Differences in early contrail evolution of two‐engine versus four‐engine aircraft: Lidar measurements and numerical simulations

Abstract

Jet‐ and vortex‐regime evolution of contrails behind cruising aircraft is investigated by focusing on the role of aircraft type. Cross‐section measurements by ground‐based lidar and observational analysis are combined with numerical simulations of fluid dynamics and microphysics in the wake of a two‐engine aircraft. Depending on ambient humidity levels, contrail evolution behind short‐/medium‐range twin‐turbofan airliners is classified into two scenarios, which is in contrast to the three scenarios observed for a wide‐body four‐turbofan aircraft. In the case of ice‐subsaturated ambient air, a short visible contrail is formed behind a two‐engine aircraft that disappears before the ice is fully entrained into the wingtip vortices (in most cases ≈4 s behind aircraft). The early evaporation of the ice is mainly due to the fast initial jet expansion, mixing the exhaust with the ambient air. Contrails behind a wide‐body four‐engine aircraft always survive at least until vortex breakdown (i.e., typically 2 min behind aircraft). This is simply due to the larger ice mass in the contrail because of the higher fuel flow rate. Generally, in the case of ice supersaturation, a diffuse secondary wake evolves above the primary vortex wake. For a two‐engine aircraft, always the whole contrail persists, while for a four‐engine aircraft, the ice inside the primary wake disappears in most cases after vortex breakdown, when the relative humidity is only slightly above ice saturation. In the more rare cases of higher ice‐supersaturation the ice in the primary wake survives vortex breakdown and becomes part of the persistent contrail.