Saulgeot et al. (2023): Effects of atmospheric stratification and jet position on the properties of early aircraft contrails

---

The impact of condensation trails, known as contrails, formed behind aircraft is an important area of research for accurately accounting for the contribution to global warming made by the aviation sector. Among the factors influencing the formation and persistence of these contrails, the composition of the plume and the atmosphere is the subject of intense research, but the role played by aircraft wakes has received little attention.

A two-dimensional parametric study of the effects of stratification and the position of the engine jet along the span was therefore carried out to assess the potential radiative impact of early contrails. This phase is unusual in that its dynamics are purely two-dimensional. It plays a particular role in the vertical dispersion of contrails due to the combined effects of stratification and jet-vortex interaction. The model consists of two counter-rotating vortices and two engine exhaust plumes. It represents a cross-section of the aircraft wake during the vortex phase.

The interaction between the jet emitted by the engines causing the contrails and the aircraft wake leads to significant changes in the altitude of the contrail (of the order of several hundred metres). This change in altitude has consequences for the radiative impact of the contrails, since the associated change in temperature influences the ice content and radiative properties. This vortex entrainment is highly dependent on the relative positioning of the jet in relation to the marginal vortices at the wingtips. Jet dispersion is also strongly influenced by buoyancy forces associated with atmospheric stratification. Three main behaviours are observed. For low levels of stratification, the ice plume descends with the wake vortices, generating little optical impact. This impact is all the weaker the closer the jets are to the wingtip vortices. For high levels of stratification and close spacing between the jets, the ice plume tends to remain at the flight altitude and spread horizontally, generating a greater optical impact. Finally, for high values of stratification and jet spacing, the plume remains concentrated in the vortices at flight altitude, resulting in a low optical impact. A threshold effect was observed, which could make it possible to feed larger-scale, less well-resolved climate models. In particular, a jet located closer to the wing tip results in contrails located at lower altitudes and a reduced optical thickness, suggesting that the position of the jet could be an interesting way of attenuating the radiative impact of the contrails.