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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About 200 pure water-vapor spectra covering the region from 800 to 3500 cm−1 were recorded with resolution of 0.1 cm−1 at temperatures 311, 318, 325, 339, 352, and 363 K using a 2 m base White cell coupled to the BOMEM DA3.002 FTIR spectrometer. The water-vapor pressure varied from 28 to 151 mbar (21–113 Torr). Under these conditions, the continuum absorbance is quite measurable with the available path lengths up to 116 m. A program was developed for spectral processing that calculates, fits, and removes ro-vibrational structure from the spectrum. The spectra obtained were used to retrieve averaged and smoothed binary absorption coefficients over the region from 800 to 1250 cm−1. Our continuum data extrapolated to room temperature are in reasonable agreement with the MT_CKD continuum model. But at higher temperatures the MT_CKD model provides very low values, which are up to 50% less than those experimentally measured.
