Advanced Spectroscopy for improved characterisation of the near-Infrared water vapour Continuum (ASPIC)

Climate change driven by, for example, changes in concentrations of carbon dioxide, leads to further changes (or feedbacks) in the concentration of water vapour in the atmosphere. Water vapour is a strong absorber of infrared energy and the additional water vapour leads to a 50-100% increase in the warming that would otherwise occur. It also leads to significant changes in the partitioning of energy absorbed by the atmosphere and surface. This is known to exert a global-level influence on the change in precipitation following climate change.

There are major knowledge gaps which inhibit a robust understanding of the global precipitation response to climate change. ASPIC will tackle some of these gaps via a wide-ranging study focusing on one important, but uncertain, component of absorption of infrared radiation by water vapour, known as the "continuum". Under ASPIC, latest technological developments will be exploited to develop a new laboratory facility for measuring the water vapour continuum. This new facility will then be used to make a novel set of measurements. These measurements will then feed into detailed state-of-the-art calculations, which will examine the implications arising from the laboratory measurements for our understanding of climate and climate change; these new detailed calculations will also incorporate other recent developments in the area, facilitating a rounded assessment.

The rather few existing laboratory measurements of water vapour continuum absorption in the so-called near-infrared wavelength region show marked disagreements, and a satisfactory explanation of these differences has proved elusive. Further, there are large uncertainties in how this absorption varies with temperature, and none of the existing measurements extend below "room temperature" to the temperatures widely experienced in the atmosphere. Following a successful pilot project, novel "super-continuum" lasers will, together with other advances, be exploited to make measurements of high-accuracy across the near-infrared wavelength region and across a temperature range more relevant to the atmosphere than earlier measurements. Two complementary spectroscopic techniques will be used to allow cross-checking of the results.

These results will then be incorporated into detailed radiative transfer calculations that will be used to significantly improve understanding of how absorption of solar radiation in the atmosphere will change as the climate warms, and water vapour concentrations increase. Existing calculations, which have used less-detailed methods, indicate strong uncertainties, with consequences for predictions of how rainfall will change in a warming climate.

The proposed measurements are inherently risky, given the difficulty of measuring the relatively weak infrared absorption under controlled laboratory conditions and the need to develop an advanced new measurement system which goes beyond the current status quo. However, ASPIC builds on an STFC-funded pilot project involving the Principal and Co-Investigator that was completed in 2015: "Measuring weak water vapour absorption using a super-continuum source" was important in clearly demonstrating the feasibility of using super-continuum lasers in such work, and therefore in mitigating the overall risk in ASPIC. The depth of experience at RAL Space (where the spectrometers will be developed and the measurements made) in developing novel spectroscopic techniques also provides an important resource for troubleshooting any difficulties that arise.

Despite this risk, the gain from doing so successfully would be significant not only from a climate perspective, but also for measurement techniques that use this same wavelength region to monitor aerosols, clouds and the Earth's surface. They would also contribute to understanding the fundamental spectroscopic properties of water vapour.

The main beneficiaries of the science proposed by ASPIC are;

(i) Operational weather forecasting organisations (such as the Met Office and the European Centre for Medium-range Weather Forecasts (ECMWF) - both of whom are project partners on ASPIC) and major climate modelling institutions such as the Met Office Hadley Centre. A core component of the models developed and used by these organisations are the radiative transfer codes which rely on robust understanding of gaseous absorption in the atmosphere.

(ii) Downstream of these organisations are many diverse users who use both weather forecasts and climate change prediction in short and long-term planning. The observations made within ASPIC have the potential to impact on: temperature biases in weather forecast models (see e.g. the support letter from ECMWF); the strength of the calculated water vapour feedbacks (which impacts the overall climate change); and hydrological feedbacks (which impacts on the changes in precipitation resulting from climate change). The results could also impact on predictions of the availability of solar energy resource, in the case of technology which exploits the near-infrared.

(iii) Operational space agencies, when their instruments use the near-infrared windows for remote sensing of the properties of the surface, aerosols and clouds, as the water vapour continuum is a cause of interference in these observations. There is a paucity of observations within these windows, especially in conditions typical of the Earth's atmosphere. The investment in the Hi-Res Spectroscopy Facility at the Rutherford Appleton Laboratory will mean that it is on a par with its EU and international equivalents and would contribute to UK competitiveness when bidding to ESA programs on spectroscopic measurements.

(iv) Industries developing and/or using optical spectroscopy as an analysis tool in environmental, geological and medical applications.