Influence of Temperature and Humidity on Contrail Formation Regions in EMAC: A Spring Case Study

Abstract. While carbon dioxide emissions from aviation often dominate climate change discussions, the significant impact of non-CO₂ effects like contrails and contrail-cirrus must not be overlooked, particularly for the mitigation of climate effects. This study evaluates key atmospheric parameters influencing contrail formation, specifically temperature and humidity, using various model setups of a general circulation model (GCM) with different vertical resolutions and two nudging methods for specified dynamics setups. Comparing simulation results with reanalysis data for March and April 2014 reveals a systematic cold bias in mean temperatures across all altitudes and latitudes, particularly in the mid-latitudes where the bias is about 3–5 K, unless mean temperature nudging is applied. In the upper-troposphere/lower stratosphere, the humidity of the nudged GCM simulations shows a wet bias, while a dry bias is observed at lower altitudes. These biases result in overestimated regions for contrail formation in GCM simulations compared to reanalysis data. A point-by-point comparison along flown trajectories with measurement aircraft data shows similar biases. Exploring relative humidity over ice (RH_ice) threshold values for identifying ice-supersaturation regions provides insights into the risks of false alarms for contrail formation, together with information on hit rates. Accepting a false alarm rate of 16 % results in a hit rate of about 40 % (RH_ice threshold 99 %), while aiming for an 80 % hit rate increases the false alarm rate to at least 35 % (RH_ice threshold 91–94 %). A comprehensive one-day case study, involving aircraft-based observations and satellite data, confirms contrail detection in regions identified as potential contrail coverage areas by the GCM.