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Identifying fast and robust numerical solvers is a critical issue that needs to be addressed in order to improve projections of polar ice sheets evolving in a changing climate. This work evaluates the impact of using advanced numerical solvers for transient ice-flow simulations conducted with the JPL-UCI Ice Sheet System Model (ISSM). We identify optimal numerical solvers by testing a broad suite of readily available solvers, ranging from direct sparse solvers to preconditioned iterative methods, on the commonly used Ice Sheet Model Intercomparison Project for Higher-Order ice sheet Models benchmark tests. Three types of analyses are considered: mass transport, horizontal stress balance, and incompressibility. The results of the fastest solvers for each analysis type are ranked based on their scalability across mesh size and basal boundary conditions. We find that the fastest iterative solvers are ĝ1/4ĝ€1.5-100 times faster than the default direct solver used in ISSM, with speed-ups improving rapidly with increased mesh resolution. We provide a set of recommendations for users in search of efficient solvers to use for transient ice-flow simulations, enabling higher-resolution meshes and faster turnaround time. The end result will be improved transient simulations for short-term, highly resolved forward projections (10-100 year time scale) and also improved long-term paleo-reconstructions using higher-order representations of stresses in the ice. This analysis will also enable a new generation of comprehensive uncertainty quantification assessments of forward sea-level rise projections, which rely heavily on ensemble or sampling approaches that are inherently expensive.

© 2015 Elsevier B.V. Following the 2002 disintegration of the northern and central parts of the Larsen B Ice Shelf, the tributary glaciers of the southern surviving part initially appeared relatively unchanged and hence assumed to be buttressed sufficiently by the remnant ice shelf. Here, we modify this perception with observations from IceBridge altimetry and InSAR-inferred ice flow speeds. Our analyses show that the surfaces of Leppard and Flask glaciers directly upstream from their grounding lines lowered by 15 to 20 m in the period 2002-2011. The thinning appears to be dynamic as the flow of both glaciers and the remnant ice shelf accelerated in the same period. Flask Glacier started accelerating even before the 2002 disintegration, increasing its flow speed by ~55% between 1997 and 2012. Starbuck Glacier meanwhile did not change much. We hypothesize that the different evolutions of the three glaciers are related to their dissimilar bed topographies and degrees of grounding. We apply numerical modeling and data assimilation that show these changes to be accompanied by a reduction in the buttressing afforded by the remnant ice shelf, a weakening of the shear zones between its flow units and an increase in its fracture. The fast flowing northwestern part of the remnant ice shelf exhibits increasing fragmentation, while the stagnant southeastern part seems to be prone to the formation of large rifts, some of which we show have delimited successive calving events. A large rift only 12 km downstream from the grounding line is currently traversing the stagnant part of the ice shelf, defining the likely front of the next large calving event. We propose that the flow acceleration, ice front retreat and enhanced fracture of the remnant Larsen B Ice Shelf presage its approaching demise.

We present the first snow avalanche dynamics model simulations to start in the middle of the avalanche path at the maximum expected speed for an extreme event. We first present a sensitivity analysis of the dynamics model to the various model inputs. A single-parameter Coulomb-type friction formulation is used in the model. This formulation is supported by various experiments and full-scale observations of avalanche flow that demonstrate a coupling between the shear and normal forces in flowing snow. The dynamics model is shown to be most sensitive to changes in the friction coefficient. We suggest that the precision in the friction coefficient necessary to confidently use a dynamics model to predict runout distances is higher than the current state of knowledge about avalanche resistance mechanisms. This result leads to the new modeling technique that starts numerical simulations at the midpoint of the length of the avalanche path at maximum speed. The Coulomb friction coefficient is chosen to produce a unique speed profile from this new starting point at maximum speed to a state of rest at an empirically pre-determined runout position. The technique reproduces the observed sharp deceleration of avalanche flow in the runout zone. Key words: avalanche dynamics, extreme events, sensitivity analysis. Nous presentons dans cet article les premieres simulations d'un modele dynamique d'avalanches de neige qui debutent au milieu du chemin de l'avalanche a la vitesse maximale attendue pour un evenement extreme. Premierement, une analyse de sensitivite du modele dynamique est presentee par rapport a plusieurs intrants du modele. Le modele utilise une formulation de friction de type Coulomb a parametre unique. Cette formulation est supportee par differents essais et observations a l'echelle reelle d'avalanche qui demontrent un couplage entre les forces de cisaillement et les forces normales dans l'ecoulement de neige. Le modele dynamique est le plus sensible aux variations du coefficient de friction. Il est suggere que la precision necessaire du coefficient de friction pour utiliser un modele dynamique pour predire les distances de parcours soit plus elevee que le niveau des connaissances actuelles sur les mecanismes de resistance aux avalanches. Ces resultats ont amene a la nouvelle technique de modelisation qui commence les simulations numeriques au milieu de la longueur du trajet de l'avalanche a vitesse maximale. Le coefficient de friction de Coulomb est choisi de facon a produire un profil de vitesse unique a partir de ce nouveau point de depart a vitesse maximale jusqu'a l'etat de repos a une position predeterminee empiriquement du parcours. La technique permet de reproduire la deceleration marquee de l'ecoulement de l'avalanche observee dans la zone de parcours. Mots-cles: dynamique d'avalanche, evenements extremes, analyse de sensibilite. [Traduit par la Redaction], Introduction Snow avalanches are a frequent winter natural hazard in alpine countries. Slab avalanches occur after the propagation of fractures in cohesive snow. The result is a large initial volume [...]

Identifying fast and robust numerical solvers is a critical issue that needs to be addressed in order to improve projections of polar ice sheets evolving in a changing climate. This work evaluates the impact of using sophisticated numerical solvers for transient ice flow simulations using the NASA-JPL/UCI Ice Sheet System Model (ISSM). We identify optimal numerical solvers by testing them on a commonly used ice flow benchmark test, the Ice Sheet Model Intercomparison Project for Higher-Order ice sheet Models (ISMIP-HOM) Experiment F. Three types of analyses are considered: mass transport, horizontal stress balance, and vertical stress balance. A broad suite of solvers is tested, ranging from direct sparse solvers to preconditioned iterative methods. The results of the fastest solvers for each analysis type are ranked based on their scalability across mesh size for each basal sliding conditions specified in Experiment F. We find that the fastest iterative solvers are ~1.5-100 times faster than the default direct solver used in ISSM with speed-ups improving rapidly with increased mesh resolution. We provide a set of recommendations for users in search of efficient solvers to use for transient ice flow simulations, enabling higher-resolution meshes and faster turnaround time. The end result will be improved transient simulations for short-term, highly resolved forward projections (10-100 year time scale) and also improved long-term paleo-reconstructions using higher-order representation of stresses in the ice. This analysis will also enable a new generation of comprehensive uncertainty quantification assessments of forward sea-level rise projections, which rely heavily on ensemble or sampling approaches that are inherently expensive. [ABSTRACT FROM AUTHOR]