- Improve the spectroscopic representation of atmospheric carbon dioxide absorption in radiative transfer models.
- Contribute to improvements in numerical weather prediction.
- Acquire a set of skills to create and analyse remote-sensing datasets.
The Infrared Atmospheric Sounding Interferometer (IASI) instruments, on board the MetOp-A and MetOp-B satellites (a third on MetOp-C should be launched in November 2018), are primarily meteorological instruments designed to provide accurate atmospheric temperature and humidity profiles with which to improve numerical weather prediction (NWP). These instruments are able to detect trace gases in the atmosphere using their distinctive spectral infrared fingerprints, thereby providing information on atmospheric chemistry, climate, and pollution.
Spectral bands of carbon dioxide (CO2) are used to retrieve atmospheric temperature profiles from IASI observations. The channels primarily used are those with minimal sensitivity to other absorbing species such as water. Most commonly the absorption bands of CO2 used are ~667 cm-1 (15 μm) and ~2350 cm-1 (4.3 μm), although the latter band in IASI spectra has higher radiometric noise. The 15 μm band is also used to determine cloud top pressure and cloud fraction, using for example the CO2-slicing method. The detection of clouds is a large source of uncertainty in infrared satellite data assimilation in NWP. Unlike optically thick clouds, optically thin clouds such as cirrus (which cover up to 25 % of the globe) are more difficult to detect. If cloud-contaminated radiances are treated as clear-sky measurements and then assimilated into NWP models, the forecasts can be significantly degraded.
The atmospheric radiative transfer models used to analyse the IASI radiances use spectroscopic line parameters from atmospheric spectroscopic databases such as HITRAN to model the absorption of trace gases, including CO2. The Voigt lineshape, the default lineshape in HITRAN, is inadequate in accurately respresenting real atmospheric spectra. New lineshape models have been proposed, for example the Hartmann–Tran profile, which accounts for effects such as Dicke narrowing and speed-dependence; the effects of collisional interferences between lines (i.e. line-mixing) can be accounted for using the Rosenkranz first-order approximation.
New spectroscopic measurements of CO2 have recently been made, from which new non-Voigt line parameters will be derived as part of this project. These will then be used in radiative transfer calculations and retrievals of temperature and cloud properties, and improvements in these retrieved quantities will be investigated.
The student will analyse laboratory infrared spectroscopic measurements of carbon dioxide and generate non-Voigt spectroscopic line parameters. These data will be utilised in several atmospheric radiative transfer codes and retrieval schemes for temperature and cloud properties: (1) the Reference Forward Model (RFM), a traditional line-by-line radiative transfer model, with the University of Leicester IASI Retrieval Scheme (ULIRS); and (2) the Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC), a fast code based on principal components, and the associated 1DVar retrieval scheme. Using these two schemes, improvements in retrieved temperature profiles and cloud information will be ascertained for (a) the new non-Voigt parameters relative to the previous HITRAN Voigt parameters for the same IASI CO2 channels as currently used in Met Office IASI assimilations; and (b) the use of additional CO2 channels in retrievals when using the new non-Voigt parameters.
Training and Skills
This studentship provides an exciting multidisciplinary opportunity to work with cutting-edge satellite observations and atmospheric radiative transfer techniques in a challenging area of atmospheric science. This project covers a range of topics: atmospheric spectroscopy; remote sensing; the interpretation of retrieved quantities; data visualisation & analysis.
The student will be affiliated to the National Centre for Earth Observation (NCEO), through which they will obtain additional training opportunities and interaction with scientists working in Earth observation. The student will also have the opportunity for short research placements at the Met Office, where they will be exposed to an “operational” environment.
Year 1: Determine new infrared spectroscopic line parameters for carbon dioxide.
Year 2: Incorporate the new line parameters into IASI retrievals of temperature and cloud properties.
Year 3: Detailed comparisons in retrieved temperature profiles and cloud information, and optimisation of CO2 channels used.
Partners and collaboration (including CASE)
This project has been co-developed with the UK Met Office, one of CENTA’s Level-2 end-user partners. The studentship will be jointly supervised by scientists from the National Centre for Earth Observation (based at the University of Leicester) and the Met Office.
The student will be expected to work closely with the Met Office (including extended visits) to learn and develop computer code.
This studentship may be a CASE award with the Met Office.
The NCEO (www.nceo.ac.uk) is a distributed NERC centre providing the UK with national capability in EO science.
Dr Jeremy Harrison is the NCEO’s spectroscopy leader and capability leader in atmospheric radiative transfer. Based in the Earth Observation Science (EOS) group at the University of Leicester, his expertise lies in atmospheric spectroscopy and the remote sensing of trace gases.
Dr Stephan Havemann is an observation based research scientist at the Met Office, specialising in the development of fast radiative transfer and variational retrieval codes.