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.

中文翻译：

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红外吸收带内水蒸气自连续体的半经验水二聚体模型


水汽连续吸收是大气辐射传输码的重要组成部分。它显着影响大气的辐射平衡，但这种吸收的物理性质仍然是一个讨论的主题。在这里，利用现有测量数据，在水蒸气的红外吸收带（从 50 到 11 200 cm-1）内考虑 H2O 自连续吸收。将此数据与许多辐射传输码中使用的 MT_CKD-3.5 连续体模型进行比较，揭示了显着的定量和定性差异。新的水蒸气自连续谱光谱来自早期使用 HITRAN-2016 在 5300 和 7200 cm-1 波段进行的 FTS 测量。先前提出的水二聚体模型根据 H 2 O 连续体的广泛最新实验数据进行了改进和统一。新模型适合合并到辐射传输码中，具有比现有模型更坚固的物理基础。它再现了 279 至 431 K 温度下带内水蒸气自连续谱的光谱行为和幅度，具体取决于波段。重要的是，在更新后的连续体模型中使用的拟合总平衡二聚化常数在所研究的整个温度和光谱区域中比独立估计高出 1.5-3 倍。讨论了造成这种情况的可能原因，这些原因对于理解连续体的物理起源很重要。据估计，水二聚体对连续体的贡献从 40% 到 90% 不等，具体取决于吸收带和温度。