scholarly journals Supplementary material to "Ice-shelf ocean boundary layer dynamics from large-eddy simulations"

Author(s):  
Carolyn Branecky Begeman ◽  
Xylar Asay-Davis ◽  
Luke Van Roekel
2021 ◽  
Author(s):  
Carolyn Branecky Begeman ◽  
Xylar Asay-Davis ◽  
Luke Van Roekel

Abstract. Small scale, turbulent flow below ice shelves is regionally isolated and difficult to measure and simulate. Yet these small scale processes, which regulate heat transfer between the ocean and ice shelves, can affect sea-level rise by altering the ability of Antarctic ice shelves to “buttress” ice flux to the ocean. In this study, we improve our understanding of turbulence below ice shelves by means of large-eddy simulations at sub-meter resolution, capturing boundary layer mixing at scales intermediate between laboratory experiments or direct numerical simulations and regional or global ocean circulation models. Our simulations feature the development of an ice-shelf ocean boundary layer through dynamic ice melting in a regime with low thermal driving, low ice-shelf basal slope, and strong shear driven by the geostrophic flow. We present a preliminary assessment of existing ice-shelf basal melt parameterizations adopted in single component or coupled ice-sheet and ocean models on the basis of a small parameter study. While the parameterized linear relationship between ice-shelf melt rate and far-field ocean temperature appears to be robust, we point out a little-considered relationship between ice-shelf basal slope and melting worthy of further study.


2020 ◽  
Author(s):  
Ji Sung Na ◽  
Taekyun Kim ◽  
Emilia Kyung Jin ◽  
Seung-Tae Yoon ◽  
Won Sang Lee ◽  
...  

Abstract. The role of the refreezing effect in the ice shelf–ocean boundary layer (IOBL) flow with a super-cooled, plume beneath the ice shelf is investigated using the large-eddy simulation. To reveal the detailed physical processes and characteristics of the IOBL flow, a model domain is initialized and forced by in situ observations and a comparison is made between two simulations, one with the refreezing effect and one without. The simulated velocity, potential temperature, and salinity field are validated with in situ observations performed in Terra Nova Bay in the western Ross Sea in 2016/2017, confirming that the vertical structures in the simulation results agree well with observations. In particular, it is evident that, when the refreezing effect is considered, the IOBL flow can be more realistically resolved, especially upward advection from the sub-ice shelf plume and the ice front eddy. Beneath the ice shelf, two district regions (the inner and outer regions) are identified based on flow characteristics and the refreezing pattern. In the inner region, stratification and stable conditions are observed with negative momentum flux and low refreezing rates. Meanwhile, in the outer region, high shear impact and unstable conditions with a heat flux of −9 to −52 W m−2 are observed, demonstrating the high refreezing rate and the entrainment of super-cooled water from the sub-ice shelf plume. A total of 94 % of the refreezing events occur in the outer region, with a maximum refreezing rate of 1.86 m yr−1 at the ice front.


2019 ◽  
Vol 49 (7) ◽  
pp. 1905-1925 ◽  
Author(s):  
Catherine A. Vreugdenhil ◽  
John R. Taylor

AbstractOcean turbulence contributes to the basal melting and dissolution of ice shelves by transporting heat and salt toward the ice. The meltwater causes a stable salinity stratification to form beneath the ice that suppresses turbulence. Here we use large-eddy simulations motivated by the ice shelf–ocean boundary layer (ISOBL) to examine the inherently linked processes of turbulence and stratification, and their influence on the melt rate. Our rectangular domain is bounded from above by the ice base where a dynamic melt condition is imposed. By varying the speed of the flow and the ambient temperature, we identify a fully turbulent, well-mixed regime and an intermittently turbulent, strongly stratified regime. The transition between regimes can be characterized by comparing the Obukhov length, which provides a measure of the distance away from the ice base where stratification begins to dominate the flow, to the viscous length scale of the interfacial sublayer. Upper limits on simulated turbulent transfer coefficients are used to predict the transition from fully to intermittently turbulent flow. The predicted melt rate is sensitive to the choice of the heat and salt transfer coefficients and the drag coefficient. For example, when coefficients characteristic of fully developed turbulence are applied to intermittent flow, the parameterized three-equation model overestimates the basal melt rate by almost a factor of 10. These insights may help to guide when existing parameterizations of ice melt are appropriate for use in regional or large-scale ocean models, and may also have implications for other ice–ocean interactions such as fast ice or drifting ice.


2020 ◽  
Author(s):  
Carolyn Branecky Begeman ◽  
Xylar Asay-Davis ◽  
Luke Van Roekel

<p>Predictions of ice shelf melting depend on dynamical insights into ocean boundary layers below ice shelves. Fundamental questions regarding the nature of stratified turbulence below the sloped and ablating ice shelf base remain. Laboratory experiments, direct numerical simulations, and observations have yielded important insights, but have yet to produce a robust relationship between ice shelf melt rates and shear- and buoyancy-driven mixing. This relationship is the target of our Large-Eddy Simulations (LES) of the ice-shelf ocean boundary layer. Several new developments were applied to the LES code PALM to produce dynamic melting as well as tides. In this presentation, we demonstrate these new model capabilities. We contrast profiles of vertical turbulent fluxes of heat, salt and momentum across different simulated ice shelf settings: cold, shear-dominated settings vs. warm, buoyancy-dominated settings. We also discuss our recent work toward a new ice-shelf melt parameterization for use in large-scale ocean models on the basis of these simulations. A new melt parameterization is a critical component of ongoing ice-ocean coupling efforts, both to place melt rate predictions on a more physical footing and to achieve convergence with vertical ocean model resolution, on which current parameterizations fail.</p>


2020 ◽  
Author(s):  
Ryan Patmore ◽  
Paul Holland ◽  
Catherine Vreugdenhil

<p>Ice shelf dynamics play a key role in the climate. Melt-rates along the ice shelf-ocean interface are an important aspect in determining the character of global sea level rise. A representation of ice shelf melt is currently implemented in various z-level General Circulation Models (GCMs) by employing parameterisations of the small scale boundary layer dynamics. However, these parameterisations are strongly dependent on the near boundary flow and at the spatial scales for which GCMs are intended the boundary layer is not well resolved. We investigate the ability of a GCM in representing these small scale boundary effects. This is done using MITgcm in an idealised setting with a sloping ice-ocean interface.</p>


2021 ◽  
Author(s):  
Carolyn Branecky Begeman ◽  
Xylar Asay-Davis ◽  
Luke Van Roekel

<p>Large-eddy simulations are used to investigate boundary layer turbulence and its control on ice-shelf basal melt rates in Antarctic settings. We present simulations at relatively low thermal driving and low ice-shelf basal slopes, resulting in simulated melt rates from 10s cm/yr to several m/yr. Our results are broadly consistent with the linear relationships between far-field thermal driving and melt rate and between ice-shelf slope and melt rate reported by previous studies. The simulated thermal exchange coefficient is lower than recommended values; thermal exchange becomes less efficient as stratification increases.  In our simulations, shear production of turbulent kinetic energy outweighs buoyant production, as found below Larsen C Ice Shelf through recent microstructure measurements. We also find that turbulent intensity and melt rate vary significantly with the orientation between the ice-shelf slope and the far-field flow, even at low ice-shelf slopes. Our results suggest that numerical ocean models employing the standard ice-shelf melt parameterization will underestimate slope effects on ice-shelf melt rates even if they capture the mean buoyancy effects on boundary layer flow. The proposed slope effects would modify feedbacks between ocean circulation and ice-shelf geometry and tidal variability in ice-shelf melt rates.</p>


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