Semi-empirical water dimer model of the water vapour self-continuum within the IR absorption bands

• Published in: Journal of quantitative spectroscopy & radiative transfer Vol. 329; p. 109198
• Main Authors: Simonova, Anna A., Ptashnik, Igor V., Shine, Keith P.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.12.2024
• Subjects: Absorption band; Continuum absorption; Line wing; Semi-empirical model; Water dimer; Water vapour; Water vapour; Line wing; Continuum absorption; Water dimer; Semi-empirical model; Absorption band
• ISSN: 0022-4073
• DOI: 10.1016/j.jqsrt.2024.109198

•H2O self-continuum absorption spectra are updated within 5300 and 7200 cm-1 bands.•Experimental H2O continuum within the IR bands, from 50 to 11 200 cm-1, is discussed.•Estimated contribution of water dimer to the in-band continuum varies from 40 to 90 %.•Semi-empirical water dimer model is developed for the IR in-band H2O self-continuum.•New in-band continuum model is suitable for incorporation into radiative transfer codes. Water vapour continuum absorption is an important component of atmospheric radiative transfer codes. It significantly impacts the radiative balance of the atmosphere, but the physical nature of this absorption remains a subject of discussion. Here the H2O self-continuum absorption is considered within the infrared absorption bands (from 50 to 11 200 cm-1) of water vapour exploiting existing measurements. Comparison of this data with the MT_CKD-3.5 continuum model, which is used in many radiative transfer codes, reveals significant quantitative and qualitative differences. New water vapour self-continuum spectra are derived from earlier FTS measurements using HITRAN-2016 in the 5300 and 7200 cm-1 bands. A previously proposed water dimer model is refined and unified based on a broad set of up-to-date experimental data on the H2O continuum. The new model, which is suitable for incorporation into radiative transfer codes, has a much firmer physical basis than existing models. It reproduces the spectral behaviour and magnitude of the in-band water vapour self-continuum for temperatures from 279 to 431 K depending on the band. Importantly, the fitted total equilibrium dimerization constant used in the updated continuum model exceeds independent estimates by a factor of 1.5–3 across the entire temperature and spectral regions studied. Possible causes for this, which are important for understanding the physical origin of the continuum, are discussed. The contribution of water dimer to the continuum is estimated to vary from 40 to 90 % depending on absorption band and temperature.