Statistics of single-particles in a numerical simulation of atmospheric turbulence in presence of rotation and stratification.

Abstract:

Here we present a detailed study of single particle Lagrangian statistics in presence of rotation and stratification. Rotating and stratified turbulent flows are obtained in this study from the integration of direct numerical simulations (DNS) of the Boussinesq equations with the addition of a Coriolis term, performed by keeping constant the ratio between the frequencies associated to rotation (f) and stratification (N), so that N/f=5 (value compatible with the ocean), for a range of Froude numbers of geophysical interest (0.03 < Fr < 0.2). When N and f are increased (yet keeping their ratio constant) a clear transition is observed in the statistics from a regime dominated by eddies to another one, dominated by waves. Moreover, Lagrangian velocity statistics exhibit anisotropy for all runs, the latter increasing as the product N τ_η increases,τ_η being the Kolmogorov time-scale based on the mean kinetic energy dissipation. In particular, when Nτ_η > 1 the displacement of particles in the vertical direction is strongly reduced, in a way such that the particles appear to be “trapped” in horizontal planes.