Water Vapour Isotopologues Measurements From FORUM Observation Using KLIMA Retrieval Code

The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, selected for the ESA's 9th Earth Explorer (EE9) mission, will be launched in 2026. For the first time, spectrally resolved radiance observations covering the Far InfraRed (FIR) band from 100 to 667 cm ?¹ with global coverage and with high spectral resolution and radiometric accuracy will be available. This spectral region is highly sensitive to upper-tropospheric-lower-stratospheric (UTLS) water vapor concentration. One of the tools used to support the FORUM proposal during the EE9 competition, and previously, for the data analysis of the REFIR-PAD (Radiation Explorer in the Far InfraRed - Prototype for Applications and Development) balloon observations, is KLIMA (Kyoto protocoL Informed Management of the Adaptation) retrieval code. KLIMA analysed the synthetic FORUM observations in clear sky providing simultaneously the vertical profile of the minor atmospheric gases (such as the water vapour), atmospheric temperature profile, surface temperature, and surface emissivity. During the EE9 phase A selection, KLIMA was employed in various ESA studies such as: FORUM-req for the assessment of the FORUM mission requirements definition; FORUM-E2E for the validation of the End-to-End Simulator (E2ES); FIRMOS for the analysis of the ground-based observations of the FORUM prototype FIRMOS (Far-Infrared Radiation Mobile Observation System). Recently, the KLIMA code was used in the context of the ASI FORUM-science project for the generation of gases optical thicknesses used by the fast code ?-FORUM. In this framework, KLIMA has been upgraded to retrieve also the concentration of the isotopologues of the atmospheric gases. In this work, we will show the assessment of the performance of the retrieval of water vapour isotopologues using the spectral coverage and radiometric accuracy of the FORUM observations, and the comparison with the results obtained when only the middle-infrared information is used. Results both on single measurement and averaged measurements will be shown to evaluate the effect of the radiometric accuracy of the FORUM observations.