Exploring the potential of MTG-IRS to diagnose convective instability for nowcasting and situational awareness.

Profiles of atmospheric temperature and humidity through a vertical column are fundamental to the assessment of convective instability. Quantities derived from such profiles, such as CAPE, CIN and various instability indices, are well-established as operational nowcasting and situational awareness products. Whilst radiosonde ascents provide the most vertically detailed measurements of upper air quantities, satellite observations are far more numerous, and geostationary orbits in particular combine the advantages of spatial coverage and temporal continuity. The MTG-IRS instrument, due to be launched aboard MTG-S in July 2025, will be the first hyperspectral sounding instrument in a geostationary orbit over Europe. A EUMETSAT Fellowship currently being hosted by the UK Met Office is exploring the potential for MTG-IRS’s sounding capability to improve the detection and characterisation of regions of convective instability. The context is operational nowcasting, where timely observations can be used by forecasters to assess the accuracy of NWP model fields before those same observations are able to influence the forecast via the next cycle of the data assimilation system.Instability indices currently generated at the UK Met Office are derived from vertical profiles of temperature and humidity retrieved from SEVIRI radiances and an NWP-derived prior estimate, using an optimal estimation algorithm. Initial work on the Fellowship project has used hyperspectral IASI data as a proxy for MTG-IRS, in place of SEVIRI, with morning MetOp orbits over Europe coinciding with the pre-convective phase of thunderstorm development. The behaviour of the optimal estimation scheme and the sensitivity of the retrievals to various aspects of the scheme, such as channel selection, relative weighting of background and observations, and surface treatment have been explored. The vertical structures of the retrieved temperature and humidity profiles are also useful when comparing the behaviour of different instability indices, and suggest adjustments to the index calculation that might better suit typical local pre-convective environments.Some aspects of validation will also be discussed. Whilst case studies are invaluable in understanding pre-convective situations and evaluating retrievals and indices, routine objective verification is also important when developing a product for operational use.