NUMERICAL SIMULATIONS OF PERSISTENT CONTRAILS

Numerical (2D) simulations of persistent contrails have been performed . The simulations begin in the vortex phase (i.e., when the aircraft w ake dynamics are dominated by the pair of downward travelling vortices ) and pursue the evolution for half an hour. Particular emphasis was l aid on the mechanisms by which contrails expand to reach the large lat eral dimensions often observed and on the ice production in these arti ficial clouds. Cross-sectional spreading rates are found to range from 120 to 290 m(2) s(-1). The expansion is mainly caused by the secondar y and higher-order vortices that develop from the reaction of the atmo sphere with the downward travelling pair of primary vortices that are themselves produced by the aircraft. An additional driving force for c ontrail expansion is the gravitational collapse that results from diff erences between the potential temperature profiles inside and outside the contrails. Humidity and temperature of the ambient air control the growth of the ice particles and, therefore, all those properties that depend on crystal size and the overall ice production rate. Radiative cooling and release of latent heat affect expansion rate or turbulenc e intensity only weakly, at least in the first 30 min.