Abstract
Abstract. Quantitative knowledge of water vapor absorption is crucial for accurate climate simulations. An open science question in this context concerns the strength of the water vapor continuum in the near infrared (NIR) at atmospheric temperatures, which is still to be quantified by measurements. This issue can be addressed with radiative closure experiments using solar absorption spectra. However, the spectra used for water vapor continuum quantification have to be radiometrically calibrated. We present for the first time a method that yields sufficient calibration accuracy for NIR water vapor continuum quantification in an atmospheric closure experiment. Our method combines the Langley method with spectral radiance measurements of a high-temperature blackbody calibration source (< 2000 K). The calibration scheme is demonstrated in the spectral range 2500 to 7800 cm−1, but minor modifications to the method enable calibration also throughout the remainder of the NIR spectral range. The resulting uncertainty (2σ) excluding the contribution due to inaccuracies in the extra-atmospheric solar spectrum (ESS) is below 1 % in window regions and up to 1.7 % within absorption bands. The overall radiometric accuracy of the calibration depends on the ESS uncertainty, on which at present no firm consensus has been reached in the NIR. However, as is shown in the companion publication Reichert and Sussmann (2016), ESS uncertainty is only of minor importance for the specific aim of this study, i.e., the quantification of the water vapor continuum in a closure experiment. The calibration uncertainty estimate is substantiated by the investigation of calibration self-consistency, which yields compatible results within the estimated errors for 91.1 % of the 2500 to 7800 cm−1 range. Additionally, a comparison of a set of calibrated spectra to radiative transfer model calculations yields consistent results within the estimated errors for 97.7 % of the spectral range.
Highlights
Solar absorption spectra in the near-infrared (NIR, 4000– 14 000 cm−1) spectral domain contain a wealth of information on atmospheric radiative processes such as the absorption of radiation by atmospheric trace gases or scattering processes by clouds and aerosols
Further alternatives include solar spectra derived from surface observations, such as the high-resolution extra-atmospheric solar spectrum (ESS) by Menang et al (2013) that covers the 4000–10 000 cm−1 spectral range and was deduced via the Langley method from solar Fourier transform infrared (FTIR) spectra radiometrically calibrated with the method by Gardiner et al (2012)
A further consistency check of the calibration error estimate provided in Sect. 4 can be obtained by a closure of calibrated spectra with synthetic solar absorption spectra obtained by radiative transfer model calculations, which enables us to detect any large deviations of the real calibration accuracy from the uncertainty estimate given in Sect
Summary
Solar absorption spectra in the near-infrared (NIR, 4000– 14 000 cm−1) spectral domain contain a wealth of information on atmospheric radiative processes such as the absorption of radiation by atmospheric trace gases or scattering processes by clouds and aerosols. Taking into account additional sources of radiance uncertainty in our closure setup (described in detail in the companion paper Sussmann et al, 2016, same issue, hereafter referred to as Part 1) and assuming the MT_CKD 2.5.2 continuum model (Mlawer et al, 2012), a calibration accuracy < 2 % is necessary to measure significant continuum absorption throughout at least 33 % of the 2500 to 7800 cm−1 spectral range covered by our measurements It is the goal of this paper to demonstrate an alternative calibration scheme which overcomes these shortcomings and meets the calibration uncertainty of < 2% required for water vapor continuum quantification in the Zugspitze closure experiment.
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