The water vapour continuum in near-infrared windows – current understanding and prospects for its inclusion in spectroscopic databases

Spectroscopic catalogues, such as GEISA and HITRAN, do not yet include information on the water vapour continuum that pervades visible, infrared and microwave spectral regions. This is partly because, in some spectral regions, there are rather few laboratory measurements in conditions close to those in the Earth’s atmosphere; hence understanding of the characteristics of the continuum absorption is still emerging. This is particularly so in the near-infrared and visible, where there has been renewed interest and activity in recent years. In this paper we present a critical review focusing on recent laboratory measurements in two near-infrared window regions (centred on 4700 and 6300 cm−1) and include reference to the window centred on 2600 cm−1 where more measurements have been reported. The rather few available measurements, have used Fourier transform spectroscopy (FTS), cavity ring down spectroscopy, optical-feedback – cavity enhanced laser spectroscopy and, in very narrow regions, calorimetric interferometry. These systems have different advantages and disadvantages. Fourier Transform Spectroscopy can measure the continuum across both these and neighbouring windows; by contrast, the cavity laser techniques are limited to fewer wavenumbers, but have a much higher inherent sensitivity. The available results present a diverse view of the characteristics of continuum absorption, with differences in continuum strength exceeding a factor of 10 in the cores of these windows. In individual windows, the temperature dependence of the water vapour self-continuum differs significantly in the few sets of measurements that allow an analysis. The available data also indicate that the temperature dependence differs significantly between different near-infrared windows. These pioneering measurements provide an impetus for further measurements. Improvements and/or extensions in existing techniques would aid progress to a full characterisation of the continuum – as an example, we report pilot measurements of the water vapour self-continuum using a supercontinuum laser source coupled to an FTS. Such improvements, as well as additional measurements and analyses in other laboratories, would enable the inclusion of the water vapour continuum in future spectroscopic databases, and therefore allow for a more reliable forward modelling of the radiative properties of the atmosphere. It would also allow a more confident assessment of different theoretical descriptions of the underlying cause or causes of continuum absorption

Water vapour self-continuum in near-visible IR absorption bands: Measurements and semiempirical model of water dimer absorption

The nature of the water vapour continuum has been of great scientific interest for more than 60 years. Here, water vapour self-continuum absorption spectra are retrieved at temperatures of 398 K and 431 K and at vapour pressures from 1000 to 4155 mbar in the 8800 and 10,600 cm−1 absorption bands using high-resolution FTS measurements. For the observed conditions, the MT_CKD-3.2 model underestimates the observed continuum on average by 1.5–2 times. We use the hypothesis that water dimers contribute to the continuum absorption to simulate the experimentally-retrieved self-continuum absorption spectra, and to explain their characteristic temperature dependence and spectral behaviour. The values of the effective equilibrium constant are derived for the observed temperatures. We find that the dimer-based model fits well to the measured self-continuum from this and previous studies, but requires a higher effective equilibrium constant compared to the modern estimates within the temperature range (268–431 K) and spectral region studied. It is shown that water dimers are likely responsible for up to 50% of the observed continuum within these bands. Possible causes of the incomplete explanation of the continuum are discussed. Extrapolating these measurements to atmospheric temperatures using the dimer-based model, we find that the newly-derived self-continuum reduces calculated surface irradiances by 0.016 W m−2 more than the MT_CKD-3.2 self-continuum in the 8800 cm−1 band for overhead-Sun mid-latitude summer conditions, corresponding to a 12.5% enhancement of the self-continuum radiative effect. The change integrated across the 10,600 cm−1 band is about 1%, but with significan

Water vapour self-continuum in near-visible IR absorption bands: measurements and semiempirical model of water dimer absorption

The nature of the water vapour continuum has been of great scientific interest for more than 60 years. Here, water vapour self-continuum absorption spectra are retrieved at temperatures of 398 K and 431 K and at vapour pressures from 1000 to 4155 mbar in the 8800 and 10,600 cm−1 absorption bands using high-resolution FTS measurements. For the observed conditions, the MT_CKD-3.2 model underestimates the observed continuum on average by 1.5–2 times. We use the hypothesis that water dimers contribute to the continuum absorption to simulate the experimentally-retrieved self-continuum absorption spectra, and to explain their characteristic temperature dependence and spectral behaviour. The values of the effective equilibrium constant are derived for the observed temperatures. We find that the dimer-based model fits well to the measured self-continuum from this and previous studies, but requires a higher effective equilibrium constant compared to the modern estimates within the temperature range (268–431 K) and spectral region studied. It is shown that water dimers are likely responsible for up to 50% of the observed continuum within these bands. Possible causes of the incomplete explanation of the continuum are discussed. Extrapolating these measurements to atmospheric temperatures using the dimer-based model, we find that the newly-derived self-continuum reduces calculated surface irradiances by 0.016 W m−2 more than the MT_CKD-3.2 self-continuum in the 8800 cm−1 band for overhead-Sun mid-latitude summer conditions, corresponding to a 12.5% enhancement of the self-continuum radiative effect. The change integrated across the 10,600 cm−1 band is about 1%, but with significant differences spectrally

Water vapor self-continuum absorption in near-infrared windows derived from laboratory measurements

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