Vertical drafts and mixing in stratified turbulence: sharp transition with Froude number

TL;DR

This study explores how stratified turbulence exhibits a sharp transition in vertical mixing and intermittency at specific Froude numbers, revealing a resonant regime with enhanced mixing and localized overturning structures.

Contribution

It introduces a simple model capturing the competition between gravity waves and nonlinear steepening, explaining the sharp transition and intermittency in stratified turbulence.

Findings

Large vertical velocity and temperature fluctuations at certain Froude numbers.

A sharp transition to non-Gaussian behavior in probability distributions.

Linear increase of mixing efficiency with Froude number.

Abstract

We investigate the large-scale intermittency of vertical velocity and temperature, and the mixing properties of stably stratified turbulent flows using both Lagrangian and Eulerian fields from direct numerical simulations, in a parameter space relevant for the atmosphere and the oceans. Over a range of Froude numbers of geophysical interest ($\approx 0.05-0.3$) we observe very large fluctuations of the vertical components of the velocity and the potential temperature, localized in space and time, with a sharp transition leading to non-Gaussian wings of the probability distribution functions. This behavior is captured by a simple model representing the competition between gravity waves on a fast time-scale and nonlinear steepening on a slower time-scale. The existence of a resonant regime characterized by enhanced large-scale intermittency, as understood within the framework of the proposed model, is then linked to the emergence of structures in the velocity and potential temperature fields, localized overturning and mixing. Finally, in the same regime we observe a linear scaling of the mixing efficiency with the Froude number and an increase of its value of roughly one order of magnitude.