Diagnosing tracer transport in convective penetration of a stably stratified layer

TL;DR

This study uses large-eddy simulations to analyze buoyant plume penetration into a stably stratified layer, developing a method to partition and quantify turbulence and mixing in different plume regions.

Contribution

The paper introduces a novel buoyancy-tracer volume distribution method to objectively identify and analyze distinct regions of plume transport and mixing in stratified environments.

Findings

Most intense buoyancy gradients occur at the plume cap, with about 50% mixing efficiency.

Environmental fluid entrainment is strongest at the plume cap, involving buoyant environmental fluid.

Radial spreading of the intrusion leads to moderate mixing of environmental fluid into the plume.

Abstract

We use large-eddy simulations to study the penetration of a buoyant plume carrying a passive tracer into a stably stratified layer with constant buoyancy frequency. Using a buoyancy-tracer volume distribution, we develop a method for objectively partitioning plume fluid in buoyancy-tracer space into three regions, each of which corresponds to a coherent region in physical space. Specifically, we identify a source region where undiluted plume fluid enters the stratified layer, a transport region where much of the transition from undiluted to mixed fluid occurs in the plume cap, and an accumulation region corresponding to a radially spreading intrusion. This method enables quantification of different measures of turbulence and mixing within each of the three regions, including potential energy and turbulent kinetic energy dissipation rates, an activity parameter, and the instantaneous mixing efficiency. We find that the most intense buoyancy gradients lie in a thin layer at the cap of the penetrating plume. This provides the primary stage of mixing between plume and environment and exhibits a mixing efficiency around 50%. Newly generated mixtures of environmental and plume fluid join the intrusion and experience relatively weak turbulence and buoyancy gradients. As the intrusion spreads radially, environmental fluid surrounding the intrusion is mixed into the intrusion with moderate mixing efficiency. This dominates the volume of environmental fluid entrained into the region containing plume fluid. However, the 'strongest' entrainment, as measured by the specific entrainment rate, is largest in the plume cap where the most buoyant environmental fluid is entrained.