Water vapor self-continuum absorption in near-infrared windows derived from laboratory measurements

Abstract

[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.

Water vapour foreign-continuum absorption in near-infrared windows from laboratory measurements

“…We present here an experimental study of the foreign-continuum absorption in a broader spectral region, 2000-9000 cm −1 , in four near-infrared windows. The work can be considered as a companion paper to that of Ptashnik et al [16], who derived the self-continuum of water vapour in the same wavelength region, using broadly the same measurement and analysis technique; indeed, as will be discussed, a consistent self-continuum is needed in order to separate the foreign continuum from the total (i.e. self plus foreign) continuum that is derived from the measurements.…”

“…Again, the retrieval technique closely follows that described in Ptashnik et al [16]. The optical depth t m of the measured spectra is given by…”

“…The background measurements with an empty cell and with air were carried out, first, to ensure that any possible cell distortion due to pressure was taken into account and, [20], light grey lines) calculated up to 25 cm −1 from the line centres without a plinth; experimental RAL (dark grey lines) spectrum; MTCKD-2.5 (dashed lines) total continuum [9,10]; optical depth of the total (i.e. self plus foreign) continuum (dots) derived as a spectrally smoothed difference between RAL and UCL08 in microwindows between lines; contribution from the self-continuum (circles) derived using data [16] for the same temperature.…”

“…The measurement technique closely follows that described in detail by Ptashnik et al [16]. Briefly, spectra were recorded using a Bruker IFS-125 highresolution Fourier transform spectrometer (FTS) at the Rutherford Appleton Laboratory (RAL) Molecular Spectroscopy Facility (http://www.msf.rl.ac.uk) using a multi-pass short-path absorption cell (SPAC), based on the White [19] arrangement.…”

“…Hence, a part of the continuum derived here belongs to spectral line absorption. This component is very small for the work reported here, contributing less than 1 per cent in the middle of the windows [16]. The contribution from the spectral lines was calculated using a line-by-line code [21] and the UCL08 (University College London, UK) compilation of H 2 O line parameters [20], which contains a number of weak water vapour lines not catalogued in the HITRAN-2008 [22] spectral line database.…”