Alternative fuel effects on the lifecycle of individual contrails from formation to dissipation

Lottermoser, Annemarie (2026): Alternative fuel effects on the lifecycle of individual contrails from formation to dissipation. Dissertation, LMU München: Fakultät für Physik

Abstract

Contrails and their evolution into contrail-cirrus clouds constitute a significant contribution to aviation's climate impact, as these anthropogenic ice clouds modify the atmospheric radiation balance. Present research investigates alternative propulsion systems, such as liquid hydrogen combustion, as potential strategies to mitigate the climate impact of contrails. These technologies can alter the physical processes involved in contrail formation and evolution. Therefore, a deep understanding of those processes is essential for assessing the potential of alternative propulsion technologies to reduce the climate impact of aviation. Recent research focuses on extending and adapting models simulating the contrail lifecycle to account for new physical processes, as well as planning and executing flight campaigns to measure emissions and contrails behind aircraft using non-conventional fuels. This thesis presents a comprehensive model investigation of the contrail lifecycle, spanning from the initial formation to the transition of line-shaped contrails into aged contrail cirrus. The aim is to advance the understanding of how contrail properties respond to changes in the propulsion system, specifically regarding hydrogen combustion or hydrogen fuel cell systems. The early formation phase is crucial for predicting the final number of nucleating ice crystals, which is one of the most important parameters of young contrails, determining the contrail's radiative impact. The Lagrangian Cloud Module (LCM) in a box model approach simulates contrail ice crystal formation; however, it lacks the accurate representation of plume dilution during the first seconds of plume evolution. With the shift to alternative fuels and the resulting changes in contrail formation processes, the existing model requires re-evaluation. Therefore, this thesis presents the development of the radial model (RadMod) that is used to simulate the early stage of the dynamical jet evolution. RadMod models the turbulent mixing of hot and moist engine exhaust with ambient air by solving the two-dimensional advection-diffusion equations for momentum, temperature, and water vapor. The model's validity is demonstrated through comparisons with theoretical and observational data across a range of conditions, including exhaust temperature, jet velocity, and aircraft velocity. Furthermore, RadMod is applied to simulate plume dilution; a scaling relation proposed in the literature is validated, and the simulated dilution is compared to measurements of the exhaust plume behind a fuel cell emulator. This work establishes the foundation for the intermediate-complexity model RadMod-LCM, which will be the subject of future research. Due to its lower computational cost compared to fully three-dimensional large-eddy simulations (LES), RadMod-LCM will enable extensive parameter studies across a wide range of atmospheric and engine conditions. In particular, this includes the modeling of aircraft configurations typical for alternative propulsion systems, e.g., smaller aircraft with lower flight and jet velocity. The formation phase is followed by the vortex phase, which covers the first few minutes of the contrail's lifecycle. Previous studies investigating the contrail's evolution during the vortex phase focused on conventional aircraft propulsion systems operating under typical cruise altitude conditions. To investigate the vortex phase of individual contrails behind aircraft powered by alternative propulsion systems, the LES model EULAG coupled to LCM is employed. The input parameters are adjusted to be representative of alternative fuels or propulsion systems. These adjustments account for increased water vapor emissions, variations in the initial number of ice particles, and an extension of the atmospheric parameter space toward higher ambient temperatures. It is shown that the processes during the vortex phase, in particular the partial sublimation of ice particles in the descending wake vortex system, reduce the initial differences in the number of ice crystals. Ice crystal loss is found to increase with higher ambient temperatures and lower humidity values. A new parameterization for ice crystal loss in hydrogen contrails is provided that is incorporated into larger-scale climate models by other research groups. Building on this work, the subsequent transition of contrails into contrail-cirrus clouds during the dispersion phase is examined using EULAG-LCM. It is assessed how a switch to alternative propulsion systems affects the lifecycle and radiative properties of contrail cirrus. Results indicate that reducing the number of initially formed ice crystals leads to fewer but larger particles, which accelerates sedimentation and shortens contrail lifetime, highlighting the potential of emission-based mitigation strategies. In contrast, the emitted water vapor mass has a comparatively minor effect on contrail-cirrus properties relative to the initial ice crystal number, ambient temperature, and aircraft type. When averaged over all meteorological conditions, a reduction of the initial ice crystal number by a factor of 100 results in a 20-fold decrease in the lifetime-integrated total extinction, which serves as a proxy for the contrail’s radiative effect. The relationship between initial ice crystal number and radiative impact is shown to be nonlinear.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: contrail lifecycle, hydrogen propulsion, climate impact, RANS modelling, high-resolution simulations
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 24. Februar 2026
• 1. Bericht­erstatter:in: Rapp, Markus