Velocity and temperature bursts in stratified geophysical fluids

Abstract:

Internal (or small-scale) intermittency is a characteristic feature of turbulent flows and is observed in the form of sporadic, intense, small-scale field fluctuations in geophysical fluids. Recent numerical investigations have shown that in stratified turbulent flows, strong enhancements of velocity and temperature develop also at large scale as a result of the interplay between gravity waves and turbulent motions. Here we present a characterization of large-scale intermittent events in stratified turbulence and their feedback on the background flow, by means of direct numerical simulations (DNS) of the Boussinesq equations. Our exploration of the parameter space shows how the kurtosis of the vertical component of the velocity and the temperature depart from the Gaussian reference in a sharp range of values of the Froude number (Fr ≈ 0.05 − 0.3) of geophysical interest. Strong vertical drafts and temperature variations are found to be associated with local instability, enhanced kinetic-potential energy exchanges (thus mixing) and small-scale intermittency. Statistics are performed on the Eulerian fields as well as on ~10^7 Lagrangian particles released in each simulation and advected for several turnover times.