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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/amt-2019-403
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-403
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 16 Dec 2019

Submitted as: research article | 16 Dec 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Atmospheric observations of the water vapour continuum in the near-infrared windows between 2500–6600 cm−1

Jonathan Elsey1, Marc D. Coleman2, Tom D. Gardiner2, Kaah P. Menang3, and Keith P. Shine1 Jonathan Elsey et al.
  • 1Department of Meteorology, University of Reading, Reading, RG6 6BB, United Kingdom
  • 2National Physical Laboratory, Teddington, London, TW11 0LW, United Kingdom
  • 3National Department of Meteorology, Ministry of Transport, Yaoundé, Cameroon

Abstract. Water vapour continuum absorption is potentially important for both closure of the Earth’s energy budget and remote sensing applications. Currently, there are significant uncertainties in its characteristics in the near-infrared atmospheric windows at 2.1 and 1.6 μm. There have been several attempts to measure the continuum in the laboratory; not only are there significant differences amongst these measurements but there are also difficulties in extrapolating the laboratory data taken at room temperature or higher to atmospheric temperatures. Validation is therefore required using field observations of the real atmosphere. There are currently few published observations in atmospheric conditions with enough water vapour to detect a continuum signal within these windows, or where the self-continuum component is significant. We present observations of the near-infrared water vapour continuum from Camborne, UK at sea level using a sun-pointing, radiometrically-calibrated Fourier transform spectrometer in the window regions between 2000–10000 cm−1. Analysis of this data is challenging, particularly because of the need to remove aerosol extinction, and the large uncertainties associated with such field measurements. Nevertheless, we present data that is consistent with recent laboratory datasets in the 4 and 2.1 μm windows (when extrapolated to atmospheric temperatures). These results indicate that the most recent revision (3.2) of the MT_CKD foreign continuum, versions of which are widely used in atmospheric radiation models, requires strengthening by a factor of ~ 5 in the centre of the 2.1 µm window. In the higher-wavenumber window at 1.6 µm, our estimated self and foreign continua are significantly stronger than MT_CKD. The possible contribution of the self and foreign continua to our derived total continuum optical depth is estimated by using laboratory or MT_CKD values of one, to estimate the other. The obtained self-continuum shows some consistency with temperature-extrapolated laboratory data in the centres of the 4 and 2.1 µm windows. The 1.6 μm region is more sensitive to atmospheric aerosol and continuum retrievals and therefore more uncertain than the more robust results at 2.1 and 4 μm. We highlight the difficulties in observing the atmospheric continuum and make the case for additional measurements from both the laboratory and field, with discussion of the requirements for any new field campaign.

Jonathan Elsey et al.
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Jonathan Elsey et al.
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Atmospheric observations of the water vapour continuum in the near-infrared windows - Data J. Elsey, T. Gardiner, M. Coleman, K. P. Menang, and K. P. Shine https://doi.org/10.5281/zenodo.3520519

Jonathan Elsey et al.
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Short summary
Water vapour is an important component in trying to understand the flows of energy between the Sun and Earth, since it is opaque to radiation emitted by both the surface and the Sun. In this paper, we study how it absorbs sunlight by way of it's continuum, a property which is poorly understood and with few measurements. Our results indicate that this continuum absorption may be more significant than previously thought, potentially impacting satellite observations and climate studies.
Water vapour is an important component in trying to understand the flows of energy between the...
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