Water vapour continuum
In addition to the spectral lines, it has long been recognized that water vapour possesses a continuum absorption which varies relatively slowly with wavelength and pervades the entire IR and microwave spectral region. This has a marked impact on the Earth's radiation balance with consequences for understanding present day weather and climate and predicting climate change. It is also important for remote sensing of the Earth and its atmosphere.
Discovered by Hettner (1918) as a low-frequency component of water vapour absorption in atmospheric transparency window 8-14 mcr, this phenomenon remained unexplained for 20 years, until Elsasser (1938) suggested that the continuum is an accumulated far-wing contribution of strong water vapour spectral lines from neighbour bands. This hypothesis was generally accepted until the end of 70th years when the strong quadratic pressure dependence of the continuum absorption (which could not be explained by Lorentz (1906) line profile) as well as the strong negative temperature dependence have been detected (Bignell et al.,1963; Penner and Varanasi,1967). In this connection Penner and Varanasi (1967) and Varanasi et al. (1968) suggested that the main contribution to the self-continuum could be caused not by far wings of water monomer lines but rather by water dimers. Similar assumption was made also by Viktorova and Zhevakin (1967) for microwave spectral region.
The dimer model have explained quite easily the pressure and temperature dependencies of the self-continuum absorption observed since then in many experiments (Mc Coy et al. 1969; Bignell, 1970; Burch, 1970; etc.). Since that time a long scientific discussion has started between adherents of the "monomer" (or "far-wings") and the "dimer" nature of the water vapour self-continuum, which is continuing up to the current time.
On the one hand, more sophisticated (than Lorentz theory) ab-initio (Tvorogov et al. 1994; Ma and Tipping 1999, 2002; etc.) and semi-empirical (Clough et al. 1989, 1995, etc; Mlawer et al. 1999; etc.) line shape models have been developed, which could explain quite well the experimental facts mentioned above, and due to which the dominating role of the far wings of water vapour lines in the continuum absorption, especially in atmospheric conditions, is most commonly accepted today.
On the other hand, water dimers have been and are being often discussed as a possible component of the water self-continuum absorption (Lowder, 1971; Penner, 1973; Roberts et al. 1976; Arefev and Dianov-Klokov 1977; Montgomery, 1978; Dianov-Klokov et al. 1981; Varanasi, 1988; Devir et al. 1994; Vigasin et al. 1989, 2000; Cormier et al. 2005, etc.).
Finally, collision-induced absorption, resulting from the generation of a short-lived complex of water vapour and colliding molecules, has been proposed as a dominant within water vapour bands in the recent MT_CKD continuum model (Mlawer et al., in preparation, http://rtweb.aer.com/continuum_frame.html).
The possibility of both collision-induced and water dimer marked contribution to the water continuum absorption is however highly disagreed by Tipping (personal communication; Brown and Tipping, 2003). This point of view is shared by Vigasin only in respect to the free pair states, which negligible role as compared to the metastable or true bound water dimers at near-room temperatures has been shown by Vigasin (1991) and by Epifanov and Vigasin (1997) on the basis of preliminary statistical partitioning of the pair states in water vapour.
Thus, a deep controversy on the nature of the water vapour continuum still remains unresolved. 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 a semi-empirical formulation of the continuum - CKD-model (Clough et al. 1989). This formulation was tuned to available (mostly laboratory) observations in rather limited (far-infrared) spectral regions.
The CKD model has served the community extremely well but we lack confidence that its semi-empirical formulation works at wavelength, or in atmospheric conditions, away from those in which it has been tested. This lack of confidence is exacerbated by the recent up-to-date theoretical (Schofield and Kjaergaard, 2003; Daniel et al. 2004; Scribano et al. 2006) and experimental (Vigasin et al. 2000, 2005; Ptashnik et al. 2004, 2005, 2006; Cormier et al. 2005; Paynter et al. 2007) studies that very well correlate and supplement each other, indicating all together the marked water dimer contribution to the water vapour self-continuum in some spectral regions.
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