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John J Cassano - One of the best experts on this subject based on the ideXlab platform.
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alexander tall tower a study of the boundary layer on the Ross Ice Shelf antarctica
Journal of Applied Meteorology and Climatology, 2017Co-Authors: Marian E Mateling, Matthew A Lazzara, Linda M Keller, George A Weidner, John J CassanoAbstract:AbstractBecause of the harsh weather conditions on the Antarctic continent, year-round observations of the low-level boundary layer must be obtained via automated data acquisition systems. Alexander Tall Tower! is an automatic weather station on the Ross Ice Shelf in Antarctica and has been operational since February 2011. At 30 m tall, this station has six levels of instruments to collect environmental data, including temperature, wind speed and direction, relative humidity, and pressure. Data are collected at 30-, 15-, 7.5-, 4-, 2-, and 1-m levels above the snow surface. This study identifies short-term trends and provides an improved description of the lowest portion of the boundary layer over this portion of the Ross Ice Shelf for the February 2011–January 2014 period. Observations indicate two separate initiations of the winter season occur annually, caused by synoptic-scale anomalies. Sensible and latent heat flux estimates are computed using Monin–Obukhov similarity theory and vertical profiles of ...
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evaluation of the amps boundary layer simulations on the Ross Ice Shelf antarctica with unmanned aircraft observations
Journal of Applied Meteorology and Climatology, 2017Co-Authors: Jonathan D Wille, Melissa A Nigro, John J Cassano, David H Bromwich, Marian E Mateling, Matthew A LazzaraAbstract:AbstractAccurately predicting moisture and stability in the Antarctic planetary boundary layer (PBL) is essential for low-cloud forecasts, especially when Antarctic forecasters often use relative humidity as a proxy for cloud cover. These forecasters typically rely on the Antarctic Mesoscale Prediction System (AMPS) Polar Weather Research and Forecasting (Polar WRF) Model for high-resolution forecasts. To complement the PBL observations from the 30-m Alexander Tall Tower! (ATT) on the Ross Ice Shelf as discussed in a recent paper by Wille and coworkers, a field campaign was conducted at the ATT site from 13 to 26 January 2014 using Small Unmanned Meteorological Observer (SUMO) aerial systems to collect PBL data. The 3-km-resolution AMPS forecast output is combined with the global European Centre for Medium-Range Weather Forecasts interim reanalysis (ERAI), SUMO flights, and ATT data to describe atmospheric conditions on the Ross Ice Shelf. The SUMO comparison showed that AMPS had an average 2–3 m s−1 high...
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the surface climatology of the Ross Ice Shelf antarctica
International Journal of Climatology, 2016Co-Authors: Carol A Costanza, Linda M Keller, Matthew A Lazzara, John J CassanoAbstract:The University of Wisconsin-Madison Antarctic Automatic Weather Station (AWS) project has been making meteorological surface observations on the Ross Ice Shelf (RIS) for approximately 30 years. This network offers the most continuous set of routine measurements of surface meteorological variables in this region. The Ross Island area is excluded from this study. The surface climate of the RIS is described using the AWS measurements. Temperature, pressure, and wind data are analysed on daily, monthly, seasonal, and annual time periods for 13 AWS acRoss the RIS. The AWS are separated into three representative regions - central, coastal, and the area along the Transantarctic Mountains - in order to describe specific characteristics of sections of the RIS. The climatology describes general characteristics of the region and significant changes over time. The central AWS experiences the coldest mean temperature, and the lowest resultant wind speed. These AWSs also experience the coldest potential temperatures with a minimum of 209.3 K at Gill AWS. The AWS along the Transantarctic Mountains experiences the warmest mean temperature, the highest mean sea-level pressure, and the highest mean resultant wind speed. Finally, the coastal AWS experiences the lowest mean pressure. Climate indIces (MEI, SAM, and SAO) are compared to temperature and pressure data of four of the AWS with the longest observation periods, and significant correlation is found for most AWS in sea-level pressure and temperature. This climatology study highlights characteristics that influence the climate of the RIS, and the challenges of maintaining a long-term Antarctic AWS network. Results from this effort are essential for the broader Antarctic meteorology community for future research.
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evaluation of the amps boundary layer simulations on the Ross Ice Shelf with tower observations
Journal of Applied Meteorology and Climatology, 2016Co-Authors: Jonathan D Wille, Melissa A Nigro, John J Cassano, David H Bromwich, Marian E Mateling, Matthew A Lazzara, Shenghung WangAbstract:AbstractFlight operations in Antarctica rely on accurate weather forecasts aided by the numerical predictions primarily produced by the Antarctic Mesoscale Prediction System (AMPS) that employs the polar version of the Weather Research and Forecasting (Polar WRF) Model. To improve the performance of the model’s Mellor–Yamada–Janjic (MYJ) planetary boundary layer (PBL) scheme, this study examines 1.5 yr of meteorological data provided by the 30-m Alexander Tall Tower! (ATT) automatic weather station on the western Ross Ice Shelf from March 2011 to July 2012. Processed ATT observations at 10-min intervals from the multiple observational levels are compared with the 5-km-resolution AMPS forecasts run daily at 0000 and 1200 UTC. The ATT comparison shows that AMPS has fundamental issues with moisture and handling stability as a function of wind speed. AMPS has a 10-percentage-point (i.e., RH unit) relative humidity dry bias year-round that is highest when katabatic winds from the Byrd and Mulock Glaciers excee...
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characteristics of the near surface atmosphere over the Ross Ice Shelf antarctica
Journal of Geophysical Research, 2016Co-Authors: Melissa A Nigro, John J Cassano, Matthew A LazzaraAbstract:Two years of data from a 30 m instrumented tower are used to characterize the near-surface atmospheric state over the Ross Ice Shelf, Antarctica. Stable stratification dominates the surface layer at this site, occurring 83% of the time. The strongest inversions occur for wind speeds less than 4 m s−1 and the inversion strength decreases rapidly as wind speed increases above 4 m s−1. In summer unstable stratification occurs 50% of the time and unstable conditions are observed in every season. A novel aspect of this work is the use of an artificial neural network pattern identification technique, known as self-organizing maps, to objectively identify characteristic potential temperature profiles that span the range of profiles present in the 2 year study period. The self-organizing map clustering technique allows the more than 100,000 observed potential temperature profiles to be represented by just 30 patterns. The pattern-averaged winds show distinct and physically consistent relationships with the potential temperature profiles. The strongest winds occur for the nearly well mixed but slightly stable patterns and the weakest winds occur for the strongest inversion patterns. The weakest wind shear over the depth of the tower occurs for slightly unstable profiles and the largest wind shear occurs for moderately strong inversions. Pattern-averaged log wind profiles are consistent with theoretical expectations. The log wind profiles exhibit a kinked profile for the strongest inversion cases indicative of decoupling of the winds between the bottom and top of the tower.
Christina L Hulbe - One of the best experts on this subject based on the ideXlab platform.
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lifting the lid nitrifying archaea sustain diverse microbial communities below the Ross Ice Shelf
Social Science Research Network, 2020Co-Authors: Clara Martinezperez, Craig Stevens, Christina L Hulbe, Christian Ohneiser, Chris Greening, Zihao Zhao, Rachael Lappan, Sean K Bay, Daniele De Corte, Blair ThomsonAbstract:The oceanic waters beneath Antarctica’s Ross Ice Shelf are among the largest unexplored habitats on Earth. Previous studies have shown active microorganisms reside in these waters, but their identity and capabilities remain unknown. Here, we combined multi-omics and biogeochemical measurements to determine the composition and metabolic activities of microbial communities in the water column under the Ross Ice Shelf. We reveal that these waters harbour diverse and distinct assemblages of microorganisms. In an ecosystem devoid of photosynthesis, the heterotrophic majority are largely sustained by highly active nitrifying archaea and bacteria (using ammonium from basal Ice) and by other chemolitoautotrophs using reduced sulfur compounds. This suggests that chemolitoautotrophy is an important ecosystem driver in the waters below the RIS. Our data provide a valuable insight into the processes that sustain the largest sub-Ice marine ecosystem, and will help predictions of the evolution of this system in a rapidly evolving climatic scenario.
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ocean mixing and heat transport processes observed under the Ross Ice Shelf control its basal melting
Proceedings of the National Academy of Sciences of the United States of America, 2020Co-Authors: Craig Stevens, Craig Stewart, Christina L Hulbe, Mike Brewer, N J Robinson, Christian Ohneiser, Stefan JendersieAbstract:The stability of large Antarctic Ice shelves has important implications for global sea level, sea Ice area, and ocean circulation. A significant proportion of Ice mass loss from these Ice shelves is through ocean-driven melting which is controlled by largely unobserved oceanic thermodynamic and circulatory processes in the cavity beneath the Ice Shelf. Here we use direct measurements to provide evidence of the changing water column structure in the cavity beneath the Ross Ice Shelf, the planet's largest Ice Shelf by area. The cavity water column data exhibit both basal and benthic boundary layers, along with evidence of tidally modulated and diffusively convecting internal mixing processes. A region of thermohaline interleaving in the upper-middle water column indicates elevated diffusion and the potential to modify the cavity circulation. The measurements were recorded using the Aotearoa New Zealand Ross Ice Shelf Program hot water drill borehole melted in the central region of the Shelf in December 2017 (HWD2), only the second borehole through the central region of the Ice Shelf, following J9 in 1977. These data, and comparison with the 1977 data, provide valuable insight into Ice Shelf cavity circulation and aid understanding of the evolution of the presently stable Ross Ice Shelf.
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mechanical analysis of pinning points in the Ross Ice Shelf antarctica
Annals of Glaciology, 2019Co-Authors: Holly Still, A J Campbell, Christina L HulbeAbstract:Ice shelves regulate the rate of Ice-sheet discharge along much of the Antarctic coastline. Pinning points, sites of localised grounding within floating Ice, can in turn, regulate the flow and thickness of an Ice Shelf. While the net resistive effect of Ice shelves has been quantified in a systematic way, few extant pinning points have been examined in detail. Here, complete force budgets are calculated and examined for Ice rises and rumples in the Ross Ice Shelf, West Antarctica. The diverse features have different effects on Ice Shelf mechanics that do not depend simply on their size but may, we conclude, depend on the properties of seafloor materials.
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structural provinces of the Ross Ice Shelf antarctica
Annals of Glaciology, 2017Co-Authors: Christine Marie Ledoux, Christina L Hulbe, Ted Scambos, Martin Forbes, Karen E AlleyAbstract:The surface of the Ross Ice Shelf (RIS) is textured by flow stripes, crevasses and other features related to Ice flow and deformation. Here, moderate resolution optical satellite images are used to map and classify regions of the RIS characterized by different surface textures. Because the textures arise from Ice deformation, the map is used to identify structural provinces with common deformation history. We classify four province types: regions associated with large outlet glaciers, shear zones, extension downstream of obstacles and suture zones between provinces with different upstream sources. Adjacent provinces with contrasting histories are in some locations deforming at different rates, suggesting that our province map is also an Ice fabric map. Structural provinces have more complicated shapes in the part of the Ice Shelf fed by West Antarctic Ice streams than in the part fed by outlet glaciers from the Transantarctic Mountains. The map may be used to infer past variations in stress conditions and flow events that cannot be inferred from flow traces alone.
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recent changes in the flow of the Ross Ice Shelf west antarctica
Earth and Planetary Science Letters, 2013Co-Authors: Christina L Hulbe, Ted Scambos, Choonki Lee, Jennifer Bohlander, Terry HaranAbstract:Abstract Comparison of surface velocities measured during the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS, 1973 to 1978) and velocities measured via feature tracking between two Moderate-resolution Imaging Spectroradiometer (MODIS) mosaics (compiled from 2003/4 and 2008/9 images) reveals widespread slowing and minor areas of acceleration in the Ross Ice Shelf (RIS) over the approximately 30 year interval. The largest changes ( − 13 ma − 2 ) occur near the Whillans and Mercer Ice Streams grounding line in the southernmost part of the Ice Shelf. Speed has increased over the interval (up to 5 ma − 2 ) between the MacAyeal Ice Stream grounding line and the Shelf front, and along the eastern Shelf front. Changes in Ice thickness computed using IceSat laser altimetry are used together with a well-tested model of the Ice Shelf to investigate underlying causes of change in the flow of the Ice Shelf over time. The observed transients represent a combination of recent forcings and ongoing response to Ice stream discharge variations over the past millennium. While evidence of older events may be present, the modern signal is dominated by shorter time scale events, including the stagnation of Kamb Ice Stream about 160 years ago, recent changes in basal drag on the Whillans Ice Stream Ice plain and, perhaps, Iceberg calving. Details in embayment geometry, for example the shallow sea floor below Crary Ice Rise, modulate the spatial pattern of Ice Shelf response to boundary condition perturbations.
Douglas R Macayeal - One of the best experts on this subject based on the ideXlab platform.
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tidal modulation of Ice Shelf flow a viscous model of the Ross Ice Shelf
Journal of Glaciology, 2014Co-Authors: Kelly M Brunt, Douglas R MacayealAbstract:Three stations near the calving front of the Ross Ice Shelf, Antarctica, recorded GPS data through a full spring-neap tidal cycle in November 2005. The data revealed a diurnal horizontal motion that varied both along and transverse to the long-term average velocity direction, similar to tidal signals observed in other Ice shelves and Ice streams. Based on its periodicity, it was hypothesized that the signal represents a flow response of the Ross Ice Shelf to the diurnal tides of the Ross Sea. To assess the influence of the tide on the Ice-Shelf motion, two hypotheses were developed. The first addressed the direct response of the Ice Shelf to tidal forcing, such as forces due to sea-surface slopes or forces due to sub-Ice-Shelf currents. The second involved the indirect response of Ice-Shelf flow to the tidal signals observed in the Ice streams that source the Ice Shelf. A finite-element model, based on viscous creep flow, was developed to test these hypotheses, but succeeded only in falsifying both hypotheses, i.e. showing that direct tidal effects produce too small a response, and indirect tidal effects produce a response that is not smooth in time. This nullification suggests that a combination of viscous and elastic deformation is required to explain the observations.
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flow of the Ross Ice Shelf antarctica is modulated by the ocean tide
Journal of Glaciology, 2010Co-Authors: Kelly M Brunt, H A Fricker, Matt A King, Douglas R MacayealAbstract:The Ice streams feeding the Ross Ice Shelf, Antarctica, have large tidally modulated (sinusoidal and stick-slip) flow, but the interaction with the Ice Shelf is poorly understood. We show that the flow of the Ross Ice Shelf front, up to ~650 km from the Ice streams, exhibits smooth, sinusoidal motions corresponding to tidal modulation. These observations suggest a possible linking of the Ice Shelf with the Ice streams to form a unified system that responds to small perturbations in stresses associated with ocean tides. If this is the case, the presence of the sinusoidal motion but the absence of stick-slip motion suggests there is damping of very high-frequency signals. The dissimilar signatures of the motions observed in the Ice streams and at the front of the Ice Shelf present challenges to model development aimed at understanding the dynamics of coupled Ice-stream/Ice-Shelf flow and the movement of Ice acRoss grounding lines.
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seismic observations of sea swell on the floating Ross Ice Shelf antarctica
Journal of Geophysical Research, 2009Co-Authors: L M Cathles, Emile A Okal, Douglas R MacayealAbstract:[1] A seismometer operating on the floating Ross Ice Shelf near its seaward Ice front (Nascent Iceberg) for 340 days (out of 730 days) during the 2004, 2005, and 2006 Antarctic field seasons recorded the arrival of 93 distantly sourced ocean swell events displaying frequency dispersion characteristic of surface gravity waves propagating on deep water. Comparison of swell event dispersion with the NOAA Wave Watch III (NWW3) ocean wave model analysis reveals that 83 of these events were linked to specific storms located in the Pacific, Southern, and Indian oceans. Nearly all major storms in the NWW3 analysis of the Pacific Ocean were linked to signals observed at the Nascent site during the period of seismometer operation. Swell-induced motion of the Ross Ice Shelf is found to increase by several orders of magnitude over the time period that sea Ice surrounding Antarctica decreases from its maximum extent (October) to its minimum extent (February). The amplitude of vertical vibration of the Ice Shelf in the frequency band between 0.025 and 0.14 Hz varies between tens of micrometers to millimeters as sea Ice decays to its minimum seasonal extent. This suggests that climate influence on sea Ice extent may indirectly modulate swell energy incident on the calving margins of the Antarctic Ice Sheet. The largest swell signals observed on the Ross Ice Shelf come from storms in the tropical Pacific and Gulf of Alaska. These remote events emphasize how the Iceberg calving margin of Antarctica is connected to environmental conditions well beyond Antarctica.
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Interactions of wind-transported snow with a rift in the Ross Ice Shelf, Antarctica
Geophysical Research Letters, 2008Co-Authors: Katherine Colby Leonard, Douglas R Macayeal, L. Bruno Tremblay, Stanley S JacobsAbstract:[1] Rifts in Ice shelves accumulate a melange of snow and firn from above and marine Ice from below, material that has been postulated to negatively influence Iceberg calving. From measurements and modeling we show that a 100 m wide rift near the front of the Ross Ice Shelf captures all wind-transported snow traveling in saltation and a substantial fraction of the snow blowing in suspension acRoss the rift. Moderate winds and snow transport appear sufficient to fill the rift in several years, whereas the melange surface profile remains relatively constant within the 30 m deep rift. Observed subsidence and ocean temperatures in the rift suggest that net basal melting dissolves the melange from below, limiting its effectiveness as an anti-calving agent near Ice fronts.
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transoceanic wave propagation links Iceberg calving margins of antarctica with storms in tropics and northern hemisphere
Geophysical Research Letters, 2006Co-Authors: Douglas R Macayeal, J N Bassis, Emile A Okal, R C Aster, Kelly M Brunt, Mac L Cathles, Robert Drucker, H A Fricker, Youngjin Kim, Seelye MartinAbstract:[1] We deployed seismometers on the Ross Ice Shelf and on various Icebergs adrift in the Ross Sea (including B15A, a large 100 km by 30 km fragment of B15, which calved from the Ross Ice Shelf in March, 2000). The data reveal that the dominant energy of these floating Ice masses is in the 0.01 to 0.1 Hz band, and is associated with sea swell generated in the tropical and extra-tropical Pacific Ocean. In one example, a strong storm in the Gulf of Alaska on 21 October 2005, approximately 13,500 km from the Ross Sea, generated swell that arrived at B15A immediately prior to, and during, its break-up off Cape Adare on 27 October 2005. If sea swell influences Iceberg calving and break-up, a teleconnection exists between the Antarctic Ice sheet mass balance and weather systems worldwide.
David H Bromwich - One of the best experts on this subject based on the ideXlab platform.
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evaluation of the amps boundary layer simulations on the Ross Ice Shelf antarctica with unmanned aircraft observations
Journal of Applied Meteorology and Climatology, 2017Co-Authors: Jonathan D Wille, Melissa A Nigro, John J Cassano, David H Bromwich, Marian E Mateling, Matthew A LazzaraAbstract:AbstractAccurately predicting moisture and stability in the Antarctic planetary boundary layer (PBL) is essential for low-cloud forecasts, especially when Antarctic forecasters often use relative humidity as a proxy for cloud cover. These forecasters typically rely on the Antarctic Mesoscale Prediction System (AMPS) Polar Weather Research and Forecasting (Polar WRF) Model for high-resolution forecasts. To complement the PBL observations from the 30-m Alexander Tall Tower! (ATT) on the Ross Ice Shelf as discussed in a recent paper by Wille and coworkers, a field campaign was conducted at the ATT site from 13 to 26 January 2014 using Small Unmanned Meteorological Observer (SUMO) aerial systems to collect PBL data. The 3-km-resolution AMPS forecast output is combined with the global European Centre for Medium-Range Weather Forecasts interim reanalysis (ERAI), SUMO flights, and ATT data to describe atmospheric conditions on the Ross Ice Shelf. The SUMO comparison showed that AMPS had an average 2–3 m s−1 high...
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evaluation of the amps boundary layer simulations on the Ross Ice Shelf with tower observations
Journal of Applied Meteorology and Climatology, 2016Co-Authors: Jonathan D Wille, Melissa A Nigro, John J Cassano, David H Bromwich, Marian E Mateling, Matthew A Lazzara, Shenghung WangAbstract:AbstractFlight operations in Antarctica rely on accurate weather forecasts aided by the numerical predictions primarily produced by the Antarctic Mesoscale Prediction System (AMPS) that employs the polar version of the Weather Research and Forecasting (Polar WRF) Model. To improve the performance of the model’s Mellor–Yamada–Janjic (MYJ) planetary boundary layer (PBL) scheme, this study examines 1.5 yr of meteorological data provided by the 30-m Alexander Tall Tower! (ATT) automatic weather station on the western Ross Ice Shelf from March 2011 to July 2012. Processed ATT observations at 10-min intervals from the multiple observational levels are compared with the 5-km-resolution AMPS forecasts run daily at 0000 and 1200 UTC. The ATT comparison shows that AMPS has fundamental issues with moisture and handling stability as a function of wind speed. AMPS has a 10-percentage-point (i.e., RH unit) relative humidity dry bias year-round that is highest when katabatic winds from the Byrd and Mulock Glaciers excee...
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a self organizing map based evaluation of the antarctic mesoscale prediction system using observations from a 30 m instrumented tower on the Ross Ice Shelf antarctica
AGUFM, 2015Co-Authors: Melissa A Nigro, John J Cassano, David H Bromwich, Jonathan D Wille, Matthew A LazzaraAbstract:AbstractAccurate representation of the stability of the surface layer in numerical weather prediction models is important because of the impact it has on forecasts of surface energy, moisture, and momentum fluxes. It also impacts boundary layer processes such as the generation of turbulence, the creation of near-surface flows, and fog formation. This paper uses observations from a 30-m automatic weather station on the Ross Ice Shelf, Antarctica, to evaluate the near-surface layer in the Antarctic Mesoscale Prediction System (AMPS), a numerical weather prediction system used for forecasting in Antarctica. The method of self-organizing maps (SOM) is used to identify characteristic potential temperature anomaly profiles observed at the 30-m tower. The SOM-identified profiles are then used to evaluate the performance of AMPS as a function of atmospheric stability.The results indicate AMPS underpredicts the frequency of near-neutral profiles and instead overpredicts the frequency of weakly unstable and weak to...
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a case study of a Ross Ice Shelf airstream event a new perspective
Monthly Weather Review, 2009Co-Authors: Daniel F Steinhoff, Saptarshi Chaudhuri, David H BromwichAbstract:Abstract A case study illustrating cloud processes and other features associated with the Ross Ice Shelf airstream (RAS), in Antarctica, is presented. The RAS is a semipermanent low-level wind regime primarily over the western Ross Ice Shelf, linked to the midlatitude circulation and formed from terrain-induced and large-scale forcing effects. An integrated approach utilizes Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery, automatic weather station (AWS) data, and Antarctic Mesoscale Prediction System (AMPS) forecast output to study the synoptic-scale and mesoscale phenomena involved in cloud formation over the Ross Ice Shelf during a RAS event. A synoptic-scale cyclone offshore of Marie Byrd Land draws moisture acRoss West Antarctica to the southern base of the Ross Ice Shelf. Vertical lifting associated with flow around the Queen Maud Mountains leads to cloud formation that extends acRoss the Ross Ice Shelf to the north. The low-level cloud has a warm signature in thermal infrare...
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climatological aspects of mesoscale cyclogenesis over the Ross sea and Ross Ice Shelf regions of antarctica
Monthly Weather Review, 1994Co-Authors: Jorge F Carrasco, David H BromwichAbstract:Abstract A one-year (1988) statistical study of mesoscale cyclogenesis near Terra Nova Bay and Byrd Glacier. Antarctica, was conducted using high-resolution digital satellite imagery and automatic weather station data. Results indicate that on average two (one) mesoscale cyclones form near Terra Nova Bay (Byrd Glacier) each week, confirming these two locations as mesoscale cyclogenesis areas. The maximum (minimum) weekly frequency of mesoscale cyclones occurred during the summer (winter). The satellite survey of mesoscale vortIces was extended over the Ross Sea and Ross Ice Shelf. Results suggest southern Maric Byrd Land as another area of mesoscale cyclone formation. Also, frequent mesoscale cyclonic activity was noted over the Ross Sea and Ross Ice Shelf, where, on average, six and three mesoscale vortIces were observed each week, respectively, with maximum (minimum) frequency during summer (winter) in both regions. The majority (70%–80%) of the vortIces were of comma-cloud type and were shallow. Only a...
Craig Stewart - One of the best experts on this subject based on the ideXlab platform.
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ocean mixing and heat transport processes observed under the Ross Ice Shelf control its basal melting
Proceedings of the National Academy of Sciences of the United States of America, 2020Co-Authors: Craig Stevens, Craig Stewart, Christina L Hulbe, Mike Brewer, N J Robinson, Christian Ohneiser, Stefan JendersieAbstract:The stability of large Antarctic Ice shelves has important implications for global sea level, sea Ice area, and ocean circulation. A significant proportion of Ice mass loss from these Ice shelves is through ocean-driven melting which is controlled by largely unobserved oceanic thermodynamic and circulatory processes in the cavity beneath the Ice Shelf. Here we use direct measurements to provide evidence of the changing water column structure in the cavity beneath the Ross Ice Shelf, the planet's largest Ice Shelf by area. The cavity water column data exhibit both basal and benthic boundary layers, along with evidence of tidally modulated and diffusively convecting internal mixing processes. A region of thermohaline interleaving in the upper-middle water column indicates elevated diffusion and the potential to modify the cavity circulation. The measurements were recorded using the Aotearoa New Zealand Ross Ice Shelf Program hot water drill borehole melted in the central region of the Shelf in December 2017 (HWD2), only the second borehole through the central region of the Ice Shelf, following J9 in 1977. These data, and comparison with the 1977 data, provide valuable insight into Ice Shelf cavity circulation and aid understanding of the evolution of the presently stable Ross Ice Shelf.
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basal melting of Ross Ice Shelf from solar heat absorption in an Ice front polynya
Nature Geoscience, 2019Co-Authors: Keith W Nicholls, Craig Stewart, Poul Christoffersen, Michael J M Williams, Julian A DowdeswellAbstract:Ice–ocean interactions at the bases of Antarctic Ice shelves are rarely observed, yet have a profound influence on Ice sheet evolution and stability. Ice sheet models are highly sensitive to assumed Ice Shelf basal melt rates; however, there are few direct observations of basal melting or the oceanographic processes that drive it, and consequently our understanding of these interactions remains limited. Here we use in situ observations from the Ross Ice Shelf to examine the oceanographic processes that drive basal ablation of the world’s largest Ice Shelf. We show that basal melt rates beneath a thin and structurally important part of the Shelf are an order of magnitude higher than the Shelf-wide average. This melting is strongly influenced by a seasonal inflow of solar-heated surface water from the adjacent Ross Sea Polynya that downwells into the Ice Shelf cavity, nearly tripling basal melt rates during summer. Melting driven by this frequently overlooked process is expected to increase with predicted surface warming. We infer that solar heat absorbed in Ice-front polynyas can make an important contribution to the present-day mass balance of Ice shelves, and potentially impact their future stability. High melt rates in a key location beneath the Ross Ice Shelf result from a seasonal inflow of water heated in the Ross Sea Polynya, according to in situ observations.
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ocean variability contributing to basal melt rate near the Ice front of Ross Ice Shelf antarctica
Journal of Geophysical Research, 2014Co-Authors: Isabella B Arzeno, Craig Stewart, Scott R Springer, Robert C Beardsley, Richard Limeburner, Brechner W Owens, Laurie Padman, Michael J M WilliamsAbstract:Basal melting of Ice shelves is an important, but poorly understood, cause of Antarctic Ice sheet mass loss and freshwater production. We use data from two moorings deployed through Ross Ice Shelf, ∼6 and ∼16 km south of the Ice front east of Ross Island, and numerical models to show how the basal melting rate near the Ice front depends on sub-Ice-Shelf ocean variability. The moorings measured water velocity, conductivity, and temperature for ∼2 months starting in late November 2010. About half of the current velocity variance was due to tides, predominantly diurnal components, with the remainder due to subtidal oscillations with periods of a few days. Subtidal variability was dominated by barotropic currents that were large until mid-December and significantly reduced afterward. Subtidal currents were correlated between moorings but uncorrelated with local winds, suggesting the presence of waves or eddies that may be associated with the abrupt change in water column thickness and strong hydrographic gradients at the Ice front. Estimated melt rate was ∼1.2 ± 0.5 m a−1 at each site during the deployment period, consistent with measured trends in Ice surface elevation from GPS time series. The models predicted similar annual-averaged melt rates with a strong annual cycle related to seasonal provision of warm water to the Ice base. These results show that accurately modeling the high spatial and temporal ocean variability close to the Ice-Shelf front is critical to predicting time-dependent and mean values of meltwater production and Ice-Shelf thinning.
