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Abstract

Numerical simulations of contrails have been performed to investigate the role of various external parameters and physical processes in the life cycle of contrails. The general idea underlying the model is that of a large-eddy model. The model explicitly represents the large-scale three-dimensional motions (10-m grid resolution), while small-scale turbulence is parameterized; it contains a detailed microphysical model and it takes into account infrared radiative cooling in cloudy conditions and the vertical shear of the ambient flow.
The model is applied to conditions typical for those under which contrails could be observed, that is, in an atmosphere which is supersaturated with respect to ice and at a temperature of 220 K. The simulations begin in the late dispersion phase (i.e., about 10(3) s after exhaust) and trace the evolution of the contrails for a hale hour period. Coherent structures can be identified within these clouds with vertical velocity fluctuations of the order of 0.1 m s(-1) that are generated mainly by buoyancy due to latent heat release. In addition, the sensitivity runs undertaken as a test of the model to a change in significant physical processes or external parameters indicate that the contrail evolution is controled primarily by humidity, temperature, and static stability of the ambient air and secondarily by the baroclinicity of the atmosphere. Moreover, it turns out that the initial ice particle concentration and radiative processes are of minor importance in the evolution of contrails, at least during the 30-min simulation period.