Sheet, stream, and shelf flow as progressive ice-bed uncoupling: Byrd Glacier, Antarctica and Jakobshavn Isbrae, Greenland

Abstract. Ice properties inferred from multi-polarization measurements, such as birefringence and crystal orientation fabric (COF), can provide insight into ice strain, viscosity, and ice flow. In 2008, the Center for Remote Sensing of Ice Sheets (CReSIS) used a ground-based VHF (very high frequency) radar to take multi-channel and multi-polarization measurements around the NEEM (North Greenland Eemian Ice Drilling) site. The system operated with 30 MHz bandwidth at a center frequency of 150 MHz. This paper describes the radar system, antenna configurations, data collection, and processing and analysis of this data set. Within the framework derived from uniaxial ice crystal model, we found that ice birefringence dominates the power variation patterns of co-polarization and cross-polarization measurements in the area of 100 km 2 around the ice core site. The phase shift between ordinary and extraordinary waves increases nonlinearly with depth. The ice optic axis lies in planes that are close to the vertical plane and perpendicular or parallel to the ice divide depending on depth. The ice optic axis has an average tilt angle of about 11.6 • vertically, and its plane may rotate either clockwise or counterclockwise by about 10 • across the 100 km 2 area, and at a specific location the plane may rotate slightly counterclockwise as depth increases. Comparisons between the radar observations, simulations, and ice core fabric data are in very good agreement. We calculated the effective colatitude at different depths by using azimuth and colatitude measurements of the c axis of ice crystals. We obtained an average effective c axis tilt angle of 9.6 • from the vertical axis, very comparable to the average optic axis tilt angle estimated from radar polarization measurements. The comparisons give us confidence in applying this polarimetric radio echo sounding technique to infer profiles of ice fabric in locations where there are no ice core measurements.

Section: Introductionmentioning

confidence: 99%

Multi-channel and multi-polarization radar measurements around the NEEM site

González

Leuschen

et al. 2018

“…Both studies employed the analytic forcebalance approach. Reusch and Hughes (2003), Hughes (2009a), and Hughes et al (2011Hughes et al ( , 2016 discuss force balance on Byrd Glacier from the geometric perspective and take issue with the analytic approach of Whillans et al (1989). None of these studies explicitly shows how the various resistive forces vary along the glacier and, instead, largely base their discussion on how the basal buoyancy, ϕ, varies upstream of the grounding line.…”

Section: Geometric Approach: Application To Byrd Glacier Antarcticamentioning

confidence: 99%

“…In a long series of papers, T. Hughes presents the geometric approach to the balance of forces acting on ice shelves, ice streams, and interior ice (Hughes, 1986(Hughes, , 1992(Hughes, , 1998(Hughes, , 2003(Hughes, , 2009a(Hughes, , b, 2012Hughes et al, 2011Hughes et al, , 2016. Rather than working his way through the basic equations, as done by most other investigators, including Van der Veen and Whillans (1989) and Van der Veen (2013), he presents derivations based on graphical interpretation of triangles representing forces acting on an ice column.…”

Section: Introductionmentioning

confidence: 99%

Basal buoyancy and fast-moving glaciers: in defense of analytic force balance

Veen

2016

Abstract. The geometric approach to force balance advocated by T. Hughes in a series of publications has challenged the analytic approach by implying that the latter does not adequately account for basal buoyancy on ice streams, thereby neglecting the contribution to the gravitational driving force associated with this basal buoyancy. Application of the geometric approach to Byrd Glacier, Antarctica, yields physically unrealistic results, and it is argued that this is because of a key limiting assumption in the geometric approach. A more traditional analytic treatment of force balance shows that basal buoyancy does not affect the balance of forces on ice streams, except locally perhaps, through bridging effects.

“…His σ F is my flotation stress, which does not appear in my 2003 paper. It appears in Hughes (2012a) and in Hughes et al (2016). Van der Veen (2016, p. 1333 states my σ F is his R xx in his Eq.…”

Section: Introductionmentioning

confidence: 99%

“…Cornelis "Kees" Van der Veen's comparison of geometric and analytic approaches to the force balance in glaciology in The Cryosphere ( Van der Veen, 2016) is most welcome because he takes seriously my geometrical approach to the longitudinal force balance (Hughes et al, 2016). The geometric force balance is useful only for one-dimensional flow along ice-sheet flow lines or flow bands of constant width.…”

Section: Introductionmentioning

confidence: 99%

Reply to “Basal buoyancy and fast-moving glaciers: in defense of analytic force balance” by C. J. van der Veen (2016)

Hughes

2017

Abstract. Two approaches to ice-sheet modeling are available. Analytical modeling is the traditional approach (Van der Veen, 2016). It solves the force (momentum), mass, and energy balances to obtain three-dimensional solutions over time, beginning with the Navier-Stokes equations for the force balance. Geometrical modeling employs simple geometry to solve the force and mass balance in one dimension along ice flow (Hughes, 2012a). It is useful primarily to provide the first-order physical basis of ice-sheet modeling for students with little background in mathematics. The geometric approach uses changes in ice-bed coupling along flow to calculate changes in ice elevation and thickness, using a floating fraction φ along a flow line or flow band, where φ = 0 for sheet flow, 0 < φ < 1 for stream flow, and φ = 1 for shelf flow. An attempt is made to reconcile the two approaches.