Kinematic properties of regions that can involve persistent contrails

Abstract. Contrails can last for many hours in the sky if they form in ice supersaturated regions (ISSRs). Contrail formation is possible once the ambient air is sufficiently moist and cold (below -40 °C) such that the water vapour in the exhaust of an aircraft engine transiently becomes water saturated, which initiates formation of droplets which immediately freeze. Contrails are persistent when the ambient relative humidity with respect to ice is at least 100 %. Cirrus clouds and contrails consist of material ice crystals, which move with the wind. Ice supersaturation is an immaterial feature, which does not generally move with the wind. However, the exact movement of ISSRs and how they move in comparison to the winds, is currently unknown. We analyse their relation using data from the German Weather Service. Our results demonstrate the kinematic differences and similarities of the movement of ISSRs in comparison to the motion of the air in and through the ISSRs. We show that the ISSRs on average move slower than the wind at the same location, the direction of movement is usually quite similar and the distributions of both velocities follow Weibull distributions. The almost identical direction of the movements is beneficial for contrail lifetimes, but the contrail lifetime is not generally determined by the lifetime of ISSRs. It happens that contrails are blown out of ISSRs with the wind. We assume that our study can be used as a basis for further analyses of the movement of ISSRs and their movement properties. Furthermore, our method of analysis is applicable to other extended features or areas, for instance for areas where aircraft non-CO₂ emissions would have a particularly large climate effect.