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&amp;lt;sup&amp;gt;−1&amp;lt;/sup&amp;gt; spectral range under atmospheric conditions

Andreas Reichert,Ralf Sussmann

doi:10.5194/acp-16-11671-2016

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.

Highlights

Atmospheric water vapor is the most important contributor to the absorption of incoming solar radiation in the near infrared (NIR; 4000–14 000 cm−1) (Kiehl and Trenberth, 1997)
The assignment of the residual optical depth (OD) to water vapor continuum absorption was made based on two arguments: as outlined in Part 1, great care was taken to construct a comprehensive uncertainty budget including thorough estimates of all relevant error contributions to the closure experiment
The integrated water vapor (IWV) dependence of the measured residual OD is consistent with that expected from water vapor continuum absorption

Summary

Introduction

Atmospheric water vapor is the most important contributor to the absorption of incoming solar radiation in the near infrared (NIR; 4000–14 000 cm−1) (Kiehl and Trenberth, 1997). Depending on the atmospheric state and the choice of continuum model, up to 6 % of the clear-sky water vapor absorption can be attributed to the continuum (Paynter and Ramaswamy, 2011). Quantitative knowledge of this contribution is a prerequisite for realistic atmospheric radiative transfer calculations employed in, e.g., climate models (Paynter and Ramaswamy, 2014; Rädel et al, 2015; Turner et al, 2012). The NIR atmospheric water vapor continuum currently still lacks sufficient experimental constraints. A number of laboratory studies based on different experimental techniques investigated this open question. Several efforts were made to quantify continuum absorption, including the contributions of both the self- and foreign-broadened continuum. Several studies made use of cell measurements with grating spectrometers (e.g., Burch, 1982, 1985; Burch and Alt, 1984) and FTIR

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