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

Lead Research Organisation: University College London
Department Name: Physics and Astronomy


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.


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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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Tennyson J (2012) An adiabatic model for calculating overtone spectra of dimers such as (H(2)O)(2). in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences

Description Work at UCL was part of a larger consortium led from Reading. At UCL we:

1. Provided detailed and appropriate line lists for the water monomer. These were able to explain all the observed long pathlength, shorter wavelength observations of experiments designed to detect the water dimer.

2. Developed a formalism and code for performing calculations of water dimer absorptions in the near-infrared.

3. Looked in detail at monomer absorption intensities in the near infrared to aid retrieval missions as the database values appeared to systematically too low.
Exploitation Route Data has been supplied for inclusion in standard atmospheric databases (BADC, HITRAN, GEISA)
Sectors Environment

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