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

Contrail cirrus is the largest contributor to aviation effective radiative forcing (ERF), but remains highly uncertain (∼ 70 %). Recent research has highlighted the critical role of cloud microphysical schemes in contrail cirrus climate modelling. In this study, we implement a contrail parameterisation in the double-moment cloud microphysics scheme, Cloud AeroSol Interacting Microphysics (CASIM), within the regional configuration of the UK Met Office Unified Model (UM). We first investigate a contrail cluster model experiment, showing that the simulated contrails retain a high ice crystal number concentration for several hours before declining. Ice water content increases during the early stage of the lifecycle before gradually decreasing. In addition, as the contrail cluster gradually sediments below flight levels, there is an increase in both contrail ice number concentration and water content. We also perform regional simulations over a European domain, estimating a regional annual mean contrail cirrus ERF of 0.93 W m−2, within the range of previous climate modelling estimates. Using a range of initial contrail width, depth and ice crystal size based on contrail observations, we estimate an annual mean European regional contrail cirrus ERF range of 0.19 to 2.80 W m−2. The seasonal cycle of contrail cirrus ERF is mainly driven by the background meteorology and the natural clouds vertical structure. Our study highlights the critical need for double-moment cloud microphysics in global climate models to realistically simulate contrail impacts. Future work should extend the simulation globally and investigate how the use of alternative fuels affects contrail microphysical properties, contrail lifetime, and climate impact.