Ablation of sloping ice faces into polar seawater

Significance 

The continued rise in the ocean and seawater levels attributed to the ablation and melting of ice faces and glaziers due to climate changes can devastating with dangerous flooding and more dangerous hurricanes and typhoons. Scientists expect the global sea levels will rise even more in the future, therefore development of appropriate mitigation measures including an understanding of the rate of ice melting is highly desirable. Presently, various techniques such as general circulation models (GCMs) have been used to investigate the ablation rate and the surrounding ocean water properties. However, the main challenge remains understanding the underlying dynamics of ice melting due to its complex nature.

On the other hand, these models are mainly effective for larger flow scales and rely on different assumptions and parameterizations that may not give the ideal roles of the convection processes responsible for controlling the ablation rate.

The underlying mechanisms governing the melting processes have been assessed based on the direct numerical simulations, which have also exhibited the potential to enhance the parameterization accuracy, especially for large-scale models. Alternatively, the ablation rate and the properties of the natural convection boundary layer have been examined based on the scaling laws taking into consideration the turbulent and laminar flows. For instance, the laminar boundary layer depends on the vertical advection, mean flow and lateral diffusion while turbulent boundary layer depends on complex motions small-scale eddies coupled with large scale motions and their associated heat-mass fluxes. Unfortunately, the three-dimensional direct numerical simulations of ice dissolution in seater have mainly focused on the natural convection, which has not properly clarified the present uncertainties regarding the effects of the slope of ice sea-water interfaces.

To this note, Mainak Mondal (Ph.D. student), Dr. Bishakhdatta Gayen, Professor Ross Griffiths and Dr. Ross Kerr from The Australian National University studied ice dissolution due to natural convection process under sloping ice face using cutting-edge direct numerical simulations. In particular, they investigated the effects of ice face slopes on the ablation rate and boundary layer properties using the turbulence resolving simulations and scaling laws. The turbulent flow at various slopes with temperature and salinity similar to that of Antarctic conditions were used. Eventually, they examined the influence of the type of boundary layer on the ablation rate and boundary layer properties which are very useful in developing reliable and accurate ice-ocean parametrization for GCMs.

The authors observed that the type of the boundary layer determined the boundary layer properties and the ablation rates. Laminar and turbulent ablation rates followed a scaling of (sin ө)1/4 and (sin ө)2/3 respectively, which were both consistent with the available theoretical predictions. For instance, the development of density stratification above the ice face in turbulent cases led to reduced ablation rate due to reduced turbulence buoyancy, while turbulent ablation rate remains height independent. Furthermore, according to the turbulent kinetic energy, shear production and buoyancy were responsible for turbulence in steep slopes while for shallow slopes, the effect of shear production was significantly dominant.

In summary, the research team demonstrated the effects of ice face slope on natural convection driven melting process. In general, the authors noted that despite the effects of other external factors such as geostrophic currents and waves, the effects of natural convection are inevitable. Altogether, the study provides a new refined scaling prediction that will advance understanding the ablation of sloping ice faces in seawater thus providing a tool for mitigating and averting some of the associated consequences. Their research work is published in Journal of Fluid Mechanics.

Authors acknowledge gratitude to the National Computational Infrastructure (NCI), Canberra for providing the necessary computation. Without the NCI’s computational resources, says Mr Mondal, “achieving such a large high-resolution model output would have been impossible. Given that we have significantly small grid spacings being used to resolve both the salinity and temperature boundary layers along with small-scale turbulent structures, the research would not have been possible without NCI’s computing power.

Movie: illustrate the temporal evolution of the turbulent boundary layer structures adjacent to the sloping ice-face.
Left side panel shows the temperature field while right side panel captures along slope velocity field. Convective flow is turbulent and predominantly in the up-slope direction.

Reference

Mondal, M., Gayen, B., Griffiths, R., & Kerr, R. (2019). Ablation of sloping ice faces into polar seawater. Journal of Fluid Mechanics, 863, 545-571.

Go To Journal of Fluid Mechanics

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