Laboratory measurements of the water vapor continuum in the 1200–8000 cm⁻¹ region between 293 K and 351 K

Abstract

[1] Laboratory Fourier transform spectroscopy of pure water vapor and water vapor mixed with air has been conducted between 1200 and 8000 cm À1 and at temperatures between 293 and 351 K with the purpose of detecting and characterizing the water vapor continuum. The spectral features of the continuum within the major water absorption bands are presented and compared where possible to those from previous experimental studies and to the commonly used MT_CKD and CKD models. It was observed that in the main, both models adequately capture the general spectral form of the continuum; however, there were a number of exceptions. Overall, there is no evidence to indicate that MT_CKD is an improvement upon the older CKD model in these spectral regions. There was generally good agreement between our results and those of other experimental investigators. The general mathematical forms of the self-continuum temperature dependence, given by both Roberts et al. (1976) and CKD/MT_CKD, fit well to the experimental continuum in these spectral regions. However, the range of temperatures over which we made measurements is not sufficient to discriminate between these two forms or to exclude the possibility of other forms of temperature dependence being more appropriate. At the same time, the actual parameters currently used in CKD/MT_CKD to describe the temperature dependence in many spectral regions cannot reproduce the observed strong spectral variation in the temperature dependence. It has not been possible to make definitive conclusions about the magnitude of the continuum absorption in the far wings of the absorption bands investigated here.

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

“…The last column of O 2 ) results [30] to account for a larger band width of 60 cm −1 (compared with 40 cm −1 in Kjaergaard et al [30]). The latter was found to be a reasonably good first approximation for water dimer Lorentzian sub-bands to describe the self-continuum spectral features within near-infrared bands [11][12][13][14], and even the strength of the continuum in the windows [14,16] at close to standard temperatures. As a result, there is quite good agreement between the radiative impact of the experimental self-and foreign CAVIAR continuum (column 3) and the modified results of Kjaergaard et al [30] for H 2 O complexes (last column in table 4) made using their assumption of Lorentzian wings.…”

“…In-band continuum measurements include those by Burch [17] (3500-4000 cm −1 ) and Paynter et al [12] (all four bands between 1300 and 7500 cm −1 ). MTCKD is in broad agreement with these measurements, although, as with the self-continuum, the measurements indicate more spectral structure.…”

Airborne and satellite remote sensing of the mid-infrared water vapour continuum

“…We consider this to be the best evidence that a weak temperature dependence to the foreign continuum is genuine. Paynter et al [9] (see their fig. 16) detected a small trend in foreign continuum coefficients between 1580 and 1610 cm −1 over the temperature range 296-351 K, which is additional evidence of a slight strengthening of the foreign continuum with decreasing temperature in this spectral interval.…”

“…This is not to say that MT_CKD is capable of reproducing all observable spectral behaviour, however. For example, the temperature dependence of the continuum has been found not to be well captured when compared with recent laboratory data [8,9]. MT_CKD also appears to underestimate the strength of the continuum in some high-transmittance atmospheric windows by as much as an order of magnitude [10].…”

“…Several definitions of the continuum exist in the literature. Following Paynter et al [9], it is possible to normalize the continuum coefficients to 1 atm pressure of the 'self' or 'foreign' gas and include the radiation term so that…”