Shear instabilities in cold lakes retain structure

In late winter, lakes at high latitudes are often covered in ice and have an inverted stratification with cold water on top of warm. But as sunlight returns in the spring, the water is warmed, inducing a convective instability. Research is increasingly showing the waters at these times are more active than previously thought.

While stratified instabilities are well studied, there is little research into their application in under-ice freshwater bodies. Stastna et al. investigated these conditions in a study of the instability of a shear flow in cold water. In this case, the water is characterized by small density differences and a nonlinear equation of state.

Using tools from turbulence theory and simulations tailored to mimic the waters in iced-over freshwater lakes, the researchers determined the detailed evolution of a three-dimensional stratified shear instability. From their results, they quantified the momentum dissipation and scalar mixing in the water and documented coherent structures in the flow.

“The most exciting part of this research is that it shows that water under ice flows retains some structure, even as they get ‘messy’ or fully three-dimensional,” said author Marek Stastna. “The natural world often does not match the world of laboratory experiments and it is important to see which ideas carry over and which do not.”

The authors hope the work can be applied to more realistic simulations, such as those with an overlying ice layer that is part of the model, or for flows chosen to match flow profiles measured in a particular lake.

Source: “The three dimensional state of stratified shear instability in cold water,” by M. Stastna, K. Bhavsar, S. Hartharn-Evans, and N. Castro-Folker, Physics of Fluids (2025). The article can be accessed at https://doi.org/10.1063/5.0268302 .

This paper is part of the 50 Years of the Canadian Symposium on Fluid Dynamics Collection, learn more here .