INVICAT is the inverse modelling framework based on the TOMCAT 3-D global chemical transport model, developed and maintained at the University of Leeds as part of the National Centre for Earth Observation (NCEO). It uses four-dimensional variational data assimilation (4D-Var) to optimise gridded distributions of sources and sinks of atmospheric trace gases from atmospheric observations. INVICAT incorporates ATOMCAT, the adjoint version of TOMCAT, to efficiently calculate sensitivities required for the optimisation. INVICAT can assimilate observations from surface monitoring networks, aircraft campaigns and satellite instruments.

INVICAT has been applied to greenhouse gases, halocarbons and other trace species, including carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), ethane (C₂H₆), carbonyl sulfide (OCS), sulfur hexafluoride (SF₆) and various chlorofluorocarbons (CFCs). The modular framework can readily be extended to additional species. Fluxes can currently be optimised at horizontal resolutions of up to 1.125° × 1.125°, and posterior uncertainty estimates can optionally be calculated.