Spinning up a highly complex, coupled Earth system model (ESM) is a time consuming and computationally demanding exercise. For models with interactive ice sheet components, this becomes a major challenge, as ice sheets are sensitive to bidirectional feedback processes and equilibrate over glacial timescales of up to many millennia. This work describes and demonstrates a computationally tractable, iterative procedure for spinning up a contemporary, highly complex ESM that includes an interactive ice sheet component. The procedure alternates between a computationally expensive coupled configuration and a computationally cheaper configuration where the atmospheric component is replaced by a data model. By periodically regenerating atmospheric forcing consistent with the coupled system, the data atmosphere remains adequately constrained to ensure that the broader model state evolves realistically. The applicability of the method is demonstrated by spinning up the preindustrial climate in the Community Earth System Model Version 2 (CESM2), coupled to the Community Ice Sheet Model Version 2 (CISM2) over Greenland. The equilibrium climate state is similar to the control climate from a coupled simulation with a prescribed Greenland ice sheet, indicating that the iterative procedure is consistent with a traditional spin-up approach without interactive ice sheets. These results suggest that the iterative method presented here provides a faster and computationally cheaper method for spinning up a highly complex ESM, with or without interactive ice sheet components. The method described here has been used to develop the climate/ice sheet initial conditions for transient, ice sheet-enabled simulations with CESM2-CISM2 in the Coupled Model Intercomparison Project Phase 6 (CMIP6). ... Read more

Ice sheets are a major component of the Earth System, however they are not yet interactively coupled to most global climate models. Here we present past achievements in this front with the CESM1.0 version as well as first results and challenges with the upcoming CESM2.0, where the Community Ice Sheet Model 2.1 is bi-directionally coupled to the atmospheric and ocean components, as opposed to only one-way coupling in CESM1.0. In both CESM versions, the surface mass balance (SMB) is calculated in the land component (CLM) with explicit albedo and refreezing calculations, and downscaled to the ice sheet model resolution via elevation classes and bi-linear (horizontal) and linear (vertical) interpolations. A major highlight of CESM is that the most important coupling process between ice sheet and atmosphere, the albedo feedback, is explicitly modeled, as opposed to state-of-the-art parameterizations of albedo and/or surface melt. Regarding future Greenland ice sheet projections with CESM1, we summarize our results on: 1) doubled end-of-the-century melt rates under RCP8.5 from increased incoming thermal radiation and turbulent fluxes despite decreased incoming solar radiation over Greenland from more clouds, 2) increase in SMB variability due to reduced accumulation to ablation area ration, 3) bimodal emergence of an anthropogenic signal on the SMB due to both increasing melt and snow accumulation, 4) reduction in ice discharge from marginal thinning. We also outline work in progress in preparation for our contribution to the Ice Sheet Model Intercomparison Project 6 (ISMIP6) and paleo-research on the last deglaciation, e.g., on model initialization, improved SMB calculation, evaluation of CESM2.0 climate over Greenland, and parameter optimization of the higher-order CISM2.1. ... Read more

The ice streams and outlet glaciers of the Greenland Ice sheet (GrIS) are dynamic active parts of the ice sheets; with the recent accelerated contribution to global sea level rise attributed to retreat and thinning from many marine-terminating outlet glaciers. For example, across the Northern margin the outlet glaciers have been seen to be retreating, and increasing in speed. (Hill, et al, 2017). Over longer time scales geomorphological evidence from previously glaciated regions, such as below the Laurentide Ice Sheet (Stokes, Nature, 2016) highlighted the very spatial dynamic nature of the paleo ice streams. In this study, we aim to investigate the variability, both spatially and temporally of the GrIS ice streams and outlet glaciers to try and understand the mechanisms that control the ice stream formation and stability. We perform an ensemble of sensitivity experiments using the new updated version 2.1 of Community Ice sheet model (CISM) for the Greenland ice sheet, at a 4kmx4km resolution. In all simulations, we force the ice sheet with the surface mass Balance (SMB) taken from RACMO2.3 with the aim of recreating a steady-state present day ice sheet. There have been a number of improvements since version2 which we will adopt: a higher-order velocity solver ‘vertically integrated approximation to the Stokes flow law’ taken from Goldberg, 2011; the introduction of a new pseudo-plastic power law sliding scheme, that relates changes in the basal shear stress to the till yield stress and a pseudo-plastic flow law exponent and finally a simple model of basal hydrology which is used to calculate the changes in till yield stress. This basal hydrology model relates changes in rheology of subglacial sediments (defined by a till friction angle) to changes in basal water pressure to produce a till yield stress that varies both in space and time. This new sliding law and basal hydrology model have a number of ... Read more