Small-scale turbulence and mixing: energy fluxes in stratified lakes

Fernández Castro, Bieito, Wüest, Alfred and Lorke, Andreas (2021) Small-scale turbulence and mixing: energy fluxes in stratified lakes. In, Reference Module in Earth Systems and Environmental Sciences. Elsevier. (doi:10.1016/B978-0-12-819166-8.00059-1).

Abstract

Aim: in this article, we describe the energetics and dynamics of small-scale turbulence and mixing in stratified water bodies. Turbulence in the stratified interior of a water body arises when wind-driven internal motions become unstable and transfer their energy towards smaller-scale motions. The resulting turbulent mixing drives weak but continuous vertical fluxes of heat, nutrients and gases, as well as any dissolved or particulate substances, playing a critical role in ecological processes. We outline the fundamental concepts describing the dynamics of stratified turbulence and provide practical guidelines for measuring turbulent fluxes in the field.

Main concepts: we first take an energy balance approach and trace the energy pathways from wind-driven basin-scale motions to small-scale turbulent mixing and compare the energy available for mixing with that stored in stratification. We then describe how turbulent eddies operate at small-scales to generate net vertical downgradient fluxes, the intensity of which can be quantified in terms of a turbulent diffusivity parameter. Finally, we illustrate the mixing mechanisms operating in different compartments of stratified water-bodies.

Main methods: the most commonly used methods to estimate vertical turbulent fluxes are described. These include direct flux measurements using the eddy-covariance technique, microstructure measurements, and basin-scale budget approaches.

Conclusion: turbulent mixing is a highly episodic phenomenon, challenging to measure with sufficient spatio-temporal resolution. Notwithstanding, a consistent picture is emerging from decades of research showing that a small fraction (close to 0.3%) of the available wind energy is invested in mixing the stratified interior of the water body. This energy is too low to cause major changes in stratification but strong enough to drive ecologically-relevant fluxes. Despite this progress, many open questions remain regarding the spatio-temporal distribution of turbulent fluxes in different systems and their evolution under climate warming, which is altering the stratification dynamics in lakes globally.

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Contributors

Author: Bieito Fernández Castro

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