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

Lead Research Organisation: Imperial College London
Department Name: Dept of Physics

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.
 
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,Energy,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

 
Description Physics talk at Engineering Headstart course 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Schools
Results and Impact talk generated questions and discussions

I have seen applications to Physics from some of the school pupils attending the Engineering Headstart courses
Year(s) Of Engagement Activity 2014,2015,2016
 
Description Women in physics talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact Presentations and special event to encourage more women to study physics at university
Year(s) Of Engagement Activity 2014,2015,2016,2017