Thin and subvisible cirrus and contrails in a subsaturated environment
- ISSN: 16807316
- DOI: 10.5194/acp-11-5853-2011
The frequency of occurrence of cirrus clouds and contrails, their\nlife time, ice crystal size spectra and thus their radiative properties\ndepend strongly on the ambient distribution of the relative humidity\nwith respect to ice (RHice). Ice clouds do not form below a certain\nsupersaturation and both cirrus and contrails need at least saturation\nconditions to persist over a longer period. Under subsaturated conditions,\ncirrus and contrails should dissipate. During the mid-latitude aircraft\nexperiment CONCERT 2008 (CONtrail and Cirrus ExpeRimenT), RHice and\nice crystals were measured in cirrus and contrails. Here, we present\nresults from 2.3/1.7 h of observation in cirrus/contrails during\n6 flights. Thin and subvisible cirrus with contrails embedded therein\nhave been detected frequently in a subsaturated environment. Nevertheless,\nice crystals up to radii of 50 mum and larger, but with low number\ndensities were often observed inside the contrails as well as in\nthe cirrus. Analysis of the meteorological situation indicates that\nthe crystals in the contrails were entrained from the thin/subvisible\ncirrus clouds, which emerged in frontal systems with low updrafts.\nFrom model simulations of cirrus evaporation times it follows that\nsuch thin/subvisible cirrus can exist for time periods of a couple\nof hours and longer in a subsaturated environment and thus may represent\na considerable part of the cirrus coverage.