Synoptic and microphysical lifetime constraints for contrails

Abstract

Contrail lifetime is constrained mainly by the sedimentation of ice crystals into lower levels that are subsaturated, by the blowing out of the ice crystals from the parent ice-supersaturated regions (ISSRs) as a result of the (horizontal) wind and by the reduction in supersaturation down to subsaturation due to large-scale subsidence. The first of these processes can be characterised by a sedimentation timescale. The second and third processes can together be characterised by a synoptic timescale. The synoptic timescale is determined in this paper by trajectory calculations for air parcels that initially reside in ice-supersaturated regions and which leave these either with the wind or where the ice supersaturation itself vanishes. It is crucial to know the timescales of contrails because their individual effect on the climate depends on their lifetime. The distinction between the two timescales is particularly important for planning flights that use alternative fuels, in order to mitigate contrail effects. This works in particular if sedimentation is the predominant contrail termination process – that is, if the sedimentation timescale is shorter than the synoptic one. Here we show that both timescales are of the order of a few hours. Actually, in nature, the three mentioned processes act simultaneously. The combined timescale is half of the harmonic mean of the two timescales in separation. Furthermore, we found as a side result that ISSRs emerge only in areas where the normalised geopotential height Z* is at least 0.98. For contrail-avoiding flight planning, this means that contrail avoidance in regions with Z* < 0.98 is not necessary.