Feasibility Study of HDO Retrieval from FORUM-EE9 Measurements

The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, in preparation for the launch as ESA's 9th Earth Explorer (EE9), will be launched in 2027. For the first time, spectrally resolved radiance observations covering the Far InfraRed (FIR) band from 100 to 667 cm ?¹, as well as the Middle InfraRed (MIR) from 667 to 1600 cm ?¹, with global coverage, high spectral resolution, and radiometric accuracy will be available. FORUM spectral region covers two emission bands of the water vapour isotopologue HDO (roughly 100-600 cm ?¹ and 1000-1600 cm ?¹) which can be used for the retrieval of its vertical profile. A deep knowledge of HDO is necessary to assess and improve the representation of water vapour-related processes. In particular, allowing to couple atmospheric circulation and precipitation components in climate models, knowledge of water vapor isotopologues can significantly improve numerical weather predictions. Indeed, it was proven [1] that the assimilation of water vapour isotopologues improves wind, humidity, and temperature fields predictions in the middle-tropophere by more than 10%. We carried out a feasibility study to evaluate the capability of FORUM measurements to provide information on H?¹?O and HDO profiles. Investigations have been also targeting the ?D parameter, which represents the fractional deviation (?) of the HDO/ H?¹?O ratio from the standard reference ratio. Since FORUM will fly in loose formation with IASI-NG, the improvements achieved when matching measurements of the two sensors are synergistically exploited have also been assessed. For this study, both synthetic observations, and retrievals were performed using the KLIMA code developed at IFAC-CNR in Florence (Italy). The study involves several atmospheric scenarios and exploits both individual and averaged observations. The quality of the retrieval is evaluated in terms of degrees of freedom, precision, and accuracy. Finally, we applied our analysis system to FORUM-like measurements such as those acquired by REFIR-PAD (Radiation Explorer in the Far InfraRed-Prototype for Applications and Development) during a balloon campaign from Teresina (Brasil, June 2005), and those acquired from ground-based stations by FIRMOS (Far-Infrared Radiation Mobile Observation System) during the Zugspitze campaign (2019). [1] Kinya Toride, Kei Yoshimura, Masataka Tada, Christopher Diekmann, Benjamin Ertl, Farahnaz Khosrawi, and Matthias Schneider, Potential of Mid-tropospheric Water Vapor Isotopes to Improve Large-Scale Circulation and Weather Predictability, Geophysical Research Letter, doi 10.1029/2020GL091698, 2021