Upper tropospheric humidity and ISSRs: near real-time flight data delivery, statistics, and application for contrail forecast model assessment

The so-called ice-supersaturated regions with air parcels in the status of ice supersaturation (ISSRs) are potential formation regions of cirrus, making them of particular interest to contrails and aviation-induced cirrus. Contrail persistence requires slight ice subsaturation to ice supersaturation (Lee et al., 2021; Li et al., 2023); otherwise, ice crystals would sublimate quickly. Contrails and contrail cirrus in regions with high relative humidity with respect to ice (RHice) have been derived to cause a net warming impact in earlier studies (Sausen et al., 2005; Stuber and Forster, 2007; Lee et al., 2021) Seasonal and regional variabilities and long-term trends of upper tropospheric RHice and ISSRs have been studied using IAGOS routine measurements from passenger aircraft (Petzold et al., 2020; Rohs et al., 2023, see also https://www.iagos.org/). With increasing awareness of the strong climate impact of contrails and contrail cirrus among other aviation emissions, re-routing aircraft to avoid contrail formation becomes an important mitigation target for the aviation industry. However, the correct representation of ISSRs in forecast models is an essential prerequisite to plan robustly more climate-friendly flight trajectories. In this work, we will show the status of weather forecast models in representing real upper tropospheric ice supersaturation. The near real-time RHice dataset from IAGOS observations, provided post-flight to Copernicus Atmosphere Monitoring Service (CAMS), are used to study the occurrence patterns and favourable spatial and temporal conditions of ISSRs in dense air traffic regions, e.g., the North Atlantic flight corridor. Cases demonstrating the assessment of ISSRs and contrail forecast models at cruise levels with in-situ aircraft observations will be showcased. Note: This work is conducted within the framework of the Horizon 2020 project ACACIA with Grant Agreement No. 875036 and the SESAR 3 JU project CICONIA with Grant Agreement No. 101114613.