High-resolution modelling of contrails formed behind hydrogen-powered aircraft

The effort to make aviation more climate-friendly requires to develop new propulsion technologies in comparison to the conventional kerosene combustion engines. Hydrogen (H₂) combustion is seen as a promising, green alternative. Assessing the climate impact of contrails, which is a major contribution to the aviation’s non-C02 effects, is an essential part of developing climate-friendly aviation. Our study investigates the properties of contrails behind H₂-powered aircraft, in particular in comparison to conventional contrails from kerosene combustion. For this, high-resolution simulations of individual contrails over their full lifecycle are performed employing the established EULAG-LCM model (a large-eddy simulation (LES) model with fully coupled particle-based ice microphysics). Young contrails and their interaction with the wake vortices are simulated as well as their transition into contrail-cirrus over time periods of several hours.

Recent simulations of contrail formation behind engines with H₂ combustion have shown that the number of created ice crystals is smaller than in conventional contrails, as the exhaust plumes are expected to be void of soot particles, on which contrail ice crystals typically form.
Previous simulations of the early contrail evolution during the vortex phase and the contrail-cirrus evolution have been performed for a broad parameter space regarding variations in meteorological and aircraft-related quantities. However, these simulations were restricted to contrails from conventional kerosene combustion.

In order to investigate the influence of a H­₂ propulsion system on the contrail evolution, two input parameters are adapted: The amount of emitted water vapour is larger and the number of initial ice crystals is smaller.  Moreover, we extend our set of atmospheric scenarios to higher ambient temperatures, as H₂ contrails can form in warmer environments where ice crystal formation in kerosene plumes does not occur.

We explore the H₂ contrail evolution for different idealised atmospheric scenarios. It is well-known that young contrails are strongly affected by the trailing wake vortices and a substantial fraction of the initially formed ice crystals can get lost due to adiabatic heating in the descending vortex pair. We clearly see that this ice crystal loss is reduced if fewer ice crystals are present in the beginning. On the other hand, ice crystal loss is more substantial for ambient temperatures above 225K.
Looking at the aged contrail-cirrus, we investigate in particular the evolution of the total extinction, which we assume to be a proxy of a change in the contrail climate impact. We observe that the initially prescribed ice crystal number, ambient temperature and relative humidity have a strong impact on the contrail lifecycle. Increasing the water vapour emission is, however, of secondary importance.

The total extinction of H₂ contrails is significantly lower than in the case of kerosene contrails. Hence, our simulations suggest that the usage of H₂ combustion as propulsion technology might strongly reduce the climate impact of a single contrail.

This work contributes to the collaborative effort of the German Aerospace Center (DLR) and Airbus in assessing the climate impact of H₂ contrails.