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Inverse Model

Overview

INVICAT is the inverse version of the TOMCAT model. It uses data assimilation (DA) methods to provide optimised gridded distributions of the sources and sinks of a range of important atmospheric trace gases. In particular, INVICAT has been used to quantify surface fluxes of important greenhouse gases and to investigate the carbon cycle and global and regional methane emissions.

In inverse modelling, the transport model (here, TOMCAT) links the surface fluxes of a species to locations where its atmospheric abundances have been observed via in situ or remote methods. Based on these flux/observation relationships, the inverse model optimally adapts the gridded surface fluxes to best match the observations. INVICAT uses four-dimensional variational (4D-Var) methods to provide high-resolution global gridded flux rates for important atmospheric species including methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), nitrous oxide (N2O) and a range of other trace gases.

Flow chart showing the methodology used by INVICAT

Flow chart showing the process followed by INVICAT to produce optimised surface fluxes.The cost function is iteratively minimised until convergence to a solution.

The first versions of INVICAT and ATOMCAT, TOMCAT’s adjoint, were developed by Chris Wilson at Leeds in 2014 (Wilson et al., 2014). Now up to version 5.0, INVICAT has assimilated data from the NOAA and AGAGE surface measurement networks and a range of satellite instruments including GOSAT, IASI, TROPOMI and MOPITT. Chris and others continue to develop INVICAT for use with a range of atmospheric species.

INVICAT has been used to show that CH4 emissions are increasing in the Amazon Basin of South America (Wilson et al., 2021); that the Amazon is likely becoming an overall source of carbon to the atmosphere due to fires (Basso et al., 2023); and that hydraulic fracturing ('fracking') in the U.S. is likely increasing leaks of ethane and methane to the atmosphere (Monks et al., 2018).

Main Model Details

  • 4D-Var inverse atmospheric model for optimised time-varying gridded surface fluxes and 4D atmospheric mixing ratios.
  • ATOMCAT – adjoint version of TOMCAT – can be run independently of INVICAT to provide backwards sensitivities of observation locations.
  • Variable spatial resolution. Standard resolution is 2.8º x 2.8º but can provide output at 5.6º x 5.6º or up to 1º x 1º.
  • Variable time resolution available for derived surface fluxes; from daily up to monthly mean emissions.
  • Uncertainties on surface fluxes can be derived by the model.
  • 60 vertical levels, from the surface up to 0.1 hPa.
  • Forced by ECMWF meteorological analyses.
  • Offline tropospheric chemistry schemes for a range of species.
  • Written in Fortran (f77, f90). Runs on most platforms. Parallelised using MPI.