Effects of atmospheric stratification and jet position on the properties of early aircraft contrails

The net impact of aircraft contrails on global climate change is a matter of controversy today. Among the many parameters potentially influencing this issue, the role played by the aircraft wake has received only little attention so far. Yet the interaction between the engine exhaust jets causing these contrails and the aircraft wake can lead to modifications in the altitude of the contrails on the order of hundreds of meters. This change in altitude heavily influences the net impact of contrails to global climate change, since it affects the ambient temperature and thereby the ice content and the radiative properties of these contrails. The wake entrainment supporting these effects basically depends on the relative positioning of the jet with respect to the tip vortices and on the buoyant forces associated with atmospheric stratification. Here we focus on these two parameters by running a large number of two-dimensional simulations of the flow from the aftermath of the jet turbulent diffusion, vortex roll-up, and initial ice formation up to the vortex destabilization stage, for a range of values of the vortex based normalization of the Brunt-Väisälä frequency and the jet to wing span ratio. The very near wake dynamics are not simulated and instead replaced by an analytical description for the vortex, the jet, and the ice plume. Ice water content is determined from the offset to ice saturation, given prescribed ambient conditions. The jet lateral spacing is considered in the range from fuselage to wing tip. The potential radiative impact of the early wake is calculated using the total extinction induced by the ice plume. The results are indicative of the impact of older contrail cirrus clouds, the largest proportion in the whole contrail radiative impact. The parametric mapping (stratification, jet spacing) highlights the important role played by the jet position on the opacity of early contrails, for regular stratification levels. In particular, a jet located closer to the wing tip results in contrails located at lower altitudes and reduced optical thickness, suggesting that jet positioning could be an interesting mean of contrail mitigation.