On Uncertainties in Forecasting Regions Where Persistent Contrails Can Occur

Kurzfassung

Rerouting air traffic to avoid ice super saturated regions (ISSRs) where contrails can persist for several hours has the potential to significantly reduce the overall climate impact of aviation. Besides operational considerations, e.g., air traffic control and airspace capacity, one of the main challenges in employing such a strategy is that numerical weather prediction (NWP) models struggle to accurately forecast ISSRs. In this thesis, the robustness of ISSR forecasts is investigated by comparing the potential of persistent contrails (PPC, a binary value that indicates whether persistent contrails can form) fields of a deterministic run of the ICON (Icosahedral non-hydrostatic) NWP model with the PPC field averaged over 40 ensemble forecasts, PPCprob. The median of the conditional distribution f(PPCprob | PPC = 1) was used as a proxy for the robustness of the ISSR forecasts. The median f(PPCprob | PPC = 1) displays a seasonal pattern with lower values, i.e., less robust forecasts, in the summer months and higher values, i.e., more robust values, in the winter months. From the annual cycle of the median f(PPCprob | PPC = 1), the ten days with the highest (robust days) and lowest median f(PPCprob | PPC = 1) (non-robust days) were chosen to investigate the synoptic situation in more detail. The robust days exhibit stronger horizontal wind and ISSRs that coincide with more active uplifting. The non-robust days are characterized by weaker horizontal wind speeds and smaller horizontal scales. Backwards Lagrangian trajectories calculated from ERA5 reanalysis data on the selected days reveal that ice supersaturation was mostly driven by cooling on the robust days and mixing on the non-robust days. These findings suggest that the main formation mechanism for ISSRs on the robust days are large-scale lifting processes that occur in the vicinity of frontal systems, while on the non-robust days, deep convective activity in the hours and days before the considered event is the source of ISSRs in the upper troposphere. Further research is required to investigate the link between deep convective systems and the robustness of ISSR forecasts. For the implementation of operational contrail avoidance strategies, it could be beneficial to deploy such strategies in the winter months, when the forecast is more robust, and the overall air traffic volume is lower.