Retrievals of water vapour and temperature exploiting the far-infrared: application to aircraft observations in preparation for the FORUM mission

Abstract. We present the extension of the RAL Infrared Microwave Sounding optimal estimation retrieval scheme to include the use of far-infrared channels in preparation for the upcoming Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission. The evolution and evaluation of the extended scheme is performed in two steps. First, clear-sky retrievals of temperature and water vapour are performed on IASI and FORUM simulations. Improvements of 2 % and 0.2 K are observed in the median mid to upper tropospheric retrievals of water vapour and temperature, respectively, using the FORUM configuration, with an increase of ~ 1 degree of freedom for water vapour and temperature. Secondly, radiances observed from an aircraft flight in the upper troposphere are modified to match the FORUM spectral characteristics. Retrievals from these radiances using the modified code show a strong agreement with contemporaneous in-situ measurements of the atmospheric state, reducing the RMSE by 18 % for water vapour from the a-priori, giving confidence in its performance. This tool is now readily available for use on FORUM observations and can be easily adapted to other far and mid-infrared instrument configurations.