The very weak water vapor self-continuum has been investigated by high sensitivity Cavity Ring Down Spectroscopy in the 1.6 µm window at five temperatures between 302 K and 340 K. The absorption cross-sections, Cs(ν, T), were retrieved for ten selected wavenumbers from a fit of the absorption coefficients measured in real time during pressure ramps, after subtraction of the contributions of the local water monomer lines and of water adsorbed on the CRDS mirrors. The values measured between 5875 and 6665 cm-1 range between 1.5 × 10-25 and 2 × 10-24 cm2 molec-1 atm-1 with a minimum around 6300 cm-1. At 302 K, an agreement within 50% is observed over the whole window with the cross-sections provided by the MT_CKD V2.5 model. Nevertheless, while our measurements show that the Cs(ν, T) decrease from 302 K to 340 K is no more than 50% for all our selected wavenumbers, the MT_CKD V2.5 model predicts a much more pronounced temperature dependence in the centre of the window, the agreement being better on the edges of the window. The obtained results are discussed in relation with theoretical modeling of the water vapor self-continuum as far-wings of monomer lines or water dimer absorption. For potential atmospheric applications, cross-sections are provided at each temperature with a sampling step of 10 cm-1 for the entire 5850 – 6700 cm-1 range.
GR-Atmospheres publishes articles that advance and improve understanding of atmospheric properties and processes, including the interaction of the atmosphere with other components of the Earth system.
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.
