Hierarchy and sea ice mechanics: A case study from the Beaufort Sea

[1] A method is presented to calculate the continuum-scale sea ice stress as an imposed, continuum-scale strain-rate is varied. The continuum-scale stress is calculated as the areaaverage of the stresses within the floes and leads in a region (the continuum element). The continuum-scale stress depends upon: the imposed strain rate; the subcontinuum scale, material rheology of sea ice; the chosen configuration of sea ice floes and leads; and a prescribed rule for determining the motion of the floes in response to the continuum-scale strain-rate. We calculated plastic yield curves and flow rules associated with subcontinuum scale, material sea ice rheologies with elliptic, linear and modified Coulombic elliptic plastic yield curves, and with square, diamond and irregular, convex polygon-shaped floes. For the case of a tiling of square floes, only for particular orientations of the leads have the principal axes of strain rate and calculated continuum-scale sea ice stress aligned, and these have been investigated analytically. The ensemble average of calculated sea ice stress for square floes with uniform orientation with respect to the principal axes of strain rate yielded alignment of average stress and strain-rate principal axes and an isotropic, continuum-scale sea ice rheology. We present a lemon-shaped yield curve with normal flow rule, derived from ensemble averages of sea ice stress, suitable for direct inclusion into the current generation of sea ice models. This continuum-scale sea ice rheology directly relates the size (strength) of the continuum-scale yield curve to the material compressive strength.

Section: Summary and Discussionmentioning

confidence: 99%

Continuum sea ice rheology determined from subcontinuum mechanics

Taylor¹,

Feltham²,

Sammonds³

et al. 2006

“…In addition, we make use of energetically consistent failure characteristics of thin ice following a yield curve with functional form close to that observed in the laboratory [Schulson and Nickolayev, 1995] for biaxial failure of columnar sea ice. The conceptual idea here is that because of the analog character of physical mechanisms, with the appropriate physical model, transcending several spatial scales rather than restricting ourselves to "adjacent" spatial scales as suggested, for example, by Overland et al [1995], should be possible.…”

Section: Failure Characteristicsmentioning

confidence: 99%

On modeling the anisotropic failure and flow of flawed sea ice

Hibler¹,

Schulson²

2000

121

131

Abstract. The failure and flow of sea ice on scales small and large is characterized by the propagation of oriented leads and cracks. In this paper a theory for the dynamical treatment of anisotropic oriented flaws in sea ice is developed and used to examine the interaction of these oriented flaws under idealized stress forcing. The essential idea of the theory is to take one or more oriented weak leads imbedded in thick ice. A constitutive law for both the thin and thick ice is taken to be similar to laboratory observations, which are consistent with Mohr Coulomb-like failure. Application of normal continuum mechanics boundary conditions leads to anisotropic flow and failure characteristics of this anisotropic composite under both near-and far-field forcing. The local failure characteristics show that there is a preferred orientation for failure with the intersection between "leads" increasing as confinement increases. For spatially separated interacting flaws the theory predicts a preference for the weakening of flaws in a narrow range of angles of-10ø-20 ø relative to the principal far-field stress. Imbedding isolated flaws leads to simulated damage directions consistent with local failure characteristics. In the case where the ice is considered to have available flaws in all directions, fracture propagation is found to proceed by picking out oriented flaws alternating in direction along large-scale damage strikes with individual flaw alignments of -20 ø relative to far-field principal stresses. The mechanisms responsible for these results and how this anisotropic sea ice rheology might be implemented in current dynamic/thermodynamic sea ice models are discussed. IntroductionThe failure and flow of sea ice on scales both small and large are characterized by the propagation of oriented leads and cracks. These patterns typically nucleate from local imperfections and flaws. Understanding and modeling the physical processes responsible for these oriented patterns is especially relevant to describing the mechanical characteristics of geophysical-scale sea ice under deformation and is relevant to the role of sea ice in global climate change and in ice forecasting applications.As Since we believe these oriented leads and slip lines to nucleate from local imperfections that are themselves oriented, an anisotropic description of individual elements of the ice is clearly critical in describing the aggregate behavior of this composite system. In much work on the small scale, anisotropic behavior has been described by fixed friction models of cracks [see, e.g., Schulson and Nickolayev, 1995]. This concept has also been proposed in modified form by Coon et al. [1992, 1998] as the foundation of an anisotropic sea ice model on the geophysical scale. In the theoretical model proposed here a different approach is used. Specifically, rather than assume a fixed lead orientation model for leads, we assume that both thick ice and thinner ice in oriented leads or flaws have a rheology similar in form to that observed for columnar ...

“…The displacements varied in October and then were nearly constant, at least for the inner array, until the end of January. We believe that large deformation and stress events were not possible until the ice strength had increased between the camp and shore, such that the coastal boundary participated in establishing the internal stress field [Overland et al, 1995]. Similar to the findings of Melling and Riedel [1996] for 1991-1992, we conclude that there are a few major weather/ice deformation events each winter, i.e., two-four, rather than a large number of events, that affect the ice dynamics on regional scales.…”

Section: Introductionmentioning

confidence: 99%

Arctic sea ice as a granular plastic

Overland¹,

McNutt²,

Salo³

et al. 1998

Self Cite

Abstract. An important consideration in understanding sea ice mechanics is the integration of observed sea ice behavior on a floe neighborhood scale (1-10 km) into ice dynamics on a regional scale 0(50 km). We investigate sea ice kinematics from October 1993 through April 1994 using relative motions from 13 drifting buoys with Global Positioning System navigation in a 20-km array centered on the Sea Ice Mechanics Initiative ice camp, and we compare these motions to synthetic aperture radar (SAR)-derived ice velocities over a 100-by 500-km region in the Beaufort Sea. There is excellent correspondence between the deformation of the buoy array and that from the SAR. Inferred ice dynamics from analysis of the two major northerly wind convergence events of the winter are consistent with a granular hardening plastic conceptual model for Beaufort sea ice. Under continued northerly winds the ice from the Alaskan shore to the camp failed in shear and convergence, in a progressive manner away from the coast. The continuum scale O(10 km) is an order of magnitude larger than the grain, i.e., floe, size O(1 km). The ice motion often forms aggregates of 20-200 km separated by narrow (<10 km) shear zones, similar to granular materials. At moderate forcing, i.e., wind stress multiplied by fetch, the ice appears to fail along slip lines that occur at an acute angle to each other and to the direction of the wind forcing, characteristic of a plastic material at critical state. With longer fetch the ice appears to fail in compression, perpendicular to the wind direction. Sea ice appeared to harden on a regional scale after the first event. During the second northerly wind event there was a sea ice breakout toward the west, apparently due to a lack of lateral confining stress. Our observations suggest that the ice floes advect through relatively stationary stress fields, created by the wind forcing and coastal boundaries. For example, while the SAR and advanced very high resolution radiometer images indicated the presence of the shear feature at the same geographic location for nearly a week, buoys would show shearing only for several days as they transited across the region of shear. There is a high correspondence between the major internal ice deformation events and persistent weather patterns on a 3-to 5-day temporal scale. This implies that SAR data collection and analysis for regional sea ice dynamics should be consistent with the wind forcing and have a sampling of less than 3 days.