Abstract
Abstract. The amplitude, the temperature dependence, and the physical origin of the water vapour absorption continuum are a long-standing issue in molecular spectroscopy with direct impact in atmospheric and planetary sciences. In recent years, we have determined the self-continuum absorption of water vapour at different spectral points of the atmospheric windows at 4.0, 2.1, 1.6, and 1.25 µm, by highly sensitive cavity-enhanced laser techniques. These accurate experimental constraints have been used to adjust the last version (3.2) of the semi-empirical MT_CKD model (Mlawer-Tobin_Clough-Kneizys-Davies), which is widely incorporated in atmospheric radiative-transfer codes. In the present work, the self-continuum cross-sections, CS, are newly determined at 3.3 µm (3007 cm−1) and 2.0 µm (5000 cm−1) by optical-feedback-cavity enhanced absorption spectroscopy (OFCEAS) and cavity ring-down spectroscopy (CRDS), respectively. These new data allow extending the spectral coverage of the 4.0 and 2.1 µm windows, respectively, and testing the recently released 3.2 version of the MT_CKD continuum. By considering high temperature literature data together with our data, the temperature dependence of the self-continuum is also obtained.
Highlights
About 60 % of the solar radiation absorbed by the Earth’s atmosphere is due to water vapour absorption
These are formed by multitudes of narrow rovibrational absorption lines that are tabulated in spectroscopic databases like HITRAN (Gordon et al, 2017) and GEISA (Jacquinet-Husson et al, 2016); (ii) a weak broadband absorption continuum with slow frequency dependence roughly following the contour of vibrational bands
The reader is referred to Cermák et al (2016) and Campargue et al (2017) for a description of our cavity ring-down spectroscopy (CRDS) spectrometer dedicated to the high sensitivity spectroscopy of atmospheric species in the 2.1 μm window
Summary
About 60 % of the solar radiation absorbed by the Earth’s atmosphere is due to water vapour absorption. We have implemented highly sensitive cavity enhanced laser techniques at different spectral points of the 4.0, 2.1, 1.6 and 1.25 μm windows to measure in the laboratory the self-continuum absorption of water vapour (Mondelain et al, 2013, 2014, 2015; Campargue et al, 2016; Ventrillard et al, 2015; Richard et al, 2017). Overall, these measurements by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OFCEAS) and Cavity Ring-down Spectroscopy (CRDS) were found closer to the MT_CKD model than to the FTS results. Using the photon lifetime measured at the end of each scan, the cavity transmission is converted into a spectrum in absolute absorption coefficient scale using the procedure detailed in Kerstel et al (2006) and Richard et al (2017)
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