Modeling and verifying ice supersaturated regions in the ARPEGE model for persistent contrail forecast

Abstract. Contrails formed by aircraft in ice supersaturated regions (ISSR) can persist and spread for several hours, evolving into cirrus which have a net positive effect on global warming. Reducing this contribution could be achieved through on-purpose flight planning, in particular by avoiding ice supersaturated regions. In this context, a modification to the cloud scheme of the ARPEGE operational numerical weather prediction (NWP) model is proposed to enable the representation of ISSRs at cruise altitude. This modification does not require any major algorithmic changes or additional computational effort, and the methodology is transferable to similar parameterizations, commonly used in global circulation models.

Humidity forecasts are evaluated using in situ aircraft humidity observations and compared with operational forecasts from ARPEGE and the Integrated Forecast System (IFS). A sensitivity study on neighborhood tolerance and humidity thresholding is carried out, enabling a comprehensive comparison between NWP forecasts. It is shown that the modified cloud scheme allows for supersaturation, significantly improving the representation of humidity with respect to ice, with ISSR discrimination skills close to IFS (hit rate ~80 % and false alarm ratio ~30 % when a neighborhood tolerance of 150 km, i.e. 10 min of flight, is applied). The spatial correspondence between observations and the modified ARPEGE model is illustrated by a commercial flight case study. The modelization of ice supersaturation in ARPEGE can therefore be used for further contrail climate impact applications, together with the associated evaluation methodology, which contributes to the definition of a shared framework for ISSR verification.