Continuum Absorption at Visible and Infrared Wavelengths and its Atmospheric Relevance (CAVIAR)

Lead Research Organisation: University of Reading
Department Name: Meteorology

Abstract

Water vapour is the most important greenhouse gas in the Earth's atmosphere. Because of its complex structure, it is unusual in that it absorbs energy across a wide range of wavelengths from the ultra-violet, to the microwave. Infrared absorption by water vapour is of particular significance. It causes a large part of the natural greenhouse effect which makes the Earth habitable, hence impacting on the present day climate. It also plays an important role in climate change. If the Earth warms, for example due to increases in CO2, water vapour concentrations increase; since water vapour is itself a greenhouse gas, this leads to a positive feedback which, models indicate, approximately doubles the warming. Unfortunately, understanding of the absorbing properties of water vapour is currently inadequate. Water vapour absorbs radiation in two ways. The first is in narrow wavelength regions (spectral lines) for which understanding is relatively good. The second is slowly varying absorption over broad spectral regions (the continuum). It is the understanding of this continuum absorption which is the subject of this proposal. The existence of the continuum has been known for decades, but an understanding of its cause, and its characteristics, is a source of controversy. One theory is that it is due to cumulative small contributions from thousands of spectral lines; an alternative, but not necessarily exclusive, theory is that it is due to absorption by pairs of weakly bound water molecules (the water dimer) and related species. Currently, most computer models used in weather forecasting, climate prediction, and to retrieve data from satellite observations, use one particular representation of the continuum developed over the past twenty years. This representation has served the community well. However, it lacks a firm theoretical basis and has only been verified using observations for a quite narrow range of wavelengths and atmospheric conditions; additionally, these observations have been made by different groups at different times and their comparability is difficult to assess. This limits confidence in its use, particularly as climate, and hence atmospheric conditions, change. Developments in the theory of continuum absorption, as well as advances in instrumentation, mean that it is timely for a concerted effort to improve our understanding and characterisation of the continuum. We bring together a consortium of 8 leading UK groups with established expertise in the theory of water vapour absorption, in the use of state-of-the-art measurement techniques in both the laboratory and the atmosphere, and in climate modelling. The programme of research involves several components. 1 Advanced calculations of vibrations and rotations of the water dimer, which will allow a better prediction of its absorption properties and its contribution to the continuum. 2 The use of a state-of-the-art laboratory instrumentation enabling the measurement of the continuum over an unprecedentedly broad range of wavelengths and conditions; an alternative technique, capable of measuring relatively weak absorption at very high precision will be deployed for detailed studies in narrower wavelength regions. 3 Field campaigns, which will use a mixture of well-calibrated ground and aircraft based instruments, and will characterise the continuum over a broad range of wavelengths under real atmospheric conditions. We propose two campaigns: one in south-west England and one at a high mountain site in Europe. This will allow measurements to be made under very different atmospheric conditions. 4 Synthesis of the results from the theory, laboratory measurements and field campaigns, drawing them together into a common framework. 5 Understanding of the impact of the new results on our understanding of present-day climate and climate change. 6 Development of a representation of the continuum data in a form that can be readily used by other researchers.

Publications

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Costa S (2012) Outgoing Longwave Radiation due to Directly Transmitted Surface Emission in Journal of the Atmospheric Sciences

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Gardiner TD (2012) Absolute high spectral resolution measurements of surface solar radiation for detection of water vapour continuum absorption. in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences

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Haywood J (2011) The roles of aerosol, water vapor and cloud in future global dimming/brightening in Journal of Geophysical Research

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Kelly R (2010) Water dimer vibration-rotation tunnelling levels from vibrationally averaged monomer wavefunctions in Journal of Quantitative Spectroscopy and Radiative Transfer

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Newman S (2012) The Joint Airborne IASI Validation Experiment: An evaluation of instrument and algorithms in Journal of Quantitative Spectroscopy and Radiative Transfer

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Newman SM (2012) Airborne and satellite remote sensing of the mid-infrared water vapour continuum. in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences

 
Description CAVIAR has led to major advances as to the role of water vapour in absorbing infrared energy. This work has consequences in weather forecasting and climate change research, as the numerical models used in these areas are heavily reliant on an accurate representation of the role of water vapour. In addition, weather prediction is highly dependent on the use of satellite data to initialise weather forecasts, as the satellite data provides global information on temperature and humidity. Since these satellite sensors mostly observe in the infrared, they too are reliant on our understanding of the fundamental properties of water vapour.



As has been long established, the absorption spectrum of water vapour is characterised by many tens of thousands of discrete spectral lines, associated with the rotation and vibration of the water molecule (hereafter called the water monomer to distinguish it from the water dimer, discussed later), which are collected into discrete "spectral bands", interspersed by a number of so-called "windows". Underlying this spectral structure is a component of absorption (the "water vapour continuum") which varies relatively smoothly with wavelength and is much less well understood - this continuum was the focus of CAVIAR. The continuum absorption is particularly important, in terms of its atmospheric impact, in the window regions, where there is relatively little other atmospheric absorption (at least in clear skies); but it is also present within the spectral bands, where its characteristics can give important clues to the causes of the continuum.



CAVIAR brought together expertise in theory, laboratory observations, field measurements and global modelling and has achieved a significant advance in understanding of the water vapour continuum, across an unprecedentedly broad range of wavelengths. This understanding includes improved characterisation of the continuum (its variations with wavelength and its temperature and pressure dependence) via laboratory and field measurements and improved understanding of the physical causes of the continuum. Clearly a number of outstanding issues remain. Further advances in laboratory measurements are required to access weak continuum absorption, especially at temperatures close to atmospheric conditions, and further theoretical developments are required to better understand dimer absorption, especially in window regions, where they may be of most significance for understanding atmospheric processes in both the present and future atmosphere. CAVIAR results are beginning to be incorporated into spectral databases, into radiation codes used in weather forecasting and climate models, and are also being considered for their impact on remote sensing applications.
Exploitation Route The results from CAVIAR are of relevance to government agencies (including the Met Office) who are involved in weather prediction, climate prediction and in using satellites, and other techniques for monitoring and observations. The Met Office was a partner in the project and is already exploiting the results within its numerical models for climate prediction and weather forecasting and also in its procedures for using satellite data as input to its weather prediction models.

The National Physical Laboratory was also a partner in CAVIAR and their work on advancing techniques for well-calibrated field observations of infrared radiation (which are of importance, for example, in environmental monitoring) is of importance to their organtisation and others involved in metrology in this area
Sectors Aerospace, Defence and Marine,Environment

URL http://www.met.reading.ac.uk/caviar/
 
Description CAVIAR has led to major advances as to the role of water vapour in absorbing infrared energy. This work has consequences in weather forecasting and climate change research, as the numerical models used in these areas are heavily reliant on an accurate representation of the role of water vapour. In addition, weather prediction is highly dependent on the use of satellite data to initialise weather forecasts, as the satellite data provides global information on temperature and humidity. Since these satellite sensors mostly observe in the infrared, they too are reliant on our understanding of the fundamental properties of water vapour. As has been long established, the absorption spectrum of water vapour is characterised by many tens of thousands of discrete spectral lines, associated with the rotation and vibration of the water molecule (hereafter called the water monomer to distinguish it from the water dimer, discussed later), which are collected into discrete "spectral bands", interspersed by a number of so-called "windows". Underlying this spectral structure is a component of absorption (the "water vapour continuum") which varies relatively smoothly with wavelength and is much less well understood - this continuum was the focus of CAVIAR. The continuum absorption is particularly important, in terms of its atmospheric impact, in the window regions, where there is relatively little other atmospheric absorption (at least in clear skies); but it is also present within the spectral bands, where its characteristics can give important clues to the causes of the continuum. CAVIAR brought together expertise in theory, laboratory observations, field measurements and global modelling and has achieved a significant advance in understanding of the water vapour continuum, across an unprecedentedly broad range of wavelengths. This understanding includes improved characterisation of the continuum (its variations with wavelength and its temperature and pressure dependence) via laboratory and field measurements and improved understanding of the physical causes of the continuum. Clearly a number of outstanding issues remain. Further advances in laboratory measurements are required to access weak continuum absorption, especially at temperatures close to atmospheric conditions, and further theoretical developments are required to better understand dimer absorption, especially in window regions, where they may be of most significance for understanding atmospheric processes in both the present and future atmosphere. CAVIAR results are beginning to be incorporated into spectral databases, into radiation codes used in weather forecasting and climate models, and are also being considered for their impact on remote sensing applications.
First Year Of Impact 2010
Sector Environment
Impact Types Policy & public services

 
Description Continuum absorption of infrared radiation by water vapour molecules
Amount £8,400 (GBP)
Funding ID 2009/R3 
Organisation The Royal Society 
Sector Charity/Non Profit
Country United Kingdom
Start 01/2010 
End 12/2012
 
Description NERC Discovery Science
Amount £799,237 (GBP)
Funding ID NE/R009848/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 05/2018 
End 05/2021
 
Description Collaboration with Met Office 
Organisation Meteorological Office UK
Country United Kingdom 
Sector Academic/University 
PI Contribution Ongoing discussions on the role of water vapour as a greenhouse gas, which is of relevance to several areas of Met Office interest, including weather forecasting, climate prediction and the exploitation of satellite data
Collaborator Contribution We provided analyses of laboratory observations for use by the Met Office especially for the exploitation of satellite data
Impact Newman SM, Green PD, Ptashnik IV, Gardiner TD, Coleman MD, McPheat RA, Smith KM 2012: Airborne and satellite remote sensing of the mid-infrared water vapour continuum Phil Trans Roy Soc A370:2611-2636 doi:10.1098/rsta.2011.0223 Newman SM, Lara AM, Smith WL, Ptashnik IV, Jones RL, Mead MI, Revercomb H Tobin DC, Taylor JK, Taylor JP 2012 The Joint Airborne IASI Validation Experiment: An evaluation of instrument and algorithms J Quant Spectrosc Radiat Transf 132:1372-1390 10.1016/j.jqsrt.2011.06.007
Start Year 2006
 
Description Collaboration with National Physical Laboratory 
Organisation National Physical Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration was mainly via PhD studentships who were either formally or informally associated with NPL research teams - scientific analysis of data produced by NPL
Collaborator Contribution Provision of data and expert advice in the area of metrology
Impact Menang KP, Coleman MD, Gardiner TD, Ptashnik IV, Shine KP 2013: A high-resolution near-infrared extraterrestrial solar spectrum derived from ground-based Fourier transform spectrometer measurements J Geophys Res: Atmos 118:5319-5531 10.1002/jgrd.50425 Gardiner TD, Coleman M, Browning H, Tallis L, Ptashnik IV, Shine KP 2012: Absolute high spectral resolution measurements of surface solar radiation for detection of water vapour continuum absorption. Phil Trans Roy Soc A 370:2590-2610 doi: 10.1098/rsta.2011.0221 Tallis L, Coleman M, Gardiner T, Ptashnik IV, Shine KP 2011: Assessment of the consistency of H2O line intensities over the near-infrared using sun-pointing ground-based Fourier transform spectroscopy J Quant Spectrosc Radiat Transf 112:2268-2280 10.1016/j.jqsrt.2011.06.007
Start Year 2006
 
Description IEEE invited talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Further discussions on water vapour continuum

Requests for further talks
Year(s) Of Engagement Activity 2012
 
Description IPCC 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Policymakers/politicians
Results and Impact Major international report widely reported in the press and considered within United Nations meetings

Informing policymaking, especially development of post-Kyoto climate change agreements of the UN Framework Convention on Climate Change
Year(s) Of Engagement Activity 2013
URL http://www.climatechange2013.org/
 
Description NERC Atmospheric Modelling Summer School 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Extensive discussions followed presentation

Requests for repeat presentations
Year(s) Of Engagement Activity 2013
URL https://www.ncas.ac.uk/index.php/en/climate-modelling-summer-school
 
Description Seminar in Lisbon 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Led to further questions about atmospheric radiative transfer

Continued dialogue
Year(s) Of Engagement Activity 2013
 
Description The greenhouse effect and global warming: celebrating the work of John Tyndall - 150th Anniversary Celebrations 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact increased discussions with colleagues

requests for similar follow up activities
Year(s) Of Engagement Activity 2011