The assimilation of satellite radiances is crucial for Numerical Weather Prediction (NWP) models with the benefit approximately evenly split between infrared and microwave data. The Infrared Atmospheric Sounding Interferometer (IASI) instruments, on board the MetOp-A, -B, and -C satellites, designed to provide accurate atmospheric temperature and humidity profiles, have the largest impact on the NWP skill. The Met Office currently uses a few hundred IASI channels in their operational data assimilation system, mainly in the CO2 absorption band around 667 cm-1 (15 μm), which benefits from low instrument radiometric noise to obtain substantial information on the vertical temperature structure. 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 (covering up to 25 % of the globe) are more difficult to detect. If cloud-contaminated radiances are misidentified as clear-sky and assimilated into NWP models, the forecasts can be significantly degraded. IASI instruments can also detect a multitude of atmospheric trace gases, thereby providing information on atmospheric chemistry, climate, and pollution.
IASI-NG will launch ~2021, with similar radiometric noise but twice the spectral resolution of IASI. To gain optimal benefit from IASI and IASI-NG in the NWP system, there is a requirement for highly accurate fast radiative transfer codes as forward model operators in the data assimilation system. These fast models are trained on line-by-line models, which use spectroscopic line parameters from atmospheric spectroscopic databases such as HITRAN to model the absorption of trace gases, including CO2. The default Voigt lineshape in HITRAN is inadequate in accurately representing real atmospheric spectra. As part of this project, new spectroscopic measurements of CO2 will be analysed to derive state-of-the-art Hartmann–Tran profile (HTp) line parameters. 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.
UK Bachelor Degree with at least 2:1 in a relevant subject or overseas equivalent.
Project Enquiries: Dr Jeremy Harrison, [email protected]
Funding Enquiries: [email protected]
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Havemann, S., Thelen, J.-C., Taylor, J.P., Harlow, R.C. (2018), The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC): A multipurpose code based on principal components. J. Quant. Spect. Rad. Trans., 220, 180-192. doi:10.1016/j.jqsrt.2018.09.008
Lavanant, L. , Fourrié, N. , Gambacorta, A. , Grieco, G. , Heilliette, S. , Hilton, F. I., Kim, M. , McNally, A. P., Nishihata, H. , Pavelin, E. G. and Rabier, F. (2011), Comparison of cloud products within IASI footprints for the assimilation of cloudy radiances. Q.J.R. Meteorol. Soc., 137: 1988-2003. doi:10.1002/qj.917