  Experimental results for passive tracer dispersion in the turbulent surface
layer under stable conditions are presented. In this case, the dispersion of
tracer particles is determined by the interplay of three mechanisms: relative
dispersion (celebrated Richardson's mechanism), shear dispersion (particle
separation due to variation of the mean velocity field) and specific
surface-layer dispersion (induced by the gradient of the energy dissipation
rate in the turbulent surface layer). The latter mechanism results in the
rather slow (ballistic) law for the mean squared particle separation. Based on
a simplified Langevin equation for particle separation we found that the
ballistic regime always dominates at large times. This conclusion is supported
by our extensive atmospheric observations. Exit-time statistics are derived
from the experimental dataset and show a reasonable match with the simple
dimensional asymptotes for different mechanisms of tracer dispersion, as well
as predictions of the multifractal model and experimental data from other
sources.
