TY - JOUR T1 - Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison JF - The Cryosphere Y1 - 2018 A1 - Goelzer, Heiko A1 - Nowicki, Sophie A1 - Edwards, Tamsin A1 - Beckley, Matthew A1 - Abe-Ouchi, Ayako A1 - Aschwanden, Andy A1 - Calov, Reinhard A1 - Gagliardini, Olivier A1 - Gillet-Chaulet, Fabien A1 - Golledge, Nicholas R. A1 - Gregory, Jonathan A1 - Greve, Ralf A1 - Humbert, Angelika A1 - Huybrechts, Philippe A1 - Kennedy, Joseph H. A1 - Larour, Eric A1 - Lipscomb, William H. A1 - Le clec'h, Sébastien A1 - Lee, Victoria A1 - Morlighem, Mathieu A1 - Pattyn, Frank A1 - Payne, Antony J. A1 - Rodehacke, Christian A1 - Rückamp, Martin A1 - Saito, Fuyuki A1 - Schlegel, Nicole A1 - Seroussi, Helene A1 - Shepherd, Andrew A1 - Sun, Sainan A1 - van de Wal, Roderik A1 - Ziemen, Florian A. AB - Abstract. Earlier large-scale Greenland ice sheet sea-level projections (e.g. those run during the ice2sea and SeaRISE initiatives) have shown that ice sheet initial conditions have a large effect on the projections and give rise to important uncertainties. The goal of this initMIP-Greenland intercomparison exercise is to compare, evaluate, and improve the initialisation techniques used in the ice sheet modelling community and to estimate the associated uncertainties in modelled mass changes. initMIP-Greenland is the first in a series of ice sheet model intercomparison activities within ISMIP6 (the Ice Sheet Model Intercomparison Project for CMIP6), which is the primary activity within the Coupled Model Intercomparison Project Phase 6 (CMIP6) focusing on the ice sheets. Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of (1) the initial present-day state of the ice sheet and (2) the response in two idealised forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without additional forcing) and in response to a large perturbation (prescribed surface mass balance anomaly); they should not be interpreted as sea-level projections. We present and discuss results that highlight the diversity of data sets, boundary conditions, and initialisation techniques used in the community to generate initial states of the Greenland ice sheet. We find good agreement across the ensemble for the dynamic response to surface mass balance changes in areas where the simulated ice sheets overlap but differences arising from the initial size of the ice sheet. The model drift in the control experiment is reduced for models that participated in earlier intercomparison exercises. VL - 12 UR - https://www.the-cryosphere.net/12/1433/2018/ ER - TY - JOUR T1 - The response of fabric variations to simple shear and migration recrystallization JF - Journal of Glaciology Y1 - 2015 A1 - Kennedy, Joseph H. A1 - Pettit, Erin C. AB - The observable microstructures in ice are the result of many dynamic and competing processes. These processes are influenced by climate variables in the firn. Layers deposited in different climate regimes may show variations in fabric which can persist deep into the ice sheet; fabric may ‘remember’ these past climate regimes. We model the evolution of fabric variations below the firn–ice transition and show that the addition of shear to compressive-stress regimes preserves the modeled fabric variations longer than compression-only regimes, because shear drives a positive feedback between crystal rotation and deformation. Even without shear, the modeled ice retains memory of the fabric variation for ~200 ka in typical polar ice-sheet conditions. Our model shows that temperature affects how long the fabric variation is preserved, but only affects the strain-integrated fabric evolution profile when comparing results straddling the thermal-activation-energy threshold (~–10°C). Even at high temperatures, migration recrystallization does not eliminate the modeled fabric’s memory under most conditions. High levels of nearest-neighbor interactions will, however, eliminate the modeled fabric’s memory more quickly than low levels of nearest-neighbor interactions. Ultimately, our model predicts that fabrics will retain memory of past climatic variations when subject to a wide variety of conditions found in polar ice sheets. VL - 61 UR - http://openurl.ingenta.com/content/xref?genre=article&issn=0022-1430&volume=61&issue=227&spage=537http://www.ingentaconnect.com/content/igsoc/jog/2015/00000061/00000227/art00011 IS - 227 JO - Journal of Glaciology ER - TY - JOUR T1 - The evolution of crystal fabric in ice sheets and its link to climate history JF - Journal of Glaciology Y1 - 2013 A1 - Joseph H Kennedy A1 - Erin C Pettit A1 - Di Prinzio, Carlos L AB - The evolution of preferred crystal-orientation fabrics is strongly sensitive to the initial fabric and texture. A perturbation in climate can induce variations in fabric and texture in the firn. Feedbacks between fabric evolution and ice deformation can enhance these variations through time and depth in an ice sheet. We model the evolution of fabric under vertical uniaxial compression and pure shear regimes typical of ice divides. Using an analytic anisotropic flow law applied to an aggregate of distinct ice crystals, the model evolves the fabric and includes parameterizations of crystal growth, polygonization and migration recrystallization. Stress and temperature history drive the fabric evolution. Using this model, we explore the evolution of a subtle variation in near-surface fabric using both constant applied stress and a stress-temperature history based on data from Taylor Dome, East Antarctica. Our model suggests that a subtle variation in fabric caused by climate perturbations will be preserved through time and depth in an ice sheet. The stress history and polygonization rate primarily control the magnitude of the preserved climate signal. These results offer the possibility of extracting information about past climate directly from ice fabrics. VL - 59 UR - http://glacierstest.gi.alaska.edu/sites/default/files/bibfiles/t12J159.pdf IS - 214 ER - TY - JOUR T1 - The crystal fabric of ice from full-waveform borehole sonic logging JF - Journal of Geophysical Research: Earth Surface (2003–2012) Y1 - 2012 A1 - Alessio Gusmeroli A1 - Erin C Pettit A1 - Joseph H Kennedy A1 - Ritz, Catherine PB - Wiley Online Library VL - 117 ER -