Modelling contrail cirrus using a double-moment cloud microphysics scheme in the UK Met Office Unified Model

Aviation currently contributes approximately 3.5% to the anthropogenic effective radiative forcing (ERF) on climate, with contrail cirrus being the largest contributor, accounting for twice the impact of aviation CO₂ emissions. However, the latest assessment of aviation's climate impact highlights significant uncertainty of around 70% in contrail cirrus ERF estimates due to process-related uncertainties. Recent research also highlights the critical role of cloud microphysical schemes in representing contrail microphysical and optical properties in climate models.

In this study, we implement a contrail parameterisation in the double-moment cloud microphysics scheme, Cloud AeroSol Interacting Microphysics (CASIM), within the UK Met Office Unified Model (UM). This enables the UM to represent the high number concentration of young contrails, which is critical for the simulation of contrail cirrus evolution and climate impacts. We use a contrail cluster model experiment to evaluate the simulated contrail cirrus evolution, demonstrating that the CASIM modelled changes in several key contrail characteristics are consistent with previous modelling and observation studies. Our analysis indicates that contrails retain a high ice crystal number concentration for a several hours, while contrail ice water content increases during the early stage of the lifecycle before gradually decreasing. In addition, as the contrail cluster descends due to sedimentation, there is an increase in both contrail ice number concentration and water content below flight levels.

We also perform a series of regional simulations over a European domain (i.e. around 35°N-58° N and 10°W-22°E) using the AEDT air traffic inventory. We find that the regional mean contrail cirrus ERF over Europe simulated in our model compares well with estimates from other climate models. Our study highlights the critical role of using a double-moment cloud microphysics scheme when simulating contrails in global climate models. In contrast, results using the UM with a single moment cloud microphysics scheme fails to capture the high ice particle number concentration in young contrails, resulting in unrealistic ERF estimates. Future work with CASIM-UM should focus on estimating contrail cirrus ERF over other regions with high air traffic to provide a more comprehensive understanding of aviation climate impact. In addition, future work should also investigate how the use of alternative fuels affects ice crystal number concentrations, contrail lifetime, microphysical and optical properties.