One of the most novel developments in the UKESM project will be the incorporation of interactive models of continental-scale ice sheets. Ice sheets play a fundamental role in the Earth’s climate and how our environment will change in the coming centuries as Greenland’s outlet glaciers speed up and the risk of Antarctic ice shelf collapse increases, both potentially leading to significant loss of mass from grounded ice sheets with large impacts on global sea level.
UKESM1 will include the option of ice sheets that interact in a physically consistent way both with the atmosphere above and also the ocean underneath the ice shelves, making it a world-leading model in this area of science. UKESM1 will use BISICLES (Cornford et al., 2013), a vertically integrated “higher order” model that can model both the slower-moving bulk of the ice as well as the dynamics of fast ice-streams and floating shelves without the significant computational expense of solving the full Stokes flow equations. BISICLES uses an adaptive grid mesh that saves resources by only putting the necessary high resolution where it is required, for example, at the grounding line where the ice shelf floats clear of the ocean floor.
Figure 1. Simulated ice velocity for Antarctica from BISICLES (right), and meshing and grounding line location for the Pine Island Glacier (left) (source: SL Cornford).
Successfully coupling BISICLES to the Global Coupled (GC) 3 configuration of the Met Office Unified Model (UM) in a physically consistent way brings a number of major scientific and technical challenges. The characteristic spatial scales of atmosphere, ocean and ice physics are different – modelling most atmosphere and ocean processes requires timescales of minutes but can be represented on spatial scales of several kilometres, but key areas of the ice must be modelled with spatial scales of a few metres but changes occur over much slower timescales of months or longer. To further complicate matters, flowing ice not only alters the active domain of the ice model, but changing sea-level turns land into ocean and back again, something that the current UM is structurally not designed to do.
Finding solutions to these issues has motivated significant developments in the UM, which are being tested in UKESM prototypes. JULES, the land surface component of UKESM1, now uses a multi-layer snow model on special sub-grid scale tiles which represent the climate at different elevations to improve the spatial resolution of the surface forcing provided to the ice sheets. NEMO, the ocean component of UKESM1, has been modified to allow circulation underneath the solid surface of the ice shelf and includes parameterisations of sub-shelf melting. Moving the land/ocean boundaries as the ice volume and extent change remains a largely unresolved issue however and the underlying spatial resolution of the UM still brings a number of restrictions to the ice simulation – for example, the narrow fjords that control many of Greenland’s outlet glaciers are simply not resolved in a global ocean model.
Figure 2. One year of snow melt (red) and accumulation (blue) on Greenland from two different UKESM-IS prototype tuning runs
Tuning of the final model is also required to ensure that significant biases are not introduced into all components by the additional degrees of freedom introduced by a fully interactive treatment of land ice. It is therefore planned to introduce the interactive BISICLES ice sheet component in a separate release of UKESM, UKESM1-IS. UKESM-IS is currently being tuned and evaluated, and will be released in early 2018. This will be the version of UKESM that will be used for the coupled ice sheet-climate simulations to be submitted under ISMIP6 to CMIP6.
Cornford, SL, Martin, DF, Graves, DT, Ranken, DF, Le Brocq, AM, Gladstone, RM, Payne, AJ, Ng, EG & Lipscomb, WH, 2013, ‘Adaptive mesh, finite volume modeling of marine ice sheets’. Journal of Computational Physics, vol 232., pp. 529-549