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 Title: Water Vapor Self-continuum By Cavity Ring Down Spectroscopy In The 1.6 Micron Transparency Window Author(s): Mondelain, Didier Contributor(s): Kassi, Samir; Campargue, Alain Subject(s): Atmospheric science Abstract: Since its discovery one century ago, a deep and unresolved controversy remains on the nature of the water vapor continuum. Several interpretations are proposed: accumulated effect of the distant wings of many individual spectral lines, metastable or true bound water dimers, collision-induced absorption. The atmospheric science community has largely sidestepped this controversy, and has adopted a pragmatic approach: most radiative transfer codes used in climate modelling, numerical weather prediction and remote sensing use the MT\_CKD model which is a semi-empirical formulation of the continuum\footnote{Mlawer, E.J., V.H. Payne, J.L. Moncet, et al. (2012), Phil. Trans. R. Soc. A, 370, 2520–2556.}. The MT\_CKD cross-sections were tuned to available observations in the mid-infrared but in the absence of experimental constraints, the extrapolated near infrared (NIR) values are much more hazardous. Due to the weakness of the broadband absorption signal to be measured, very few measurements of the water vapor continuum are available in the NIR windows especially for temperature conditions relevant for our atmosphere. This is in particular the case for the 1.6 $\mu$m window where the very few available measurements show a large disagreement. Here we present the first measurements of the water vapor self-continuum cross-sections in the 1.6 $\mu$m window\footnote{Mondelain, D., A. Aradj, S. Kassi, et al. (2013), JQSRT, 130, 381–391.} by cavity ring down spectroscopy (CRDS). The pressure dependence of the absorption continuum was investigated during pressure cycles up to 12 Torr for selected wavenumber values. The continuum level is observed to deviate from the expected quadratic dependence with pressure. This deviation is interpreted as due to a significant contribution of water adsorbed on the super mirrors to the cavity loss rate. The pressure dependence is well reproduced by a second order polynomial. We interpret the linear and quadratic terms as the adsorbed water and vapour water contribution, respectively. The derived self-continuum cross sections, measured between 5875 and 6450 \wn, shows a minimum value around 6300 \wn. These cross sections will be compared to the existing experimental data and models, especially to recent FTS measurements and to the last version of the MT\_CKD 2.5 model. Issue Date: 2014-06-17 Publisher: International Symposium on Molecular Spectroscopy Citation Info: Mondelain, D.; Kassi, S.; Campargue, A. WATER VAPOR SELF-CONTINUUM BY CAVITY RING DOWN SPECTROSCOPY IN THE 1.6 MICRON TRANSPARENCY WINDOW. Proceedings of the International Symposium on Molecular Spectroscopy, Urbana, IL, June 16-21, 2014. DOI: 10.15278/isms.2014.TI12 Genre: CONFERENCE PAPER/PRESENTATION Type: Text Language: English URI: http://hdl.handle.net/2142/50897 DOI: 10.15278/isms.2014.TI12 Rights Information: Copyright 2014 by the authors. Licensed under a Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/ Date Available in IDEALS: 2014-09-172015-04-14
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