CAVIAR

The project CAVIAR (Continuum Absorption at Visible and Infrared wavelengths and its Atmospheric Relevance) addresses the component of the absorption of electromagnetic radiation by water vapour which varies relatively slowly with wavelength, known as the water vapour continuum.

The continuum has a significant impact on fluxes of infrared radiation and atmospheric cooling rates, with consequences for understanding the effect of water vapour on both the present-day climate, in climate change, and in remote sensing of the atmosphere.

Currently most widely used representations of the continuum CKD or MT_CKD model have been tested so far over only a narrow range of wavelengths (far-infrared) and atmospheric conditions. Advances in understanding of the spectroscopy of thewater molecule, and in instrumentation capabilities, means that it is timely to invest significant effort in placing our understanding and characterisation of the continuum on a much firmer basis.

Consortium objectives

• To use state-of-the-art theory, and co-ordinated and intercalibrated laboratory and field measurements, to quantify the strength of the water vapour continuum in a consistent manner across the wavelength range 0.5-125 microns, to characterise its dependence on temperature and water abundance, and to ascribe a cause or causes to this absorption;
• To use state-of-the-art radiative transfer schemes to quantify the impact of this water vapour continuum on our understanding of the present day Earth radiation budget and climate change.

Research programme 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 fourier transform spectrometer enabling the measurement of the continuum over an unprecedentedly broad range of wavelengths and conditions; an alternative technique, cavity ring down spectroscopy, capable of measuring relatively weak absorption at very high precision will bedeployed 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. There will be 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.