Can Measurements of the Near‐Infrared Solar Spectral Irradiance be Reconciled? A New Ground‐Based Assessment Between 4,000 and 10,000 cm⁻¹

Abstract

The near‐infrared solar spectral irradiance (SSI) is of vital importance for understanding the Earth's radiation budget, and in Earth observation applications. Differences between previously published solar spectra (including the commonly used ATLAS3 spectrum) reach up to 10% at the low wavenumber end of the 4,000–10,000 cm−1 (2.5–1 μm) spectral region. The implications for the atmospheric sciences are significant, since this spectral region contains 25% of the incoming total solar irradiance. This work details an updated analysis of the CAVIAR SSI, featuring additional analysis techniques and an updated uncertainty budget using a Monte Carlo method. We report good consistency with ATLAS3 in the 7,000–10,000 cm−1 region where there is confidence in these results due to agreement with other spectra, but ~7% lower in the 4,000–7,000 cm−1 region, in general agreement with several other analyses.

Assessment of the Impact of Solar Spectral Irradiance on Near‐Infrared Clear‐Sky Atmospheric Absorption and Heating Rates

“…Differences of up to about 10.6 W m −2 (8.2%) reported by Menang and Shine () were also obtained here. However, a reanalysis of the CAVIAR spectra by Elsey et al () has reduced the difference between the absorbed solar irradiances calculated using the ATLAS 3 and CAVIAR spectra (which is now called the CAVIAR2 SSI) to about 6.2 W m −2 (4.8%). It can be seen from Table that there is a difference of 3.6 W m −2 (2.8%) between the absorbed irradiances for ATLAS 3 and the new “reference” spectrum, the SOLAR‐ISS spectrum.…”

“…The Langley technique was also applied to these relatively low‐resolution measurements in atmospheric windows. The resolution of the instrument used for measuring this SSI was 10 nm. CAVIAR2 spectrum (with a spectral resolution of 0.03 cm −1 ); a ground‐based observed SSI obtained from an updated analysis of CAVIAR spectrum (Elsey et al, ). The biggest difference between the CAVIAR and CAVIAR 2 solar spectra as a result of this reanalysis is at the highest wavenumbers as Figure shows.…”

“…CAVIAR2 spectrum (with a spectral resolution of 0.03 cm −1 ); a ground‐based observed SSI obtained from an updated analysis of CAVIAR spectrum (Elsey et al, ). The biggest difference between the CAVIAR and CAVIAR 2 solar spectra as a result of this reanalysis is at the highest wavenumbers as Figure shows.…”

“…Considering the fact that there are significant disagreements between ground‐based, aircraft‐based, and satellite‐based measured SSI in the near IR (see, for example, Elsey et al, , and associated references), it is timely and important to further assess the effect of these differences on radiative transfer calculations, particularly since several very recent publications have presented new SSI, which significantly update earlier analyses. In this study, we update and extend the work of Menang and Shine () but limit our analysis to observation‐based SSI since they are generally more reliable than modeled spectra.…”

“…The LBL model is chosen because, first, it accounts for all atmospheric absorbers in a large spectral region (about 0.4–100 μm) and second, its results can be used to validate results from less detailed global climate models (Collins et al, ). We shall focus on clear‐sky absorption and heating rates in the near IR, from about 4,000–10,000 cm −1 (2.5–1 μm), a spectral region that contains a significant amount (about 25%) of available solar energy (Elsey et al, ). In section 2, we shall present the sources of SSI data as well as the radiative transfer calculations, while section 3 will focus on results.…”