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Quantitative knowledge of water vapor absorption is crucial for accurateclimate simulations. An open science question in this context concerns thestrength of the water vapor continuum in the near infrared (NIR) atatmospheric temperatures, which is still to be quantified by measurements.This issue can be addressed with radiative closure experiments using solarabsorption spectra. However, the spectra used for water vapor continuumquantification have to be radiometrically calibrated. We present for thefirst time a method that yields sufficient calibration accuracy for NIR watervapor continuum quantification in an atmospheric closure experiment. Ourmethod combines the Langley method with spectral radiance measurements of ahigh-temperature blackbody calibration source (<  2000 K). The calibrationscheme is demonstrated in the spectral range 2500 to 7800 cm−1, butminor modifications to the method enable calibration also throughout theremainder of the NIR spectral range. The resulting uncertainty (2σ)excluding the contribution due to inaccuracies in the extra-atmospheric solarspectrum (ESS) is below 1 % in window regions and up to 1.7 % withinabsorption bands. The overall radiometric accuracy of the calibration dependson the ESS uncertainty, on which at present no firm consensus has beenreached in the NIR. However, as is shown in the companion publicationReichert and Sussmann (2016), ESS uncertainty is only of minor importance forthe specific aim of this study, i.e., the quantification of the water vaporcontinuum in a closure experiment. The calibration uncertainty estimate issubstantiated by the investigation of calibration self-consistency, whichyields compatible results within the estimated errors for 91.1 % of the2500 to 7800 cm−1 range. Additionally, a comparison of a set ofcalibrated spectra to radiative transfer model calculations yields consistentresults within the estimated errors for 97.7 % of the spectral range

atmospheric measurement techniques

The Zugspitze radiative closure experiment for quantifying water vapor absorption over the terrestrial and solar infrared – Part 2: Accurate calibration of high spectral-resolution infrared measurements of surface solar radiation