  The turbulent diffusion of Lagrangian tracer particles has been studied in a
flow on the surface of a large tank of water and in computer simulations. The
effect of flow compressibility is captured in images of particle fields. The
velocity field of floating particles has a divergence, whose probability
density function shows exponential tails. Also studied is the motion of pairs
and triplets of particles. The mean square separation <Delta(t)^2> is fitted to
the scaling form <Delta(t)^2> ~ t^alpha, and in contrast with the
Richardson-Kolmogorov prediction, an extended range with a reduced scaling
exponent of alpha=1.65 pm 0.1 is found. Clustering is also manifest in strongly
deformed triangles spanned within triplets of tracers.
