Abstract
AbstractIn spite of decades of extensive studies, the role of water dimers (WD) in the atmospheric radiation budget is still controversial. In order to search for evidence of the dimer in the solar near infrared, high spectral resolution pure water vapour absorption spectra were obtained in laboratory conditions for two different pressures and temperatures in the spectral region 5000–5600 cm−1 (1.785 to 2 µm). The residual was derived as a difference between the measured optical depth and the calculated one for water monomer, using the modified HITRAN database and two different representations of the water vapour continuum: CKD‐2.4 (Clough–Kneizys–Davies) and the Ma and Tipping continuum. In both cases the residuals obtained are very similar to those expected from a recent theoretical calculation of the WD absorption. However, the WD band half‐width at half maximum (HWHM) and dimerization equilibrium constant, Keq, required to provide a best fit to the residual, differ for each case. To be in best agreement with the residual calculated by using the Ma and Tipping continuum, the WD bands HWHM should be ∼28 cm−1, and Keq=0.02±0.0035 atm−1 and 0.043±0.0055 atm−1 for temperatures 342 and 299 K respectively. For the residual calculated using the CKD‐2.4 continuum the fitted value of the HWHM is ∼18cm−1, and Keq=0.011±0.0025 atm−1 (342 K) and 0.018±0.003 atm−1 (299 K). It is concluded that a substantial part of the WD absorption is already implicitly included within the CKD‐2.4 continuum model. The increase in estimated clear‐sky global mean absorption of solar radiation due to WD varies from 0.5% to 2.0%, depending on the set of WD parameters used. On the basis of a comparison of the derived Keq values with others in the literature, the higher estimate is favoured. Copyright © 2004 Royal Meteorological Society
Highlights
Water vapour is one of the most important substances in the atmosphere
The observed laboratory spectra were compared with calculated absorbance spectra, generated for the measured laboratory conditions using HITRAN v.11, the line-by-line (LBL) code of Mitsel et al (1995), and the Reference Forward Model (RFM) line-by-line code of Dudhia (1997)
It means that using CKD-2.4 with HITRAN v.11 is quite justified in this case, in spite of the fact that originally HITRAN-96 is supposed to be used with this version of CKD continuum
Summary
Water vapour is one of the most important substances in the atmosphere. The absorption of solar radiation in the near infrared and visible spectral region is dominated by H2O molecular absorption. In addition to the water monomer (WM), water clusters, such as water dimers (WD) or trimers, have been investigated very intensively in recent years (Mhin et al 1993; Huisken et al 1996; Low and Kjaergaard 1999; Vaida and Headrick 2000; Evans and Vaida 2000). The role of the water dimer in the atmosphere is still controversial, in spite of many theoretical and experimental investigations. There are two interrelated issues in estimating the role of WD in the radiation budget. It is necessary to know the parameters (intensity, width, position and shape) of the WD absorption bands. It is important to know the abundance of the dimers.
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More From: Quarterly Journal of the Royal Meteorological Society
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