Improved humidity treatment and trajectory–grid mapping in the EMAC contrail submodel and their implications for contrail climate change functions

Contrails are a major contributor to aviation’s non-CO₂ climate effects, and trajectory-based mitigation concepts depend on robust estimates of contrail formation, persistence, optical properties, and radiative forcing. In EMAC, contrails evolve along Lagrangian trajectories and are subsequently transformed back to grid-point space to derive contrail climate change functions (CCFs; Frömming et al., 2021). However, the baseline EMAC contrail scheme has previously shown very low optical thickness values, which can be linked to low humidity values and to artefacts introduced by sampling and mapping between the Lagrangian and Eulerian frameworks.

We present targeted developments of the EMAC contrail submodel that address (i) humidity and saturation consistency and (ii) trajectory–grid coupling. First, we revise the humidity formulation by introducing an H₂O compensation factor and an explicit humidity threshold for contrail processes, implemented with a consistent treatment of saturation specific humidity and timestep handling. Second, we extend the grid-point-to-Lagrangian mapping by adding four-point bilinear horizontal interpolation of meteorological variables at the exact Lagrangian positions, reducing step-like gradients when trajectories cross grid boxes. Third, we update the Lagrangian-to-grid transformation to mitigate mapping artefacts affecting contrail ice water content and optical properties. In addition to the previously investigated North Atlantic flight region, we also analyse contrail climate change function fields for multiple days over Asia.

In the shown test cases, the added humidity threshold systematically reduces contrail persistence: the baseline setup (no threshold) yields a characteristic lifetime of ~7.8 h, while threshold-based setups reduce lifetimes to ~4.0–4.2 h and down to ~2.9–3.5 h depending on threshold strength. These developments improve numerical consistency and reduce sampling/mapping artefacts, providing a more robust basis for EMAC-based contrail RF estimates and for constructing contrail climate change functions for aviation applications.

The project leading to this study was funded by the European SESAR programme under Grant Agreement No. 101114785 (CONCERTO). High performance supercomputing resources were used from the DKRZ Cluster in Hamburg.

References:  

[1] Matthes, S., Lührs, B., Dahlmann, K., Grewe, V., Linke, F., Yin, F., Klingaman, E. and Shine, K. P.: Climate-Optimized Trajectories and Robust Mitigation Potential: Flying ATM4E, Aerospace 7(11), 156, 2020.

[2] Frömming, C., Grewe, V., Brinkop, S., Jöckel, P., Haslerud, A. S., Rosanka, S., Van Manen, J., and Matthes, S.: Influence of weather situation on non-CO₂ aviation climate effects: The REACT4C climate change functions, ACP, 21, 9151 – 9172, 2021.