Inversion of CO2 sources and sinks using satellite data: application to TOVS

The current observation surface network for CO2 poorly informs about space and time variations of CO2 fluxes over land at a global scale. Additional information is sought to compensate this lack of knowledge: the surface observations gets denser and efforts are being made to measure atmospheric CO2 from space. The HIRS instrument has been operated since 1979 on-board the polar obiting NOAA satellites. Eventhough it was designed for meteorological purposes, CO2 variations in the upper troposphere noticeably impact the radiances that it measures. CO2 retrievals in the tropics (20N-20S) have been produced only recently and first validations have shown that such data agree with aircraft observations reasonably well. Properly handling them to constrain the inversion of CO2 sources and sinks is an important challenge given the length of the instrument record. In this paper we describe how CO2 retrievals from HIRS can contribute to the monitoring of the CO2 surface fluxes. A variational inverse scheme has been designed, that uses the LMDZT transport model. Error characteristics of the CO2 retrievals are estimated using both model computations and in situ observations. Results are shown and prospects for other CO2-sensitive satellite instruments, like AIRS, SCIAMACHY and the CO2-dedicated OCO instrument are discussed.