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