UK RESEARCH AND INNOVATION (GB)
Global distributions of the methane (CH4) column average and carbon monoxide (CO) total column are observable by satellite shortwave infrared (SWIR) spectrometers through detection of surface-reflected solar radiation. Observations by ENVISAT SCIAMACHY and GOSAT-TANSO have been exploited extensively to investigate biogenic, pyrogenic and anthropogenic sources and, in the case of methane, to quantify emissions through inverse modelling. ESA’s S5P offers a major advance on these preceding satellite SWIR spectrometers for identification and quantification of sources on finer scales by providing the first contiguous, daily global coverage at high spatial resolution (7 x 7 km). However, for inverse modelling of emission sources, height-resolved information would offer a major innovation on column information; particularly resolution of the lower tropospheric layer. This Fellowship proposes to develop and apply a scheme to achieve this by combining SWIR and TIR information on CH4 and CO.
RAL has developed a state-of-the-art scheme to retrieve global height-resolved methane distributions from thermal infrared (TIR) measurements in the Infrared Atmospheric Sounding Interferometer (IASI) 7.9 µm band (Siddans et al., 2017). While providing information on two independent vertical layers in the troposphere, sensitivity in this band decreases towards the ground, due to decreasing thermal contrast between the atmosphere and surface. A combined retrieval scheme exploiting in addition the high signal-to-noise information from S5P (SWIR/column) with that from IASI or CrIS (TIR/height-resolved) would enable lower tropospheric distributions of methane and CO to be resolved. Lower tropospheric concentrations are more closely-related to emission sources than are column measurements and inverse modelling of surface fluxes should be less sensitive to errors in representation of transport at higher altitudes; a limiting factor for current schemes.
As baseline, ESA S5P Level 2 (L2) products will be combined with retrievals from RAL’s IASI scheme; either as additional prior information for the IASI retrieval (L2-L1) or by combining retrieved L2 products (L2-L2). The IASI TIR scheme will then be modified and applied to CrIS (Suomi-NPP or NOAA-20), whose observations are separated by ~5 minutes from S5P compared to ~4 hours for IASI. Test data sets will be compared with analyses, models and surface measurements. Possibilities to improve on: (a) ESA’s S5P products, (b) the TIR scheme or (c) the SWIR-TIR scheme will then be assessed. Finally, the best performing scheme will be run to produce a fully-sampled 1-year CH4 and CO height-resolved dataset which will be made accessible to the science community.