Probing Surface Flows with Lagrangian Tracers

Abstract

We discuss the properties of a two-dimensional flow v in a free-slip surface above three-dimensional volume turbulence and probe structures by advecting passive Lagrangian tracers. The dynamics of the flow is determined by changing patterns of sinks and sources due to downwelling and upwelling fluid, respectively. The divergence patches are found to be exponentially distributed in magnitude. The degree of compressibility of the surface flow characterized by the dimensionless number, C = / , is about ½ for different drivings of volume turbulence. The advected tracers cluster exponentially fast in an initial phase until particle depletion becomes important; then a slower cluster evolution takes over. An almost ballistic pair dispersion regime is found for larger times in contrast to a regime where incompressible flows show decorrelated motion. The exponential probability density function of the divergence can be related to an algebraic distribution for the particle densities.