Estimation of Footprints of Observation Sites Using NIES and FLEXPART Atmospheric Transport Models

Abstract:

The Total Carbon Column Observing Network (TCCON) is a network of ground-based Fourier Transform Spectrometers (FTS) that record near-infrared (NIR) spectra of the sun. From these spectra, accurate and precise column-averaged abundances of atmospheric constituents including CO₂, CH₄, H₂O, and others are retrieved. Use of TCCON FTS observations is an appropriate way to validate satellite observations (SCIAMACHY, GOSAT, OCO-2). However, our knowledge about TCCON observations is limited. The retrievals from the ground based FTSs report column-averaged abundances, so the concentration distribution along the vertical profile as well as value and location of sources affecting the concentration are mainly unknown. Studying the footprint is especially important for sites located near regions with different emissions or sinks of GHG’s, e.g. for coastal stations, where marine air masses are mixed with continental ones.

In this work, we use the National Institute for Environmental Studies (NIES) Eulerian three-dimensional transport model (TM) and the FLEXPART Lagrangian plume diffusion model (LPDM) to determine footprints of operational and future TCCON FTS observational sites. The NIES model was previously used to study the seasonal and inter-annual variability of CO₂ and CH₄. In paper by Belikov at al. (ACP, 2013) we shows, that the NIES model is able to successfully reproduce the vertical profile of greenhouse gases (CO₂, CH₄, SF₆) and the seasonal and inter-annual variability of XCO₂ and XCH₄. Using this fact, NIES TM CO₂concentrations sampled at the level of 1 km above ground and at 13:00 of local time were used to initialize backward tracer simulation with FLEXPART model. The sampling time is selected to fit the overpass time of satellites measuring greenhouse gases and because the NIES TM is more successful at reproducing diurnal tracer variations.

Based on the FLEXPART back-trajectory simulations, we estimate footprints of operational and future TCCON FTS sites. These footprints are used to test different methods of co-location of XCO₂ from TCCON and the NIES model, and TCCON and GOSAT products.