Colorado, Lamont, Oklahoma (the ARM program SGP site), and Beltsville, Maryland. At each site we deploy and continuously operate a 1-m cascaded additive-double Czerny-Turner scanning monochromator with a bialkali photomultiplier and photon-counting detection. Lambertian fore-optic errors are less than 1% over the range of zenith angles from 0 to 80°. The instruments use photon counting and make measurements at 290 nm not affected by stray light under typical conditions. The basic performance specifications of the instrument were demonstrated by a prototype at the 1997 North-American UV Spectroradiometer Intercomparison. Data shown here demonstrate that these are met in routine operation. The fundamental instrument performance specifications are: Optical resolution: 0.1 nm FWHM, triangular slit-function. Wavelength reproducibility: ′0.0025-nm 2<T with 296-nm Hg retrace-scan corrections applied, ′0.007 nm 2σ over typical diurnal variability, without correction. Wavelength accuracy: Limited by calibration systematic errors. Believed to be 0.005-nm worst case. Stray light: < 10 - 7 at 4 FWHM, 10 - 1 0 at 20 nm, slit-scattering function versus 325 nm HeCd. Angular response: less than 1% error from cosine over the range of zenith angles from 0 to 80°. Signal linearity: The instrument uses a photomultiplier with 2-ns rise-time and photon counting detection. The dual-threshold discriminator has a 700-Mhz synchronous signal counting limit. The maximum counting rates seen at the longest wavelengths are less than 10 MHz; less than 1/5 of the frequency where nonlinearity can be detected, as tested for the 1997 Intercomparison. 2000 was the first full year of operation of our instrument at the NOAA Table Mountain site (140.177 °N 105.276 °W, 1900 m asl) for which the operational and calibration frequencies justify making the data accessible to outside users for scientific application. We show performance in routine operation and issues of calibration over the period April 2000 to 31 December 2001.

Shortwave clear-sky diffuse irradiance in the 300- to 1100-nm range: comparison of models with UV-VIS-NIR and broadband radiometer measurements at the Southern Great Plains ARM site in September/October 2001

Broadband shortwave diffuse horizontal irradiance models overestimate measurements by between 7 and 14% using the most reliable input data for the models and the best available broadband measurements of diffuse irradiance. This paper uses spectral irradiance measurements and models as opposed to broadband measurements and models to investigate the contributions to this difference from various regions of the spectrum. The data are from the first Atmospheric Radiation Measurement (ARM) diffuse irradiance intensive observation period (IOP) held in September and October of 2001 at the Oklahoma ARM site near Ponca City. Visible and ultraviolet (UV) rotating shadowband spectroradiometers (RSS) acquired data during the IOP. Diffuse measurements with conventional broadband diffuse pyranometers and direct irradiance measurements using an absolute cavity radiometer are also available for analysis. Integrated spectral measurements are consistent with broadband measurements and, therefore, confirm the earlier results that models over predict diffuse. The wavelength dependent differences in models and measurements are illustrated and discussed.

Transfer of UV irradiance calibration to our field spectroradiometers: current performance and operational experience at Table Mountain, Colorado

At present the United States Department of Agriculture (USDA) Reference Spectroradiometric Network consists of 4 sites: Table Mt. CO, Ft. Collins CO, Lamont OK (The ARM program SGP site), and Beltsville MD. At each site we operate a 1-meter cascaded additive-double Czerny-Turner scanning monochromator with a bi-alkali Photomultiplier and photon-counting detection. Irradiance calibrations are provided for the instrument at Table Mt CO by NOAAs Central Ultraviolet Calibration Facility (CUCF) from NIST-traceable standards. Calibrations are transferred from this instrument to others in the network (and additional stability monitoring of the primary instrument conducted) using shippable transfer calibrators we have designed. Here we describe these transfer calibrators, and our operational experience seen at the Table Mt. Site in 2002 and 2003 to date.