Moderate spectral resolution solar irradiance measurements, aerosol optical depth, and solar transmission, from 360 to 1070 nm, using the refurbished rotating shadow band spectroradiometer (RSS)

Joseph J Michalsky,Peter W Kiedron

doi:10.5194/amt-15-353-2022

Abstract

Abstract. This paper reports on a third-generation rotating shadow band 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 Atmospheric Radiation Measurement (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, spectral irradiances and direct normal solar transmission, as well as normalized diffuse and global irradiances.

Highlights

The rotating shadowband spectroradiometer was developed to provide spectrally-resolved measurements of the shortwave spectrum for the Atmospheric Radiation Measurement (ARM) program (Stokes and Schwartz, 1994)
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 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

Summary

Introduction

The rotating shadowband spectroradiometer was developed to provide spectrally-resolved measurements of the shortwave spectrum for the Atmospheric Radiation Measurement (ARM) program (Stokes and Schwartz, 1994). The band that shadows the diffuser is positioned below the horizon at the beginning of every cycle where dark and global horizontal measurements are made; it moves to three positions near the sun and samples at each of these Two of these stops are near and on either side of the sun, but do not block it; the mid-stop totally blocks direct sunlight. The sideband measurements are used to calculate a first order correction for excess skylight blocked by the band during the measurement with totally blocked direct sunlight Using these measurements and pre-deployment, wavelength-dependent cosine response corrections, global and diffuse horizontal and direct normal irradiances can be calculated for 1002 continuous wavelengths. The initial five weeks of data showed suspicious wavelength dependencies in the AODs, perhaps caused by a few, poorly determined extraterrestrial response retrievals at the beginning of the measurement set, and, we suggest that data taken before 1 October 2009 have a large uncertainty; irradiances and transmissions for the first five weeks were not removed from the database and should be used with caution

Wavelength Registration

Findings

Irradiance Calculations