Numerical simulations of homogeneously nucleated natural cirrus and contrail-cirrus. Part 1: How different are they?

Abstract

The evolution of contrail-cirrus and natural cirrus formed by homogeneous nucleation is studied over up to ten hours by means of a Large-Eddy Simulation (LES) model equipped with a Lagrangian ice microphysics module. This is the first time that both cloud types are investigated in a single study. Characteristics of their life cycles depend strongly on the synoptic scenario. Weak, but enduring updraughts allow for the longest life times of contrail-cirrus. For cirrus clouds, the updraught speed during their formation is most crucial. Once contrails lose their linear shape they are hardly distinguishable from natural cirrus which makes it difficult to evaluate the extent and effect of the anthropogenic cloud modification. Despite their different formation mechanisms (contrails are generated locally and have initially much higher ice crystal number concentrations than natural cirrus) we could not single out microphysical criteria that could help to distinguish in general between both cloud types in observations.