Kinematic properties of ice supersaturated regions and lifetime limitations for contrails

Kurzfassung

Contrails can last for many hours in the sky if they form in so-called ice supersaturated regions (ISSRs). While both contrails and cirrus clouds consist of ice crystals that move with the wind, ice supersaturation is an immaterial feature, which does not necessarily follow this motion. If ISSRs move in a similar direction as the wind, this is beneficial for contrail lifetimes, but the lifetime of contrails (on which the individual radiative effect and thus their effect on climate depends) is not generally determined by the lifetime of ISSRs. Instead, contrail lifetime is constrained mainly by three processes: (1) the sedimentation of ice crystals into subsaturated layers, (2) the horizontal advection of ice crystals out of ISSRs by the wind and (3) the reduction of supersaturation through large-scale subsidence. The first of these processes can be characterised by a sedimentation timescale. The second and third processes can be characterised by a synoptic timescale. The distinction between the two timescales is important for planning flights with alternative fuels to mitigate contrail effects. This study analyses the kinematic differences and similarities between ISSR motion and air movement within them. It provides estimates of the two relevant contrail timescales. The results show that 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. ISSRs mainly occur where the normalised geopotential height (Z*) is ≥ 0.98, meaning contrail avoidance below this threshold is generally unnecessary.