The use of variable CO2 in the data assimilation of AIRS and IASI radiances

An important component of the assimilation of radiance observations from the AIRS and IASI satellite instruments is the radiative transfer modelling. Currently, the RTTOV model used in the ECMWF IFS system uses a fixed value for CO2. Neglecting the spatio-temporal variability of CO2 introduces an error in the simulation of the satellite radiances, which could affect the quality of the analyses and forecasts. The current assumption is that variational bias correction corrects most of this error and therefore minimizes the impact on the forecast scores. This paper investigates the possibility of modelling CO2 within the IFS to improve the radiative transfer modelling. Results show that the required bias correction is significantly reduced when using more realistic CO2 values. The impact on the analysis quality and forecast scores is mostly neutral with some indication of improvement in the Tropics and the stratosphere.