On the Weather Impact of Contrails: New Insights from Coupled ICON–CoCiP Simulations

Abstract. Contrail forecasts typically neglect feedbacks with the atmosphere. Here, we investigate the contrail-weather interaction using a two-way coupling of the Contrail Cirrus Prediction model (CoCiP) with the global non-hydrostatic numerical weather model ICON. ICON includes a new two-moment cloud ice microphysics scheme that enables skillful predictions of ice supersaturation, validated against radiosonde observations and compared with ECMWF forecasts. The CoCiP model uses a new method to limit the uptake of ambient ice supersaturation when many contrails form. Radiative effects of contrails are calculated using the ecRad radiation scheme within ICON. The models are coupled using the YAC coupler to exchange atmospheric and contrail state variables after each ICON time step. The coupled system results are broadly consistent with offline CoCiP simulations, but captures additional feedbacks. The significance of the computed contrail effects is tested by comparison to numerical noise perturbation or twin experiments of the results of two forecasts differing by small random factors in the initial values. The instantaneous radiative forcing (RF) by the contrails exhibits slightly higher global mean values and a more nonlinear dependence on optical depth than previous standalone CoCiP estimates. Contrails induce a butterfly effect that reduces weather predictability after a few days. Hence, contrails are predictable – but only for a finite period. The global mean forecast simulations reveal short-term atmospheric impacts of contrails, including warming at flight levels, as expected. Effects on surface temperature and precipitation appear regionally random, with negligible global mean values before the butterfly effect dominates the results.