Optimising water vapour retrievals by exploiting sensitivity within the far-infrared: A study in support of the ESA FORUM mission.

<p>Theoretical models suggest that &#8764;55% of the outgoing longwave radiation from Earth is within the far-infrared region, 100-666 cm^&#8722;1. Nevertheless, the top-of-atmosphere radiation spectrum in this region has never been measured, something that will change with ESA&#8217;s Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, launching in 2027. Studies have indicated that absorption within this region is dominated by tropospheric water vapour, significantly impacting Earth&#8217;s radiation budget. Quantifying these concentrations plays a vital role in estimating its radiative effects and associated feedbacks.</p> <p>The Infrared and Microwave Sounding (IMS) retrieval scheme developed at RAL Space is an optimal estimation scheme currently using channels within the mid- and near-infrared as well as the microwave region to obtain simultaneous retrievals of the vertical atmospheric profile and cloud properties. Current retrievals using this scheme have been performed on observations from Infrared Atmospheric Sounding Interferometer (IASI), Microwave Humidity Sounder (MHS) and the Advanced Microwave Sounding Unit (AMSU) onboard the MetOp satellites. Temperature and water vapour retrievals using this framework have been validated against radiosonde data with biases within 1 K and 10% of the reference respectively.</p> <p>This work seeks to extend IMS into the far-infrared and exploit the known sensitivity of upwelling radiation within this region to improve current retrievals of water vapour. This would enhance our understanding of the spatial and temporal variations of water vapour within the atmosphere, and its role in Earth's radiation budget. Unique clear-sky airborne measurements will be used to analyse channel sensitivity within this region and maximise the information content for the retrieval. This retrieval capability would be the first of its kind to be thoroughly validated in this way and would be available for use on FORUM observations.</p>