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

Lead Research Organisation: University of Reading
Department Name: Meteorology


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.

Planned Impact

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.


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Description The development of the new laboratory system is ongoing (and delayed by COVID-related laboratory closures), but while this is still in progress, we have exploited previously acquired data that was partially analysed as part of a NERC PhD studentship, and completed under ASPIC. These used atmospheric measurements to improve understanding of how solar radiation ("sunlight") is absorbed in the near-infrared part of the spectrum. It focused on parts of the spectrum that are relatively transparent to sunlight, but where there is known to be weak absorption by atmospheric water vapour. Current laboratory measurements disagree about aspects of this absorption (and resolving these differences is a primary goal of ASPIC) and essentially none have been made below room temperature; our atmospheric observations allowed us to further assess the laboratory observations. The results strongly indicate that the methods used to represent this absorption in many weather forecast and climate prediction models underestimate that absorption, with implications for the atmospheric energy budget. The work also has implications for satellite "remote sensing" methods that exploit these relatively transparent regions to observe the atmospheric and surface properties.

We have also collaborated with (i) German research institute (DLR, near Munich) to assist in the analysis of their new laboratory data which has led to improved understanding of the causes of water vapour continuum absorption in regions of the near-infrared with strong water vapour absorption. We provided data from previous laboratory experiments for comparison. The work provides additional evidence that of absorption by the so-called water dimer - i.e. two water molecules loosely bound together. (ii) the Institute of Atmospheric Optics in Tomsk in the processing of previously-unanalysed data from an earlier NERC grant, which has increased understanding of atmospheric absorption by water vapour in the near-visible infrared region of the spectrum (iii) University College London who are working on the atmospheres of exoplanets - our data was relevant to their analysis and their work has illustrated the importance of water vapour absorption on understanding exoplanet atmospheres.
Exploitation Route The work is of direct interest to weather forecast and climate prediction centres and staff from the Met Office and ECMWF are Project Partners of ASPIC and are kept informed of developments. Both institutes have been provided with an updated analysis of earlier measurements.
Sectors Aerospace, Defence and Marine,Environment,Other

Description FiWe have provided updated analysis of earlier laboratory measurements to the Met Office for incorporation into the radiative transfer code used in their Unified Model used for weather and climate prediction. This replaces an earlier analysis which we deemed to be updated based on more recent evidence. This is likely to be incorporated in the next version of the Unified Model. We have provided the same data, for the same reason, to the European Centre for Medium-range Weather Forecasts and also to a group at NASA Goddard Space Flight Center working on retrievals of aerosol properties from satellite data.
Sector Aerospace, Defence and Marine,Environment,Other
Impact Types Societal

Title Atmospheric observations of the water vapour continuum in the near-infrared windows - Data 
Description Dataset for our AMT paper (Atmospheric observations of the water vapour continuum in the near-infrared windows) 10.5194/amt-2335-2020 Included is the best estimate of the continuum optical depth (18 September 2008). the ratio of this best estimate to the corresponding MT_CKD 3.2 optical depth from 18 September 2008, and the CAVIAR-field estimated continuum absorption coefficients using both MT_CKD and CAVIAR-lab. Other data are available from the corresponding author upon request. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Impact None to record as yet 
Description Provision of data on infrared absorption to the European Centre for Medium-Range Weather Forecasts (2020 - Still Active) 
Organisation European Centre for Medium Range Weather Forecasting ECMWF
Country United Kingdom 
Sector Public 
PI Contribution provision of data to test/improve radiative transfer code used in the ECMWF weather forecast model
Collaborator Contribution Collation of previously derived data
Impact Continuing dialogues with colleagues at ECMWF on their needs
Start Year 2021
Description Provision of data on infrared absorption to the Met Office to enhance their Unified (Climate and Weather Prediction) Model 
Organisation Meteorological Office UK
Country United Kingdom 
Sector Academic/University 
PI Contribution We provided an updated analysis of our laboratory-based measurements of the near-infrared continuum, in a form suitable for inclusion in upcoming versions of the radiative transfer code used in Met Office climate and numerical prediction models.
Collaborator Contribution The Met Office ingested our data into their radiation model and provided us with updated "spectral files" which for use in our version of their model - this is actively used in, for example, NERC-funded PhD work at Reading
Impact No outputs yet
Start Year 2020
Description Organisation of Online International Workshop on the Water Vapour Continuum 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact International workshop involving scientists currently active in the spectroscopy of water vapour continuum, which is a controversial area. The workshop was a vehicle for presenting most recent results and also for clarifying areas where there was broad agreement, areas of continued disagreement and possible future activities. Participants came from the UK, USA, Cameroon, France, Germany, Russia and Denmark.
Year(s) Of Engagement Activity 2021