LARGE-EDDY SIMULATION OF AIRCRAFT EXHAUST PLUMES IN THE FREE ATMOSPHERE - EFFECTIVE DIFFUSIVITIES AND CROSS-SECTIONS

The effective diffusion of aircraft emissions in the free stably strat ified atmosphere is investigated by means of large-eddy simulation in a domain of size 4.3 x 1.1 x 1.1 km(3). On that scale, the atmosphere is represented by a weakly turbulent flow under uniformly stratified a nd shearless conditions. It is assumed that aircraft induced turbulenc e has ceased. The exhaust plumes of aircraft are represented by line s ources with Gaussian cross-sections. The stratification has been varie d between 0.006 s(-1) and 0.03 s(-1). The computed effective horizonta l diffusion coefficient lies between II +/- 2 m(2)s(-1) and 21 +/- 4 m (2)s(-1), depending on the level of stratification. Likewise the verti cal diffusivity ranges from 2.3 +/- 0.6 m(2)s(-1) to 0.37 +/- 0.04 m(2 )s(-1) in the beginning of the diffusion process, and amounts almost i ndependently of the stratification to about 0.15 +/- 0.01 m(2)s(-1) la ter on. The plume cross-sections increase with time and stratification . The scattering of the results for a given stratification reflects th e action of decaying, anisotropic, and homogeneous turbulence. The res ults confirm very well data from recent in-situ flight measurements in the North-Atlantic flight corridor in the tropopause region.