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

Abstract. Contrail forecasts typically neglect feedbacks with the atmosphere. Here, we couple the Contrail Cirrus Prediction model (CoCiP) with the global Icosahedral Non-hydrostatic (ICON) numerical weather model in a two-way mode accounting for contrail-weather interaction. The models exchange atmospheric and contrail state variables after each time step using the coupler YAC. ICON includes a new two-moment cloud ice microphysics scheme that enables skillful predictions of ice supersaturation. CoCiP now limits the uptake of ambient ice supersaturation when many contrails form. Radiation is calculated in ICON using the ECMWF radiation scheme ecRad. Contrail radiative forcing is computed from the difference of ICON results with and without contrail feedback. The coupled system results are broadly consistent with offline CoCiP simulations. The ICON results are validated against radiosonde observations and compared with ECMWF forecasts showing improved score values. The significance of the computed contrail effects is tested by numerical noise perturbation or twin experiments comparing the results of forecast pairs with initial values differing randomly. Contrails induce a butterfly effect with disturbances growing with time. Contrails induce disturbances similar to random disturbances. Within the first 5 d, contrails warm the ambient air at contrail levels. Contrails change the surface temperature and precipitation locally by an order 1 K and 10 mm d−1, with pattern similar to random disturbances and with magnitude depending on ambient weather, with negligible global mean changes. After 5 d, the weather changes are dominated by the butterfly effect. The slow response of the surface temperature to contrail RF deserves further investigations.