a list of all "PISM Applications of the Month"

All inversions reproduce the overall pattern of observed surface velocities, which shows that, in general, our data and model choices are capable of reproducing the observations by only adjusting basal yield stress. In the lower 5 km of the glacier a clear trend from higher to lower basal yield stress values is visible.

see the youtube video

compared. The new PISM “regional” mode, the pismo executable in stable0.5, was applied. The model captures the high velocities near the terminus qualitatively, but even at high 2 km model resolution the distinct fast flowing arms are not well modelled, and the modelled velocities and fluxes are overall lower than than observed. The question remains: Are there are deep troughs in the bed topography where the surface velocity is very high?

To assess the sensitivity of the projections to the ice sheet model initial state, four initialisaton methods were used. Analyses of these 100 years simulations indicate that the mass changes due to climate forcing are decoupled from the changes due to dynamic response and the initialisation procedure. The simulated mass loss has a relatively large range, 0.5 to 6.5 cm sea level rise equivalent, which is to a large extent due to the range in the projected climate forcing from the regional climate models that were used to downscale the climate fields.

Our models indicate that Greenland is more sensitive than Antarctica to likely atmospheric changes in temperature and precipitation, while Antarctica is most sensitive to increased ice-shelf basal melting. An experiment approximating the IPCC’s RCP8.5 scenario produces first century contributions to sea level of 22.3 and 8.1 cm from Greenland and Antarctica, respectively, with a range among models of 62 and 14 cm, respectively. By 200 years, projections increase to 53.2 and 26.7 cm, respectively, with ranges of 79 and 43 cm.

We interactively couple the atmosphere-ocean-vegetation general circulation model ECHAM5/MPIOM/LPJ with the ice sheet model mPISM, a modified version of the Parallel Ice Sheet Model, without flux correction or anomaly maps in our models. We run ECHAM5 in T31 resolution and mPISM on a 20 km grid covering most of the northern hemisphere. For comparison, we also perform an experiment using the PMIP2 protocol and the ICE-5G ice sheet reconstruction (Peltier, 2004) instead of mPISM. In runs using pre-industrial as well as LGM boundary conditions, the shape of the ice sheets has a strong influence on the wind systems and thereby on the global climate. Our model shows ice sheet collapses as regular part of the ice sheet behavior. These pulses create strong signals in the ocean.

Large uncertainties exist in the potential changes of dynamic ice discharge from Antarctica from precipitation. Here we show that snowfall and discharge are not independent, but that future ice discharge will increase by up to three times as a result of additional snowfall under global warming. Our results, based on PISM-PIK runs forced by climate simulations through to the end of 2500, show that enhanced discharge exceeds the effect of surface warming as well as that of basal ice-shelf melting. In an ensemble of simulations designed to capture ice-physics uncertainty, the additional dynamic ice loss along the coastline compensates for between 30 and 65 per cent of the ice gain due to enhanced snowfall over the entire continent. This results in a dynamic ice loss of up to 1.25 metres in the year 2500 for the strongest warming scenario.

The build-up of the Greenland Ice Sheet from ice-free conditions is studied using PISM driven by fields from an atmospheric GCM. Experiments where the two are coupled off-line are augmented by one where an intermediate ice sheet configuration is coupled back to the GCM. The ice sheet regrows from the ice-free state but this is halted when the intermediate recoupling step is included. This inhibition of further growth is due to a Föhn effect of moist air parcels being lifted over the intermediate ice sheet and arriving in the low-lying Greenland interior with high temperatures. This demonstrates that two-way coupling between the atmosphere and the ice sheet is essential for understanding its dynamics. Conditions cooler than those of today may be necessary for the GrIS to regrow to the present volume.

Retreat of the Last Glacial Maximum (LGM) Antarctic ice sheet is thought to have been initiated by changes in ocean heat and eustatic sea level from northern ice sheets melted under rising atmospheric temperatures. The extent to which spatial variability in ice dynamics may have modulated the resultant pattern and timing of decay of the Antarctic ice sheet has so far received little attention, however. We use a 5-km resolution whole-continent PISM model to assess whether differences in the mechanisms governing ice sheet flow could account for discrepancies between geochronological studies in different parts of the continent. We simulate the geometry and flow characteristics of an equilibrium LGM ice sheet. Then we perturb the system with sea level and ocean heat flux increases to investigate ice-sheet vulnerability. We find that although ocean warming and sea-level rise bring about ocalized glacier acceleration, drawdown of ice from neighboring areas leads to widespread thinning of entire glacier catchments.

Linear response functions can separately estimate the sea-level contributions of thermal expansion and solid ice discharge from Greenland and Antarctica. This formalism introduces a time-dependence which allows for future rates of sea-level rise to be influenced by past climate variations. The linear response function for the solid ice discharge is computed with the Potsdam Parallel Ice Sheet Model PISM-PIK (Winkelmann et al. 2011) under surface warming scenarios. Different from earlier studies we conclude that solid ice discharge from Greenland due to dynamic thinning is bounded by 0.42 m sea-level equivalent. Ice discharge induced by surface warming on Antarctica is best captured by a model which reflects the fact that ice loss increases with the cumulative amount of heat available for softening the ice in our model.

Inverse methods are used to estimate model parameters from observations, here basal shear stress from the surface velocity of an ice sheet. One starts with an initial estimate of the model parameters and then updates them to improve the match to observations in an iterative process. Large-scale spatial features are adjusted first. A stopping criterion prevents the overfitting of data. In this paper, iterative inverse methods are applied to the shallow-shelf approximation forward model. A new incomplete Gauss–Newton method is introduced and compared to the steepest descent and nonlinear conjugate gradient methods. Two different stopping criteria, the discrepancy principle and a recent-improvement threshold, are compared. The IGN method shows faster convergence than the others. Though PISM is not mentioned by this paper, and the experiments were done in python, code supporting these inversion methods is already present in the PISM dev branch.

In an attempt to constrain the climate of the Last Glacial Maximum period (LGM, c. 30–20 ka before present), a simulation of the New Zealand Southern Alps icefield is presented. PISM is applied at 500 m-resolution using empirical glaciological, climatological and geological data specific to the model domain, the entire icefield. An LGM cooling of at least 6–6.5 °C is necessary to bring about valley glaciers that extend beyond the mountains. However, climate–topography thresholds related to the elevation and hypsometry of individual catchments control the gradient of the rate of glacier expansion in the domain. In order to remain within geologically-reconstructed LGM limits we find that the LGM cooling was most likely associated with a precipitation regime up to 25% drier than today.

We use output from a general circulation model (CAM3+CLM) to construct adaptive temperature and precipitation patterns to force PISM off-line, taking into consideration that the patterns change in a non-uniform way, both spatially and temporally, as the geometry of the ice sheet evolves and as climate changes. In a series of experiments we investigate retreat from the present day configuration and build-up from ice free conditions during a warmer-than-present climate. We find that the ice sheet is able to survive and build up at higher temperatures using the more realistic adaptive patterns compared to the classic constant patterns. The ice sheet is multistable at least for certain temperature forcings, so it does not necessarily return to its initial configuration after a temperature excursion.

Numerical experiments are conducted on a synthetic topography with PISM, in which stress balance is connected to evolving thermodynamics and hydrology. The sensitivity of cyclic behaviour to changes in sliding-law parameters and the climate input is studied. Multiple types of oscillations were found, with strong variations in both amplitude and frequency. High-frequency oscillations (period 114–169 years), which are shown to have a major impact on ice velocities and a small effect on the ice volume, are related to variations in the water distribution at the base. Low-frequency cycles (period 1000+ years), which have a major impact on both velocities and ice volume, are linked to changes in the thermal regime.

Observed large-scale disintegration of Antarctic ice shelves has moved their fronts closer towards grounded ice, accelerating ice-sheet discharge and contributing to global sea-level rise. Here we describe the first-order large-scale kinematic contribution to calving which is consistent with large-scale observation. This calving law depends only on local ice properties which are, however, determined by the full topography of the ice shelf. Simulations in PISM-PIK using the parameterization reproduces multiple stable fronts as observed for the Larsen A and B Ice Shelves, including abrupt transitions between them. We also find multiple stable states of the Ross Ice Shelf.

Polythermal conditions are ubiquitous among glaciers and ice sheets. Fortunately, temperature and liquid water fraction are functions of a single enthalpy variable: a small enthalpy change in cold ice is a change in temperature, while a small enthalpy change in temperate ice is a change in liquid water fraction. The unified enthalpy formulation described in this paper models the mass and energy balance for the three-dimensional ice fluid, for the surface runoff layer and for the subglacial hydrology layer, together in a single energy-conserving theoretical framework. It is implemented in PISM. Results for the Greenland ice sheet are compared with those from a cold-ice scheme.

A new regional mode in PISM is applied to the Jakobshavn outlet glacier. This mode is best suited for high spatial resolutions (< 1 km) and short timescales (< 1000 a). The first step is the identification of a drainage basin based on the surface gradient. Boundary conditions along the basin outline then partially-isolate the outlet glacier flow from the rest of the ice sheet. The ice dynamics model applied within the basin is the full enthalpy-based, SSA-sliding model. Both slow and fast ice flow are captured, as shown by a comparison to observations.

A macroscopic fracture-density field is introduced into PISM. Its evolution includes the initiation and growth of fractures as well as their advection with two-dimensional ice flow. To first approximation, fracture growth is assumed to depend on the spreading rate only, while fracture initiation is defined in terms of principal stresses. The inferred fracture-density fields compare well with observed elongate surface structures. The aim of this study is to introduce the field and investigate which of the observed surface structures can be reproduced by the simplest physically motivated fracture source terms.

The Antarctic Research Centre is using PISM to study Antarctic ice sheet behaviours during key periods of the past, particularly the LGM and the mid-Pliocene.

In a short News & Views article in Nature, Levermann reviews the potential for tsunami-genic iceberg calving as a phenomenon and a possible hazard. PISM-PIK Antarctic ice sheet simulations gave the frequency distribution of iceberg height and energy in discharge events per decade; see box. Iceberg discharge was computed from the vertical extent of the ice sheet and the marginal velocity distribution in a present-day equilibrium state. The results show a peak in the abundance of icebergs with a height of around 400 metres, and a distribution of energies up to several kilotonnes of TNT.

A study of the extent of the Greenland ice sheet during the Mid-Pliocene Warmth (3.3–3.0 Ma), its advance across the continental shelf during the late Pliocene to early Pleistocene glaciations (3.0–2.4 Ma) as implied by offshore geological studies, and the transition from glacial to interglacial conditions around 2.4 Ma as deduced from the deposits of the Kap København Formation, North Greenland.

This poster presents outcomes of step-cooling perturbation from current conditions using the Parallel Ice Sheet Model (PISM).

These simplified first experiments clearly demonstrate that ice sheet nucleation centers are consistent with the geological record. This as an encouraging start towards increased model complexity (ice shelves, lithospheric rebound) and transient model runs using past temperature and sea level reconstructions.

This paper presents a dynamic equilibrium simulation of the ice sheet-shelf system on Antarctica with the Potsdam Parallel Ice Sheet Model (PISM-PIK) with a focus on the Ronne-Filchner and Ross ice shelf areas as well as on the whole ice-sheet system.

PISM-PIK (and now PISM) allows free movement of grounding lines and calving fronts thanks to a physically-motivated calving law based on horizontal spreading rates and an implementation of a calving-front stress boundary condition.

The results support the approach of superposition of SIA and SSA for the representation of fast motion of grounded ice.