The Water Vapour Continuum: Brief History and Recent Developments

V.E. Zuev Institute of Atmospheric Optics, SB RAS, 1, Academician Zuev Square, Tomsk, 634021 Russia
Surveys in Geophysics (Impact Factor: 4.13). 01/2014; 33(3-4):1-21. DOI: 10.1007/s10712-011-9170-y

ABSTRACT The water vapour continuum is characterised by absorption that varies smoothly with wavelength, from the visible to the microwave.
It is present within the rotational and vibrational–rotational bands of water vapour, which consist of large numbers of narrow
spectral lines, and in the many ‘windows’ between these bands. The continuum absorption in the window regions is of particular
importance for the Earth’s radiation budget and for remote-sensing techniques that exploit these windows. Historically, most
attention has focused on the 8–12μm (mid-infrared) atmospheric window, where the continuum is relatively well-characterised,
but there have been many fewer measurements within bands and in other window regions. In addition, the causes of the continuum
remain a subject of controversy. This paper provides a brief historical overview of the development of understanding of the
continuum and then reviews recent developments, with a focus on the near-infrared spectral region. Recent laboratory measurements
in near-infrared windows, which reveal absorption typically an order of magnitude stronger than in widely used continuum models,
are shown to have important consequences for remote-sensing techniques that use these windows for retrieving cloud properties.

KeywordsEarth radiation budget–Water vapour spectroscopy–Water dimers–Remote sensing

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    ABSTRACT: Recent laboratory measurements show that absorption by the water vapour continuum in near-infrared windows may be about an order of magnitude higher than assumed in many radiation codes. The radiative impact of the continuum at visible and near-infrared wavelengths is examined for the present day and for a possible future warmer climate (with a global-mean total column water increase of 33%). The calculations use a continuum model frequently used in climate models (“CKD”) and a continuum model where absorption is enhanced at wavelengths greater than 1 µm based on recent measurements (“CAVIAR”). The continuum predominantly changes the partitioning between solar radiation absorbed by the surface and the atmosphere; changes in top-of-atmosphere net irradiances are smaller. The global-mean clear-sky atmospheric absorption is enhanced by 1.5 W m−2 (about 2%) and 2.8 W m−2 (about 3.5%) for CKD and CAVIAR respectively, relative to a hypothetical no-continuum case, with all-sky enhancements about 80% of these values. The continuum is, in relative terms, more important for radiation budget changes between the present day and a possible future climate. Relative to the no-continuum case, the increase in global-mean clear-sky absorption is 8% higher using CKD and almost 20% higher using CAVIAR; all-sky enhancements are about half these values. The effect of the continuum is estimated for the solar component of the water vapour feedback, the reduction in downward surface irradiance and precipitation change in a warmer world. For CKD and CAVIAR respectively, and relative to the no-continuum case, the solar component of the water vapour feedback is enhanced by about 4% and 9%, the change in clear-sky downward surface irradiance is 7% and 18% more negative, and the global-mean precipitation response decreases by 1% and 4%. There is a continued need for improved continuum measurements, especially at atmospheric temperatures and at wavelengths below 2 µm.
    Quarterly Journal of the Royal Meteorological Society 04/2014; · 3.33 Impact Factor
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    ABSTRACT: The gaseous absorption of solar radiation within near-infrared atmospheric windows in the Earth's atmosphere is dominated by the water vapour continuum. Recent measurements by Baranov et al. (2011) [17] in 2500 cm−1 (4 μm) window and by Ptashnik et al. (2011) [18] in a few near-infrared windows revealed that the self-continuum absorption is typically an order of magnitude stronger than given by the MT_CKD continuum model prior to version 2.5. Most of these measurements, however, were made at elevated temperatures, which makes their application to atmospheric conditions difficult. Here we report new laboratory measurements of the self-continuum absorption at 289 and 318 K in the near-infrared spectral region 1300–8000 cm−1, using a multipass 30 m base cell with total optical path 612 m. Our results confirm the main conclusions of the previous measurements both within bands and in windows. Of particular note is that we present what we believe to be the first near-room temperature measurement using Fourier Transform Spectrometry of the self-continuum in the 6200 cm−1 (1.6 μm) window, which provides tentative evidence that, at such temperatures, the water vapour continuum absorption may be as strong as it is in 2.1 μm and 4 μm windows and up to 2 orders of magnitude stronger than the MT_CKD-2.5 continuum. We note that alternative methods of measuring the continuum in this window have yielded widely differing assessment of its strength, which emphasises the need for further measurements.
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    ABSTRACT: Free tropospheric humidity (FTH) is a key parameter of the radiation budget of the Earth. In particular, its distribution over the intertropical belt has been identified as an important contributor to the water vapour feedback. Idealized radiative transfer computations are performed to underscore the need to consider the whole probability distribution function (PDF) rather than the arithmetical mean of the FTH. The analysis confirmed the overwhelming role of the dry end of the PDF in the radiative perturbation of the top of atmosphere longwave budget. The physical and dynamical processes responsible for the maintenance of this dry part of the FTH distribution are reviewed, and the lateral mixing between the tropics and the extra-tropics is revealed as a major element of the dry air dynamics. The evolution of this lateral mixing in the framework of the global warming is discussed, and perspectives of work are listed as a mean of a conclusion.
    Surveys in Geophysics 33(3-4). · 4.13 Impact Factor


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May 15, 2014