Turbulence comes in bursts in stably stratified flows

TL;DR

This paper reveals that in stably stratified flows, intense turbulent bursts can occur more frequently when the flow is more stable, due to nonlinear wave energy amplification, challenging traditional assumptions.

Contribution

The study introduces a simple model and numerical simulations demonstrating that stronger turbulence bursts can occur in more stable stratified flows, a counterintuitive finding.

Findings

Stronger turbulent bursts occur in more stable flows.

Bursts are linked to nonlinear amplification of wave energy.

Numerical simulations confirm the paradoxical behavior.

Abstract

There is a clear distinction between simple laminar and complex turbulent fluids. But in some cases, as for the nocturnal planetary boundary layer, a stable and well-ordered flow can develop intense and sporadic bursts of turbulent activity which disappear slowly in time. This phenomenon is ill-understood and poorly modeled; and yet, it is central to our understanding of weather and climate dynamics. We present here a simple model which shows that in stably stratified turbulence, the stronger bursts can occur when the flow is expected to be more stable. The bursts are generated by a rapid non-linear amplification of energy stored in waves, and are associated with energetic interchanges between vertical velocity and temperature (or density) fluctuations. Direct numerical simulations on grids of 2048^3 points confirm this somewhat paradoxical result of measurably stronger events for more stable flows, displayed not only in the temperature and vertical velocity derivatives, but also in the amplitude of the fields themselves.