[1] In 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 mm (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/m 2 (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.

SUMMARYIn spite of decades of extensive studies, the role of water dimers (WD) in the atmospheric radiation budget is still controversial. In order to search for evidence of the dimer in the solar near infrared, high spectral resolution pure water vapour absorption spectra were obtained in laboratory conditions for two different pressures and temperatures in the spectral region 5000-5600 cm −1 (1.785 to 2 µm). The residual was derived as a difference between the measured optical depth and the calculated one for water monomer, using the modified HITRAN database and two different representations of the water vapour continuum: CKD-2.4 (Clough-Kneizys-Davies) and the Ma and Tipping continuum. In both cases the residuals obtained are very similar to those expected from a recent theoretical calculation of the WD absorption. However, the WD band half-width at half maximum (HWHM) and dimerization equilibrium constant, K eq , required to provide a best fit to the residual, differ for each case. To be in best agreement with the residual calculated by using the Ma and Tipping continuum, the WD bands HWHM should be ∼28 cm −1 , and K eq = 0.02 ± 0.0035 atm −1 and 0.043 ± 0.0055 atm −1 for temperatures 342 and 299 K respectively. For the residual calculated using the CKD-2.4 continuum the fitted value of the HWHM is ∼18 cm −1 , and K eq = 0.011 ± 0.0025 atm −1 (342 K) and 0.018 ± 0.003 atm −1 (299 K). It is concluded that a substantial part of the WD absorption is already implicitly included within the CKD-2.4 continuum model. The increase in estimated clear-sky global mean absorption of solar radiation due to WD varies from 0.5% to 2.0%, depending on the set of WD parameters used. On the basis of a comparison of the derived K eq values with others in the literature, the higher estimate is favoured.

For a long time, it has been believed that atmospheric absorption of radiation within wavelength regions of relatively high infrared transmittance (so-called 'windows') was dominated by the water vapour self-continuum, that is, spectrally smooth absorption caused by H 2 O−H 2 O pair interaction. Absorption due to the foreign continuum (i.e. caused mostly by H 2 O−N 2 bimolecular absorption in the Earth's atmosphere) was considered to be negligible in the windows. We report new retrievals of the water vapour foreign continuum from high-resolution laboratory measurements at temperatures between 350 and 430 K in four near-infrared windows between 1.1 and 5 mm (9000-2000 cm −1 ). Our results indicate that the foreign continuum in these windows has a very weak temperature dependence and is typically between one and two orders of magnitude stronger than that given in representations of the continuum currently used in many climate and weather prediction models. This indicates that absorption owing to the foreign continuum may be comparable to the self-continuum under atmospheric conditions in the investigated windows. The calculated global-average clear-sky atmospheric absorption of solar radiation is increased by approximately 0.46 W m −2 (or 0.6% of the total clearsky absorption) by using these new measurements when compared with calculations applying the widely used MTCKD (Mlawer-Tobin-Clough-Kneizys-Davies) foreigncontinuum model.