The Zugspitze radiative closure experiment for quantifying water vapor absorption over the terrestrial and solar infrared – Part 3: Quantification of the mid- and near-infrared water vapor continuum in the 2500 to 7800 cm<sup>−1</sup> spectral range under atmospheric conditions

Abstract

Abstract. We present a first quantification of the near-infrared (NIR) water vapor continuum absorption from an atmospheric radiative closure experiment carried out at the Zugspitze (47.42° N, 10.98° E; 2964 m a.s.l.). Continuum quantification is achieved via radiative closure using radiometrically calibrated solar Fourier transform infrared (FTIR) absorption spectra covering the 2500 to 7800 cm−1 spectral range. The dry atmospheric conditions at the Zugspitze site (IWV 1.4 to 3.3 mm) enable continuum quantification even within water vapor absorption bands, while upper limits for continuum absorption can be provided in the centers of window regions. Throughout 75 % of the 2500 to 7800 cm−1 spectral range, the Zugspitze results agree within our estimated uncertainty with the widely used MT_CKD 2.5.2 model (Mlawer et al., 2012). In the wings of water vapor absorption bands, our measurements indicate about 2–5 times stronger continuum absorption than MT_CKD, namely in the 2800 to 3000 cm−1 and 4100 to 4200 cm−1 spectral ranges. The measurements are consistent with the laboratory measurements of Mondelain et al. (2015), which rely on cavity ring-down spectroscopy (CDRS), and the calorimetric–interferometric measurements of Bicknell et al. (2006). Compared to the recent FTIR laboratory studies of Ptashnik et al. (2012, 2013), our measurements are consistent within the estimated errors throughout most of the spectral range. However, in the wings of water vapor absorption bands our measurements indicate typically 2–3 times weaker continuum absorption under atmospheric conditions, namely in the 3200 to 3400, 4050 to 4200, and 6950 to 7050 cm−1 spectral regions.

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

“…Furthermore, mispointing leads to erroneous air mass input for the Langley fit. As outlined in Reichert et al (2015), systematic mispointing can be determined using multiple measurements of solar line Doppler shifts at different orientations of the solar rotation axis. Using this method, mispointing-corrected air mass values can be calculated as input to the Langley fits.…”

A Fourier transform spectroradiometer for ground-based remote sensing of the atmospheric downwelling long-wave radiance

“…The main data product of the REFIR-PAD spectroradiometer is the calibrated atmospheric emitted radiance integrated in the field of view of the instrument (a cone with an aperture of about 10 • ) and spectrally resolved with a 0.4 cm −1 resolution in the 100-1500 cm −1 range. The calibration procedure, described in detail in Bianchini and Palchetti (2008), follows the complex calibration described by Revercomb et al (1988). The onboard RBB sources are simulated using a specific mathematical model shown in Palchetti et al (2008).…”

“…Spectral observations in the thermal infrared have been used to retrieve atmospheric state from both top-ofatmosphere (see, e.g., Ridolfi et al (2000)) and ground-based observations (Smith et al, 1999) and to perform radiative closure experiments (Turner et al, 2004;Reichert and Sussmann, 2016). However, all these observations typically cover only the mid-infrared.…”

Recent advances in collisional effects on spectra of molecular gases and their practical consequences

“…Nevertheless, the retrieval of water-vapor continuum absorption from atmospheric spectra is challenging due to the difficulty in discriminating it from aerosol scattering and absorption. Recent atmospheric validation in the infrared can be found in [539][540][541][542][543]. Table 6 regroups the few recent laboratory studies of the foreign continuum associated with mixtures of H 2 O with N 2 (or air).…”