Influence of aircraft-induced dilution dynamics on near-field ice crystal formation

Abstract. The influence of aircraft-geometry-induced dynamics on ice crystal formation in contrails is investigated using a detailed 1D microphysics code combined with results from several 3D CFD simulations. Two configurations are considered: one including a complete aircraft geometry and another with an engine alone. For each configuration, two ambient temperatures and two combustion modes are considered, corresponding to soot-poor and soot-rich regimes. These simulations are used to determine the temperature and dilution profiles within the plume, which are then applied to the 1D microphysics code MoMiE. This code represents the various processes of ice crystal formation, notably ice nucleation from volatile particles and from hydrated soot particles. The results show that aircraft-geometry-induced dilution and temperature changes directly impact ice crystal formation, in particular ice particle concentration, number, and size. The difference observed between the engine-only configuration and the complete aircraft geometry is less than 10% for the number of ice crystals per meter of flight in the soot-poor regime, whereas it ranges from less than 1% to 21% in the soot-rich regime. Comparison with results obtained using Schumann’s analytical dilution law, commonly used to represent dilution in 1D box models, also shows significant differences. In the soot-poor regime, the CFD-based dilution leads to a 35% underestimation of the ice crystal number relative to Schumann’s law at high ambient temperatures, whereas it leads to a 118% overestimation at colder ones. Given that ice crystal number, concentration, and size are key parameters for determining the radiative impact of contrails, these results highlight the importance of the aircraft dynamics in ice crystal formation, but also the need for further study into the role of the dilution process in estimating contrail properties.