scholarly journals An Oceanographic Study of the Cavity Beneath the McMurdo Ice Shelf, Antarctica

2021 ◽  
Author(s):  
◽  
Natalie J Robinson
Keyword(s):  
Boundary Layer ◽  
Water Column ◽  
General Circulation ◽  
Open Water ◽  
Fine Sand ◽  
Freezing Temperature ◽  
Ocean Model ◽  
Ice Shelf ◽  
Climate History ◽  
Mcmurdo Sound

<p><b>This thesis reports the first observations of currents, temperature and salinity beneaththe McMurdo Ice Shelf, Antarctica. They are reviewed and discussed here in conjunctionwith results of a numerical modelling study used to simulate current flow and to investigatelocal sediment deposition. The McMurdo Ice Shelf lies behind Ross Island off theVictoria Land coast of Antarctica, and represents the northwest corner of the much largerRoss Ice Shelf. The site will be drilled by the ANDRILL consortium in 2006, passingthrough the ice shelf, the water column, and 1000 m into the sea floor, to obtain a recordof ice shelf and climate history in this area.</b></p> <p>This study stems from a site survey carried out in early 2003, for which access holeswere melted at two locations on the McMurdo Ice Shelf. Current meters surveyed multipledepths simultaneously during spring tides, and profiles of temperature and salinitywere collected through a diurnal tidal cycle at each site. Maximum currents were recordedin the boundary layer at the base of the ice shelf, reaching 0.22 m s−1 during the flood tide.</p> <p>The salinity and temperature profiles were similar at the two sites, with greater temporalvariability observed at the site closer to the open water of McMurdo Sound. Supercooling,due to the pressure-dependence of the in-situ freezing temperature, was observed at oneof the sites. At the second site, where the draft of the ice shelf was deeper, temperaturescorresponding to basal melting were observed.</p> <p>At a third site on the sea ice at the northwestern edge of the McMurdo Ice Shelf, acurrent meter surveyed the water column to 320 metres below sea level for 23 days. Thisallowed comparison of current behaviour through spring and neap tides, and between subseaice and sub-ice shelf environments in the same season. Net throughflow over springtides at each of the three sites was consistent with transport eastwards from McMurdoSound along the channel defined by local bathymetry. Profiles of temperature and salinityfrom beneath the ice shelf were likewise consistent with McMurdo Sound being the sourceof the observed water masses.</p> <p>Flow along the sub-ice shelf channel was further investigated using an adaptation of atwo-dimensional thermohaline ocean model. Year-long profiles of temperature and salinityfrom southern McMurdo Sound were used to seasonally force the model, resulting in annual variation in all parameters. The rate of melting decreased monotonically from∼0.6 m yr−1 at the deep end of the ice shelf, into a region of freezing associated withsupercooling closer to the McMurdo Sound end of the domain. This change in regimemirrored the observations from the boundary layer beneath the McMurdo Ice Shelf.</p> <p>Sediment transport and deposition were investigated, with settling velocities used to representsediment sizes ranging from biogenic pellets and fine sand through algal flocs to finemud, particle types known and described from the present day environment. This methodof incorporating sedimentation processes gave results similar to observations from surfacesediment cores collected beneath the ice shelf. The larger grains were preferentially depositedclose to the open water McMurdo Sound source, whereas fine-grained materialwas entrained into the general circulation and deposited by regions of down-welling. Asettling velocity of ∼1x10−4 m s−1, corresponding to a grain size of ∼5 μm, defined theboundary between these depositional behaviours.</p> <p>Characteristics of the water beneath the ice shelf suggest that it had been transportedfrom McMurdo Sound, being modified through interaction with the base of the ice shelf.</p> <p>This pattern of throughflow was also seen in the current meter data, with a strong tidalsignal throughout the water column superimposed on the net transport eastward fromMcMurdo Sound and under the ice shelf. This net flow pattern was supported by theresults of the longer-term simulation experiments.</p>

scholarly journals An Oceanographic Study of the Cavity Beneath the McMurdo Ice Shelf, Antarctica

2021 ◽  
Author(s):  
◽  
Natalie J Robinson
Keyword(s):  
Boundary Layer ◽  
Water Column ◽  
General Circulation ◽  
Open Water ◽  
Fine Sand ◽  
Freezing Temperature ◽  
Ocean Model ◽  
Ice Shelf ◽  
Climate History ◽  
Mcmurdo Sound

<p><b>This thesis reports the first observations of currents, temperature and salinity beneaththe McMurdo Ice Shelf, Antarctica. They are reviewed and discussed here in conjunctionwith results of a numerical modelling study used to simulate current flow and to investigatelocal sediment deposition. The McMurdo Ice Shelf lies behind Ross Island off theVictoria Land coast of Antarctica, and represents the northwest corner of the much largerRoss Ice Shelf. The site will be drilled by the ANDRILL consortium in 2006, passingthrough the ice shelf, the water column, and 1000 m into the sea floor, to obtain a recordof ice shelf and climate history in this area.</b></p> <p>This study stems from a site survey carried out in early 2003, for which access holeswere melted at two locations on the McMurdo Ice Shelf. Current meters surveyed multipledepths simultaneously during spring tides, and profiles of temperature and salinitywere collected through a diurnal tidal cycle at each site. Maximum currents were recordedin the boundary layer at the base of the ice shelf, reaching 0.22 m s−1 during the flood tide.</p> <p>The salinity and temperature profiles were similar at the two sites, with greater temporalvariability observed at the site closer to the open water of McMurdo Sound. Supercooling,due to the pressure-dependence of the in-situ freezing temperature, was observed at oneof the sites. At the second site, where the draft of the ice shelf was deeper, temperaturescorresponding to basal melting were observed.</p> <p>At a third site on the sea ice at the northwestern edge of the McMurdo Ice Shelf, acurrent meter surveyed the water column to 320 metres below sea level for 23 days. Thisallowed comparison of current behaviour through spring and neap tides, and between subseaice and sub-ice shelf environments in the same season. Net throughflow over springtides at each of the three sites was consistent with transport eastwards from McMurdoSound along the channel defined by local bathymetry. Profiles of temperature and salinityfrom beneath the ice shelf were likewise consistent with McMurdo Sound being the sourceof the observed water masses.</p> <p>Flow along the sub-ice shelf channel was further investigated using an adaptation of atwo-dimensional thermohaline ocean model. Year-long profiles of temperature and salinityfrom southern McMurdo Sound were used to seasonally force the model, resulting in annual variation in all parameters. The rate of melting decreased monotonically from∼0.6 m yr−1 at the deep end of the ice shelf, into a region of freezing associated withsupercooling closer to the McMurdo Sound end of the domain. This change in regimemirrored the observations from the boundary layer beneath the McMurdo Ice Shelf.</p> <p>Sediment transport and deposition were investigated, with settling velocities used to representsediment sizes ranging from biogenic pellets and fine sand through algal flocs to finemud, particle types known and described from the present day environment. This methodof incorporating sedimentation processes gave results similar to observations from surfacesediment cores collected beneath the ice shelf. The larger grains were preferentially depositedclose to the open water McMurdo Sound source, whereas fine-grained materialwas entrained into the general circulation and deposited by regions of down-welling. Asettling velocity of ∼1x10−4 m s−1, corresponding to a grain size of ∼5 μm, defined theboundary between these depositional behaviours.</p> <p>Characteristics of the water beneath the ice shelf suggest that it had been transportedfrom McMurdo Sound, being modified through interaction with the base of the ice shelf.</p> <p>This pattern of throughflow was also seen in the current meter data, with a strong tidalsignal throughout the water column superimposed on the net transport eastward fromMcMurdo Sound and under the ice shelf. This net flow pattern was supported by theresults of the longer-term simulation experiments.</p>

scholarly journals Response to Filchner-Ronne Ice Shelf cavity warming in a coupled ocean–ice sheet model. Part I: The ocean perspective

2017 ◽  
Author(s):  
Ralph Timmermann ◽  
Sebastian Goeller
Keyword(s):  
World Ocean ◽  
Ice Sheet ◽  
Coupled Model ◽  
Freezing Temperature ◽  
Ocean Model ◽  
Climate Forcing ◽  
Global Ocean ◽  
Ice Shelf ◽  
Marginal Seas ◽  
Grounding Line

Abstract. A Regional Antarctic and Global Ocean (RAnGO) model has been developed to study the interaction between the world ocean and the Antarctic ice sheet. The coupled model is based on a global implementation of the Finite Element Sea-ice Ocean Model (FESOM) with a mesh refinement in the Southern Ocean, particularly in its marginal seas and in the sub-ice shelf cavities. The cryosphere is represented by a regional setup of the ice flow model RIMBAY comprising the Filchner-Ronne Ice Shelf and the grounded ice in its catchment area up to the ice divides. At the base of the RIMBAY ice shelf, melt rates from FESOM's ice-shelf component are supplied. RIMBAY returns ice thickness and the position of the grounding line. The ocean model uses a pre-computed mesh to allow for an easy adjustment of the model domain to a varying cavity geometry. RAnGO simulations with a 20th-century climate forcing yield realistic basal melt rates and a quasi-stable grounding line position close to the presently observed state. In a centennial-scale warm-water-inflow scenario, the model suggests a substantial thinning of the ice shelf and a local retreat of the grounding line. The potentially negative feedback from ice-shelf thinning through a rising in-situ freezing temperature is more than outweighed by the increasing water column thickness in the deepest parts of the cavity. Compared to a control simulation with fixed ice-shelf geometry, the coupled model thus yields a slightly stronger increase of ice-shelf basal melt rates.

First results from coupling the Parallel Ice Sheet Model with the Modular Ocean Model via an Antarctic ice-shelf cavity module

2021 ◽  
Author(s):  
Moritz Kreuzer ◽  
Ronja Reese ◽  
Willem Huiskamp ◽  
Stefan Petri ◽  
Torsten Albrecht ◽  
...  
Keyword(s):  
Time Scales ◽  
General Circulation ◽  
Ice Sheet ◽  
Circulation Model ◽  
Ocean Model ◽  
Global Ocean ◽  
Ice Shelf ◽  
Antarctic Ice Sheet ◽  
First Results ◽  
The Antarctic

&lt;p&gt;The past and future evolution of the Antarctic Ice Sheet is largely controlled by interactions between the ocean and floating ice shelves. To investigate these interactions, coupled ocean and ice sheet model configurations are required. Previous modelling studies have mostly relied on high resolution configurations, limiting these studies to individual glaciers or regions over short time scales of decades to a few centuries. To study global and long term interactions, we developed a framework to couple the dynamic ice sheet model PISM with the global ocean general circulation model MOM5 via the ice-shelf cavity module PICO. Since ice-shelf cavities are not resolved by MOM5, but parameterized with the box model PICO, the framework allows the ice sheet and ocean model to be run at resolution of 16&amp;#8201;km and 3 degrees, respectively. We present first results from our coupled setup and discuss stability, feedbacks, and interactions of the Antarctic Ice Sheet and the global ocean system on millennial time scales.&lt;/p&gt;

scholarly journals Response to Filchner–Ronne Ice Shelf cavity warming in a coupled ocean–ice sheet model – Part 1: The ocean perspective

Ocean Science ◽  
2017 ◽  
Vol 13 (5) ◽  
pp. 765-776 ◽  
Author(s):  
Ralph Timmermann ◽  
Sebastian Goeller
Keyword(s):  
World Ocean ◽  
Ice Sheet ◽  
Coupled Model ◽  
Freezing Temperature ◽  
Ocean Model ◽  
Climate Forcing ◽  
Global Ocean ◽  
Ice Shelf ◽  
Marginal Seas ◽  
Grounding Line

Abstract. The Regional Antarctic ice and Global Ocean (RAnGO) model has been developed to study the interaction between the world ocean and the Antarctic ice sheet. The coupled model is based on a global implementation of the Finite Element Sea-ice Ocean Model (FESOM) with a mesh refinement in the Southern Ocean, particularly in its marginal seas and in the sub-ice-shelf cavities. The cryosphere is represented by a regional setup of the ice flow model RIMBAY comprising the Filchner–Ronne Ice Shelf and the grounded ice in its catchment area up to the ice divides. At the base of the RIMBAY ice shelf, melt rates from FESOM's ice-shelf component are supplied. RIMBAY returns ice thickness and the position of the grounding line. The ocean model uses a pre-computed mesh to allow for an easy adjustment of the model domain to a varying cavity geometry. RAnGO simulations with a 20th-century climate forcing yield realistic basal melt rates and a quasi-stable grounding line position close to the presently observed state. In a centennial-scale warm-water-inflow scenario, the model suggests a substantial thinning of the ice shelf and a local retreat of the grounding line. The potentially negative feedback from ice-shelf thinning through a rising in situ freezing temperature is more than outweighed by the increasing water column thickness in the deepest parts of the cavity. Compared to a control simulation with fixed ice-shelf geometry, the coupled model thus yields a slightly stronger increase in ice-shelf basal melt rates.

scholarly journals Observation and Parameterization of Ablation at the Base of Ronne Ice Shelf, Antarctica

2010 ◽  
Vol 40 (10) ◽  
pp. 2298-2312 ◽  
Author(s):  
Adrian Jenkins ◽  
Keith W. Nicholls ◽  
Hugh F. J. Corr
Keyword(s):  
Boundary Layer ◽  
Open Water ◽  
Ablation Rate ◽  
Ocean Currents ◽  
Turbulent Transfer ◽  
Transfer Coefficients ◽  
Ice Shelf ◽  
Direct Measurements ◽  
Oceanic Boundary Layer ◽  
The Impact

Abstract Parameterizations of turbulent transfer through the oceanic boundary layer beneath an ice shelf are tested using direct measurements of basal ablation. Observations were made in the southwestern part of Ronne Ice Shelf, about 500 km from open water. The mean basal ablation rate was measured over a month-long and a year-long period using phase-sensitive radar to record the thinning of the ice shelf. Ocean temperatures were observed within about 25 m of the ice shelf base over the period of the radar observations, while the tidally dominated ocean currents were estimated from tidal analysis of collocated current observations from an earlier period. Ablation rates derived using these ocean data and a number of bulk parameterizations of turbulent transfer within the boundary layer are compared with the direct measurements. The ablation rates derived using a parameterization that explicitly includes the impact of ocean currents on the turbulent transfer of heat and salt match the observations to within 40%; with suitable tuning of the drag coefficient, the mismatch can be reduced below the level of the observational errors. Equally good agreement can be obtained with two slightly simpler, current-dependent parameterizations that use constant turbulent transfer coefficients, and the optimal values for the coefficients at this particular location on Ronne Ice Shelf are given.

Effects of model resolution on ice shelf-ocean boundary layer

2020 ◽  
Author(s):  
Alena Malyarenko ◽  
Stefan Jendersie ◽  
Mike Williams ◽  
Natalie Robinson ◽  
Pat Langhorne
Keyword(s):  
Boundary Layer ◽  
Layer Structure ◽  
Ablation Rate ◽  
Ocean Model ◽  
Turbulent Transfer ◽  
Model Resolution ◽  
Transfer Coefficients ◽  
Ice Shelf ◽  
Boundary Layer Structure ◽  
Ocean Boundary

&lt;p&gt;&lt;span&gt;Boundary layer mixing at the ice-ocean thermodynamic interface is represented by&amp;#160;turbulent transfer coefficients, &amp;#915;&lt;sub&gt;T&lt;/sub&gt;&amp;#160;and &amp;#915;&lt;sub&gt;S&lt;/sub&gt;. Commonly used expressions for these are based on observations at the sea ice-ocean and ice shelf-ocean boundaries, and result in values ranging over 5 orders of magnitude (10&lt;sup&gt;-7&lt;/sup&gt;&lt; &amp;#915;&lt;sub&gt;T&lt;/sub&gt;&lt; 10&lt;sup&gt;-2&lt;/sup&gt;). To demonstrate the potential effect of the choice of turbulent transfer parameterisation we applied all of the available transfer coefficient values (12) to an idealised ice shelf-ocean cavity model experiment using the ISOMIP domain with ROMS. &lt;/span&gt;The mean ablation rate in warm cavity scenarios varies between 2.1 and 4.7 m/year, and in cold cavity scenarios between 0.03 and 0.17 m/year.&lt;/p&gt;&lt;p&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;&amp;#915;&lt;sub&gt;T&lt;/sub&gt;&amp;#160;and &amp;#915;&lt;sub&gt;S &lt;/sub&gt;not only directly determine the ablation rate, but have effects on fresh water distribution in the ocean boundary layer. High &amp;#915; values develop deep mixed layers, while low &amp;#915; values stratify the top ocean grid cells. Thus the ocean boundary layer structure directly depends on vertical resolution in the ocean model and how well the mixing scheme can handle the stratification effects.&amp;#160;&lt;/span&gt;&lt;span&gt;The experiment results we are presenting here include comprehensively tested and quantified effects of tidal forcing, mixing schemes, vertical flux distribution and ocean model resolution on the ablation rates and the ocean boundary layer structure.&lt;/span&gt;&lt;/p&gt;

scholarly journals Observations of turbulence beneath sea ice in southern McMurdo Sound, Antarctica

Ocean Science ◽  
2009 ◽  
Vol 5 (4) ◽  
pp. 435-445 ◽  
Author(s):  
C. L. Stevens ◽  
N. J. Robinson ◽  
M. J. M. Williams ◽  
T. G. Haskell
Keyword(s):  
Sea Ice ◽  
Water Column ◽  
Supercooled Water ◽  
Turbulent Energy ◽  
Small Scale ◽  
Initial Growth ◽  
Ice Shelf ◽  
Mcmurdo Sound ◽  
Frazil Ice ◽  
Vertical Diffusivity

Abstract. The first turbulence profiler observations beneath land fast sea ice which is directly adjacent to an Antarctic ice shelf are described. The stratification in the 325 m deep water column consisted of a layer of supercooled water in the upper 40 m lying above a quasi-linearly stratified water column with a sharp step in density at mid-depth. Turbulent energy dissipation rates were on average 3×10−8 m2 s−3 with peak bin-averaged values reaching 4×10−7 m2 s−3. The local dissipation rate per unit area was estimated to be 10 m Wm−2 on average with a peak of 50 m Wm−2. These values are consistent with a moderate baroclinic response to the tides. The small-scale turbulent energetics lie on the boundary between isotropy and buoyancy-affected. This will likely influence the formation and aggregation of frazil ice crystals within the supercooled layer. The data suggest that the large crystals observed in McMurdo Sound will transition from initial growth at scales smaller than the Kolmogorov lengthscale to sizes substantially (1–2 orders of magnitude) greater than the Kolmogorov scale. An estimate of the experiment-averaged vertical diffusivity of mass Kρ yields a coefficient of around 2×10−4 m2s−1 although this increased by a factor of 2 near the surface. Combining this estimate of Kρ with available observations of average and maximum currents suggests the layer of supercooled water can persist for a distance of ~250 km from the front of the McMurdo Ice Shelf.

scholarly journals Coupling framework (1.0) for the PISM (1.1.4) ice sheet model and the MOM5 (5.1.0) ocean model via the PICO ice shelf cavity model in an Antarctic domain

2021 ◽  
Vol 14 (6) ◽  
pp. 3697-3714
Author(s):  
Moritz Kreuzer ◽  
Ronja Reese ◽  
Willem Nicholas Huiskamp ◽  
Stefan Petri ◽  
Torsten Albrecht ◽  
...  
Keyword(s):  
General Circulation ◽  
Ice Sheet ◽  
Ocean Model ◽  
Global Ocean ◽  
Cavity Model ◽  
Energy Fluxes ◽  
Ice Shelf ◽  
Antarctic Ice Sheet ◽  
Wide Range ◽  
The Antarctic

Abstract. The past and future evolution of the Antarctic Ice Sheet is largely controlled by interactions between the ocean and floating ice shelves. To investigate these interactions, coupled ocean and ice sheet model configurations are required. Previous modelling studies have mostly relied on high-resolution configurations, limiting these studies to individual glaciers or regions over short timescales of decades to a few centuries. We present a framework to couple the dynamic ice sheet model PISM (Parallel Ice Sheet Model) with the global ocean general circulation model MOM5 (Modular Ocean Model) via the ice shelf cavity model PICO (Potsdam Ice-shelf Cavity mOdel). As ice shelf cavities are not resolved by MOM5 but are parameterized with the PICO box model, the framework allows the ice sheet and ocean components to be run at resolutions of 16 km and 3∘ respectively. This approach makes the coupled configuration a useful tool for the analysis of interactions between the Antarctic Ice Sheet and the global ocean over time spans of the order of centuries to millennia. In this study, we describe the technical implementation of this coupling framework: sub-shelf melting in the ice sheet component is calculated by PICO from modelled ocean temperatures and salinities at the depth of the continental shelf, and, vice versa, the resulting mass and energy fluxes from melting at the ice–ocean interface are transferred to the ocean component. Mass and energy fluxes are shown to be conserved to machine precision across the considered component domains. The implementation is computationally efficient as it introduces only minimal overhead. Furthermore, the coupled model is evaluated in a 4000 year simulation under constant present-day climate forcing and is found to be stable with respect to the ocean and ice sheet spin-up states. The framework deals with heterogeneous spatial grid geometries, varying grid resolutions, and timescales between the ice and ocean component in a generic way; thus, it can be adopted to a wide range of model set-ups.

scholarly journals On the Conditional Frazil Ice Instability in Seawater

2015 ◽  
Vol 45 (4) ◽  
pp. 1121-1138 ◽  
Author(s):  
James R. Jordan ◽  
Satoshi Kimura ◽  
Paul R. Holland ◽  
Adrian Jenkins ◽  
Matthew D. Piggott
Keyword(s):  
Sea Ice ◽  
Water Column ◽  
Freezing Point ◽  
Initial Perturbation ◽  
Ocean Model ◽  
Ice Shelf ◽  
High Background ◽  
Ice Shelves ◽  
Ice Growth ◽  
Frazil Ice

AbstractIt has been suggested that the presence of frazil ice can lead to a conditional instability in seawater. Any frazil forming in the water column reduces the bulk density of a parcel of frazil–seawater mixture, causing it to rise. As a result of the pressure decrease in the freezing point, this causes more frazil to form, causing the parcel to accelerate, and so on. This study uses linear stability analysis and a nonhydrostatic ocean model to study this instability. The authors find that frazil ice growth caused by the rising of supercooled water is indeed able to generate a buoyancy-driven instability. Even in a gravitationally stable water column, the frazil ice mechanism can still generate convection. The instability does not operate in the presence of strong density stratification, high thermal driving (warm water), a small initial perturbation, high background mixing, or the prevalence of large frazil ice crystals. In an unstable water column, the instability is not necessarily expressed in frazil ice at all times; an initial frazil perturbation may melt and refreeze. Given a large enough initial perturbation, this instability can allow significant ice growth. A model shows frazil ice growth in an Ice Shelf Water plume several kilometers from an ice shelf, under similar conditions to observations of frazil ice growth under sea ice. The presence of this instability could be a factor affecting the growth of sea ice near ice shelves, with implications for Antarctic Bottom Water formation.

scholarly journals Adjoint sensitivities of sub-ice-shelf melt rates to ocean circulation under the Pine Island Ice Shelf, West Antarctica

2012 ◽  
Vol 53 (60) ◽  
pp. 59-69 ◽  
Author(s):  
Patrick Heimbach ◽  
Martin Losch
Keyword(s):  
Ocean Circulation ◽  
General Circulation ◽  
Three Dimensional ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Ocean Model ◽  
West Antarctica ◽  
Ice Shelf ◽  
Algorithmic Differentiation ◽  
Freshwater Fluxes

AbstractWe investigate the sensitivity of sub-ice-shelf melt rates under Pine Island Ice Shelf, West Antarctica, to changes in the oceanic state using an adjoint ocean model that is capable of representing the flow in sub-ice-shelf cavities. The adjoint code is based on algorithmic differentiation (AD) of the Massachusetts Institute of Technology’s ocean general circulation model (MITgcm). The adjoint model was extended by adding into the AD process the corresponding sub-ice-shelf cavity code, which implements a three-equation thermodynamic melt-rate parameterization to infer heat and freshwater fluxes at the ice-shelf/ocean boundary. The inferred sensitivities reveal dominant timescales of 30–60 days over which the shelf exit is connected to the deep interior via advective processes. They exhibit rich three-dimensional time-evolving patterns that can be understood in terms of a combination of the buoyancy forcing by inflowing water masses, the cavity geometry and the effect of rotation and topography in steering the flow in the presence of prominent features in the bedrock bathymetry. Dominant sensitivity pathways are found over a sill, as well as ‘shadow regions’ of very low sensitivities. To the extent that these transient patterns are robust they carry important information for decision-making in observation deployment and monitoring.

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