Modeling the Impact of Alternative Fuels and Hydrogen Propulsion on Contrail‐Cirrus: A Parameter Study

Abstract Line‐shaped contrails formed behind aircraft can evolve into broadly spread and long‐living cirrus clouds under favorable conditions. These contrail‐cirrus contribute significantly to the aviation‐induced radiative forcing. While past modeling studies have examined contrail‐cirrus across various atmospheric and aircraft‐type dependent parameters, they have focused on conventional kerosene combustion. In this study, we investigate how the switch to an alternative propulsion system, such as hydrogen combustion, may alter contrail‐cirrus properties using the large‐eddy simulation (LES) model EULAG coupled with the Lagrangian Cloud Module (LCM), a particle‐based microphysics module. Building on prior work that modeled hydrogen contrails during the vortex phase, we use those results to initialize the subsequent contrail‐cirrus evolution. We explore a wide range of background meteorological conditions, including variations of ambient temperature, relative humidity with respect to ice, vertical wind shear, and updraft velocity, and assess two aircraft types. Key contrail properties, such as total ice crystal number and mass, are found to be most sensitive to the initial number of ice crystals and ambient temperature. We show that reducing the number of initially formed ice crystals substantially decreases contrail radiative impact. This is primarily due to a shorter contrail‐cirrus lifetime, driven by the earlier onset and more efficient sedimentation of the fewer but larger ice crystals. Moreover, the relationship between radiative impact and initial ice crystal number is nonlinear, consistent with previous studies.