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: 3.45). 01/2014; 33(3-4):1-21. DOI: 10.1007/s10712-011-9170-y


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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Available from: Igor Ptashnik, Jan 13, 2014
    • "This description of the gas absorption coefficients is valid if the absorption coefficient is only composed of lines and do not contain any absorption continuum. This approximation is not valid in most parts of the thermal infrared spectrum because of the importance in strength and in spectral width of the water vapor absorption continuum [38] "
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    ABSTRACT: 1-D radiative transfer code MOMO (Matrix-Operator Model), has been extended from [0.2−3.65μm] band to the whole [0.2−100μm] spectrum. MOMO can now be used for computation of full range radiation budgets (shortwave and longwave). This extension to the longwave part of the electromagnetic radiation required to consider radiative transfer processes that are features of the thermal infrared: the spectroscopy of the water vapor self- and foreign- continua of absorption at 12μm and the emission of radiation by gases, aerosol, clouds and surface. MOMO's spectroscopy module, CGASA (Coefficient of Gas Absorption), has been developed for computation of gas extinction coefficients, considering continua and spectral line absorption. The spectral dependences of gas emission/absorption coefficients and of Planck's function are treated using a k-distribution. The emission of radiation is implemented in the adding-doubling process of the matrix operator method using Schwarzschild's approach in the radiative transfer equation (a pure absorbing/emitting medium, namely without scattering). Within the layer, the Planck-function is assumed having an exponential dependence on the optical-depth. In this paper, validation tests are presented for clear air case studies: Comparisons to the analytical solution of a monochromatic Schwarzschild's case without scattering show an error of less than 0.07% for a realistic atmosphere with an optical depth and a blackbody temperature that decrease linearly with altitude. Comparisons to radiative transfer code RTTOV are presented for simulations of top of atmosphere brightness temperature for channels of the space-borne instrument MODIS. Results show an agreement varying from 0.1 K to less than 1 K depending on the channel. Finally MOMO results are compared to CALIPSO Infrared Imager Radiometer (IIR) measurements for clear air cases. A good agreement was found between computed and observed radiance: biases are smaller than 0.5 K and RMSE (Root Mean Square Error) varies between 0.4 K and 0.6 K depending on the channel. The extension of the code allows the utilization of MOMO as forward model for remote sensing algorithms in the full range spectrum. Another application is full range radiation budget computations (heating rates or forcings).
    No preview · Article · Sep 2014 · Journal of Quantitative Spectroscopy and Radiative Transfer
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    • "Although we have confidence that the continuum is significantly stronger than given by CKD in these windows, especially at elevated temperatures, the primary uncertainty in the 2.1 and 3.8 μm windows is in the extrapolation of these values to atmospheric temperatures, which has to be done in the absence of a robust physical understanding of the causes of the continuum. The absorption is plausibly due to water dimers, or other water complexes, and is stronger than can be explained by existing far-wing line theories (see Ptashnik et al. (2011b) or Shine et al. (2012) for more discussion). The continuum strength in the CKD model generally lies at the lower end of the uncertainty range in the laboratory measurements, and so the results using CKD could be regarded as a lower limit to the impact of the continuum. "
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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.
    Full-text · Article · Jun 2014 · Quarterly Journal of the Royal Meteorological Society
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    • "Although the continuum absorption in these windows, and its dependence on pressure and temperature , is now much better observed and characterised, there remains a lively debate on the underlying physical causes of this continuum. The historical perspective underpinning the development of a theory of water vapour continuum absorption has led to recent developments and experimental campaigns leading to new insights into this uncertain aspect of how water vapour influences the flows of radiative energy through the atmosphere (Shine et al. 2012). Although improved representation of the water vapour continuum is likely to reduce uncertainty in Earth's radiative energy budget only marginally, it may have implications for the retrieval of physical quantities from space and therefore how we observe Earth's climate system. "
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    ABSTRACT: Water vapour modulates energy flows in Earth's climate system through transfer of latent heat by evaporation and condensation and by modifying the flows of radiative energy both in the longwave and shortwave portions of the electromagnetic spectrum. This article summarizes the role of water vapour in Earth's energy flows with particular emphasis on (1) the powerful thermodynamic constraint of the Clausius Clapeyron equation, (2) dynamical controls on humidity above the boundary layer (or free-troposphere), (3) uncertainty in continuum absorption in the relatively transparent "window" regions of the radiative spectrum and (4) implications for changes in the atmospheric hydrological cycle.
    Preview · Article · Jul 2012 · Surveys in Geophysics
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