Variability of ice supersaturated regions at flight altitudes: evaluation of ERA5 reanalysis using IAGOS in situ measurements

Abstract. Contrail cirrus is one of the largest contributors to aviation's radiative forcing, which arises from long-lived persistent contrails. Avoiding persistent contrail formation has been suggested as a measure to reduce the climate impact of aviation, requiring accurate forecasts of ice supersaturated conditions, i.e. where the relative humidity over ice (RHi) exceeds 100 %. Numerical weather prediction (NWP) models often underestimate or do not account for ice supersaturation. This study evaluates ice supersaturated regions (ISSRs) in the ECMWF ERA5 reanalysis using In-service Aircraft for a Global Observing System (IAGOS) measurements over tropical and extratropical regions in the upper troposphere and lower stratosphere for the period 2011–2022. It considers cloudy and clear-sky conditions and how North Atlantic weather patterns affect the ERA5 ability to predict ISSRs. ERA5 generally underestimates ISSR occurrence due to a dry bias in RHi; the equitable threat score (ETS) is 0.2–0.4, indicating a weak to mediocre relationship with IAGOS. Clear-sky conditions result in an ETS of 0.05-0.18 and generally below 0.1 in cloudy conditions, indicating an almost random relationship. The latter is the result of the saturation adjustment used by the NWP model underlying the reanalysis. North Atlantic winter weather patterns appear to affect the ability of ERA5 to predict ISSRs, particularly along eastbound routes. This may result from varying ISSR distributions relative to the jet stream. North Atlantic summer weather patterns show little impact due to weaker teleconnection patterns. Overall, the underestimation of ISSRs in ERA5 is most critical in the upper troposphere, where their occurrence is highest.