n most near‐infrared atmospheric windows, absorption of solar radiation is dominated by the water vapor self‐continuum, and yet there is a paucity of measurements in these windows. We report new laboratory measurements of the self‐continuum absorption at temperatures between 293 and 472 K and pressures from 0.015 to 5 atm in four near‐infrared windows between 1 and 4 μm (10000–2500 cm−1); the measurements are made over a wider range of wavenumbers, temperatures, and pressures than any previous measurements. They show that the self‐continuum in these windows is typically one order of magnitude stronger than given in representations of the continuum widely used in climate and weather prediction models. These results are also not consistent with current theories attributing the self‐continuum within windows to the far wings of strong spectral lines in the nearby water vapor absorption bands; we suggest that they are more consistent with water dimers being the major contributor to the continuum. The calculated global average clear‐sky atmospheric absorption of solar radiation is increased by ∼0.75 W/m2 (which is about 1% of the total clear‐sky absorption) by using these new measurements as compared to calculations with the MT_CKD‐2.5 self‐continuum model.
JGR-Atmospheres includes physics and chemistry of the atmosphere, as well as the atmospheric-biospheric, lithospheric, and hydrospheric interface.
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We present experimental and theoretical studies of medium infrared absorption by pure water vapor. Measurements have been made in the 1900–2600 cm-1 and 3900–4600 cm-1 regions, for temperatures and pressures in the 500–900 K and 0–70 atm ranges, respectively. They are consistent with available data and enable the determination of continuum absorption parameters. It is shown that calculations with line shapes derived from the impact approximation are very inaccurate. Models accounting for the finite durations of collisions and line-mixing through wave-number dependent effective broadening parameters are introduced. The latter have been determined using two different approaches, which are (i) empirical determinations from fits of experimental data and (ii) direct predictions from first principles using a statistical approach. Effective broadening parameters obtained using these two different approaches are in satisfactory agreement for both the temperature and wavenumber dependencies. These data are tested by calculations of continua in various spectral regions and the agreement with measured values is satisfactory. The remaining discrepancies probably result from the influence of the internal structures of the absorption bands considered and thus from the influence of line-mixing. Nevertheless, accurate predictions are obtained in wide temperature and spectral ranges when the total absorption at elevated density is considered. This agreement, which is due to the relatively weak continuum absorption and large contributions of nearby lines, makes the present models suitable for most practical applications involving elevated densities.
