scholarly journals Ice-shelf ocean boundary layer dynamics from large-eddy simulations

2021 ◽  
Author(s):  
Carolyn Branecky Begeman ◽  
Xylar Asay-Davis ◽  
Luke Van Roekel
Keyword(s):  
Boundary Layer ◽  
Ocean Circulation ◽  
Large Eddy Simulations ◽  
Global Ocean ◽  
Small Scale ◽  
Parameter Study ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Large Eddy ◽  
Ocean Boundary

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.

Toward a new ice-shelf melt rate parameterization with large-eddy simulations

2020 ◽  
Author(s):  
Carolyn Branecky Begeman ◽  
Xylar Asay-Davis ◽  
Luke Van Roekel
Keyword(s):  
Large Scale ◽  
Ocean Model ◽  
Large Eddy Simulations ◽  
Stratified Turbulence ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Ocean Coupling ◽  
Large Eddy ◽  
Dynamic Melting ◽  
Ocean Boundary

<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>

Turbulent dynamics and ice-shelf basal melt rates from large-eddy simulations

2021 ◽  
Author(s):  
Carolyn Branecky Begeman ◽  
Xylar Asay-Davis ◽  
Luke Van Roekel
Keyword(s):  
Boundary Layer ◽  
Ocean Circulation ◽  
Large Eddy Simulations ◽  
Far Field ◽  
Ice Shelf ◽  
Thermal Exchange ◽  
Boundary Layer Turbulence ◽  
Shelf Slope ◽  
Linear Relationships ◽  
Large Eddy

<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>

scholarly journals Large-eddy simulation of the ice shelf-ocean boundary layer and heterogeneous refreezing rate by sub-ice shelf plume

2020 ◽  
Author(s):  
Ji Sung Na ◽  
Taekyun Kim ◽  
Emilia Kyung Jin ◽  
Seung-Tae Yoon ◽  
Won Sang Lee ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Outer Region ◽  
Ice Shelf ◽  
Salinity Field ◽  
Eddy Simulation ◽  
Large Eddy ◽  
In Situ Observations ◽  
Ocean Boundary

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.

scholarly journals Case study of a humidity layer above Arctic stratocumulus using balloon-borne turbulence and radiation measurements and large eddy simulations

2020 ◽  
Author(s):  
Ulrike Egerer ◽  
André Ehrlich ◽  
Matthias Gottschalk ◽  
Roel A. J. Neggers ◽  
Holger Siebert ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Sensible Heat Flux ◽  
Large Eddy Simulations ◽  
Cloud Layer ◽  
Specific Humidity ◽  
The Arctic ◽  
Small Scale ◽  
Humid Air ◽  
Field Campaign ◽  
Large Eddy

Abstract. Specific humidity inversions occur frequently in the Arctic. The formation of these inversions is often associated with large scale advection of humid air. However, small-scale boundary layer processes interacting with the humidity inversions are not fully understood yet. In this study, we analyze a three-day period of a persistent layer of increased specific humidity above a stratocumulus cloud observed during an Arctic field campaign in June 2017. The tethered balloon system BELUGA (Balloon-bornE moduLar Utility for profilinG the lower Atmosphere) recorded high-resolution vertical profile measurements of turbulence and radiation in the atmospheric boundary layer. We find that the humidity inversion and the cloud layer are coupled by eddy dissipation, extending above the cloud boundary and linking both layers through turbulent mixing. One case reveals a strong negative virtual sensible heat flux at cloud top (eddy covariance estimate of −15 W/m2), indicating entrainment of humid air from above into the cloud layer. Large Eddy Simulations (LES) based on field campaign data are conducted to supplement the flux measurements. Independent experiments for two days confirm the observed entrainment of humid air, reproducing the observed negative turbulent fluxes of heat and moisture at cloud top. The LES realizations suggest that in the presence of a humidity layer the cloud layer remains thicker and the inversion height is slightly raised, reproducing results from previous idealized LES studies. While this acts to prevent cloud collapse, it remains unclear how the additional moisture is processed in the cloud and how exactly it contributes to the longevity of Arctic cloud layers.

scholarly journals On the Application of the Dynamic Smagorinsky Model to Large-Eddy Simulations of the Cloud-Topped Atmospheric Boundary Layer

2006 ◽  
Vol 63 (2) ◽  
pp. 526-546 ◽  
Author(s):  
M. P. Kirkpatrick ◽  
A. S. Ackerman ◽  
D. E. Stevens ◽  
N. N. Mansour
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Dynamic Model ◽  
Potential Temperature ◽  
Large Eddy Simulations ◽  
Small Scale ◽  
Subgrid Scale ◽  
Smagorinsky Model ◽  
Marine Stratocumulus ◽  
Large Eddy

Abstract In this paper the dynamic Smagorinsky model originally developed for engineering flows is adapted for simulations of the cloud-topped atmospheric boundary layer in which an anelastic form of the governing equations is used. The adapted model accounts for local buoyancy sources, vertical density stratification, and poor resolution close to the surface and calculates additional model coefficients for the subgrid-scale fluxes of potential temperature and total water mixing ratio. Results obtained with the dynamic model are compared with those obtained using two nondynamic models for simulations of a nocturnal marine stratocumulus cloud deck observed during the first research flight of the second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field experiment. The dynamic Smagorinsky model is found to give better agreement with the observations for all parameters and statistics. The dynamic model also gives improved spatial convergence and resolution independence over the nondynamic models. The good results obtained with the dynamic model appear to be due primarily to the fact that it calculates minimal subgrid-scale fluxes at the inversion. Based on other results in the literature, it is suggested that entrainment in the DYCOMS-II case is due predominantly to isolated mixing events associated with overturning internal waves. While the behavior of the dynamic model is consistent with this entrainment mechanism, a similar tendency to switch off subgrid-scale fluxes at an interface is also observed in a case in which gradient transport by small-scale eddies has been found to be important. This indicates that there may be problems associated with the application of the dynamic model close to flow interfaces. One issue here involves the plane-averaging procedure used to stabilize the model, which is not justified when the averaging plane intersects a deforming interface. More fundamental, however, is that the behavior may be due to insufficient resolution in this region of the flow. The implications of this are discussed with reference to both dynamic and nondynamic subgrid-scale models, and a new approach to turbulence modeling for large-eddy simulations is proposed.

scholarly journals Stratification Effects in the Turbulent Boundary Layer beneath a Melting Ice Shelf: Insights from Resolved Large-Eddy Simulations

2019 ◽  
Vol 49 (7) ◽  
pp. 1905-1925 ◽  
Author(s):  
Catherine A. Vreugdenhil ◽  
John R. Taylor
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Rectangular Domain ◽  
Large Eddy Simulations ◽  
Intermittent Flow ◽  
Transfer Coefficients ◽  
Ice Shelf ◽  
Obukhov Length ◽  
Mixed Regime ◽  
Large Eddy

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.

Assessing z-level modelling of the ice shelf - ocean boundary layer

2020 ◽  
Author(s):  
Ryan Patmore ◽  
Paul Holland ◽  
Catherine Vreugdenhil
Keyword(s):  
Boundary Layer ◽  
General Circulation ◽  
Spatial Scales ◽  
General Circulation Models ◽  
Small Scale ◽  
Ice Shelf ◽  
Shelf Dynamics ◽  
Boundary Layer Dynamics ◽  
Global Sea Level ◽  
Ocean Boundary

<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>

Onset of Double-Diffusive Convection in the Ice Shelf/Ocean Boundary Layer

2020 ◽  
Author(s):  
Leo Middleton ◽  
Catherine Vreugdenhil ◽  
Paul Holland ◽  
John Taylor
Keyword(s):  
Boundary Layer ◽  
Turbulent Flow ◽  
Flow Regimes ◽  
Turbulent Convection ◽  
Double Diffusive Convection ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Diffusive Convection ◽  
Ocean Boundary ◽  
Double Diffusive

<p>The interaction between Ice-Shelves and the Ocean is an important component of the response of ice sheets to future warming oceans. Observational data in the ocean boundary layer beneath ice shelves is limited and the turbulent flow in the boundary layer is not well characterised. Our work uses small scale (9m depth) direct numerical simulations (DNS) of the Ice-Shelf-Ocean Boundary Layer, inspired by field observations made beneath the George VI Ice Shelf. Here, warm water has been observed directly beneath the ice shelf, and yet the observed melt rates are modest. To study this scenario, we simulate a forced turbulent flow underlying an ice shelf where the ice base is represented by a dynamic melting boundary condition. As the ice melts, a pool of relatively cold, fresh water develops below the ice base. Thermal diffusion causes the underlying water to cool and can drive turbulent convection. At the same time, the salinity gradient in the halocline is stabilising, but develops over a longer time scale. As a result, two flow regimes exist: one with active turbulent convection driven by double-diffusion of heat and salt, and the other with stratified turbulence leading to mixing of the halocline. By varying control parameters, we identify the transition between the flow regimes in terms of the temperature contrast (thermal driving) and the level of turbulence in the far field. We consider the behaviour of the diapycnal buoyancy flux near the ice base, and it provides insight into the drivers of both the double-diffusive convection and its modification by ambient turbulence. Finally, we discuss how the double-diffusive process we have described applies to real-world ice-shelf ocean boundary layers, and how it may be quantified within observations.</p>

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