Contrail formation for aircraft with hydrogen combustion – Part 1: A systematic microphysical investigation

Abstract. The number of ice crystals formed during the contrail’s jet phase has a long-lasting impact on the life cycle and radiative forcing of contrail cirrus clouds. Contrail formation for conventional kerosene combustion is well studied, and suitable parametrizations for the early ice crystal number have been used to estimate the climate impact of contrail cirrus with a general circulation model. However, a parametrization for the number of ice crystals formed is lacking for hydrogen combustion. To develop such a parametrization, we present a comprehensive set of contrail formation simulations using the particle-based Lagrangian Cloud Module in a box model approach. Unlike kerosene combustion, no soot particles are emitted in the hydrogen combustion case. Thus, ice crystal formation is assumed to occur on ambient aerosols entrained into the exhaust plume. The results show that coarse mode particles have negligible influence on ice crystal number due to their low abundance. Furthermore, ice crystal formation involving multiple co-existing aerosol populations with different properties (number, size, solubility) can be reconstructed from simulations involving single aerosol populations. We also identify atmospheric conditions where homogeneous droplet nucleation can be safely neglected as potential ice formation pathway. Based on more than 20,000 simulations covering a broad range of atmospheric conditions and aerosol properties, we identify a regime where ice crystal formation becomes nearly independent of ambient relative humidity, aerosol size, and solubility. Our results provide a basis for a data-driven parametrization of ice crystal number in contrails from hydrogen combustion, to be presented in a companion paper.