The Dilemma of RESOLUTION: How Good is Good Enough: A Case Study from Greenland James L Fastook University of Maine We thank the NSF, which has supported the development of this model over many many years through several different grants, and especially CReSIS (ANT ) for the excellent data product used here. James L Fastook University of Maine We thank the NSF, which has supported the development of this model over many many years through several different grants, and especially CReSIS (ANT ) for the excellent data product used here.

SeaRISE ● Whole-icesheet modelers are predicting the response of the Greenland and Antarctic Ice Sheets to projected climate change. ● With a database provided by Jesse Johnson – ● These datasets include surface elevation, thickness, bedrock elevation, and other boundary conditions necessary for the models. ● Both Greenland and Antarctica are provided at 5 km resolution. ● Whole-icesheet modelers are predicting the response of the Greenland and Antarctic Ice Sheets to projected climate change. ● With a database provided by Jesse Johnson – ● These datasets include surface elevation, thickness, bedrock elevation, and other boundary conditions necessary for the models. ● Both Greenland and Antarctica are provided at 5 km resolution.

An Accurate Representation ● We have seen from previous work that an accurate representation of the bed is essential if models are to produce reasonable results. ● As we look at the response of the ice sheets to projected climate change, we wonder how good our resolution must be, especially in the fast-flow areas of the ice streams, many of which are topographically controlled.

A High-Resolution Dataset ● CRESIS has provided us with a very high- resolution bed for the Jakobshavn catchment. ● Joel Plummer, PhD student, Dept. Geography, U. Kansas ● 1294 X 438 rectangular grid – 566,772 points, – 565,041 quadrilaterals – Average spacing 100 X 300 m. ● Covering 16,000 square kilometers – equal to 4 “Rhode Islands” or 1.1 “Connecticuts” – one-and-a-half B-15 icebergs

CReSIS Flightlines

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A High-Resolution Dataset ● We show the results of – Using this dataset at a degraded resolution comparable to the whole-icesheet dataset provided by Jesse Johnson – Using progressively higher resolutions to more accurately capture the deep channel in which the Jakobshavn Ice Stream flows. – Doing this using the embedded-model feature of UMISM, whereby a higher-resolution, small-domain model (the ice stream) is run inside a lower- resolution, broader-domain model (the whole icesheet).

Ice Shelf Physics ● Weertman (1957), the spreading of an ice shelf in a parallel-walled channel is proportional to h 3. – Thinning therefore is proportional to h 4.

Thinning Rate ●

Ice Shelf Parameter ● Add Weertman thinning rate times a parameter between 0 and 1. – 0: fully buttressed, no thinning. – 1: no buttressing, maximum thinning rate. ● To the accumulation rate (source or sink of mass) in the right-hand side of the continuity equation in the Shallow Ice Approximation Formulation.

The Canyon ● The most remarkable feature is the ice stream channel – Approximately 100 km long – Incised as much as 1750 m into the surrounding terrain – More than 10 km wide in places ● It is on the same scale as the Grand Canyon – 446 km long – m deep – km wide

The Canyon

Spinup ● A very abbreviated spinup of just 5000 years – with climate held constant at climate knob +1

Spinup

Velocities after spinup ● A Logarithmic scale – 1 m/a = 0 (10 0 ) – 10 m/a = 1 (10 1 ) – 100 m/a = 2 (10 2 ) – 1000 m/a = 3 (10 3 ) – m/a = 4 (10 4 )

Perturbation scenario ● With captured spunup configurations as the starting point of the perturbation scenario – Hold steady for 100 years – Increase temperature knob by 4 C sinusoidally over 100 years – Hold steady for further 100 years

Response: Area

Response: Volume

Response: Area

350 km^2 lost in under 5 years

Conclusions: The representation of ice stream “Canyons” is essential, even at the 5 km resolution of whole ice sheet models. At 2 km resolution, even a simple mass-balance climate-initiated retreat (simply warming temperature) produces different results. The abrupt retreat episode occurring in the Jakobshavn Canyon clears 350 km^2 in under 5 years, with velocities approaching 10 km/yr. an event commensurate with the observed retreat in the last decade. The representation of ice stream “Canyons” is essential, even at the 5 km resolution of whole ice sheet models. At 2 km resolution, even a simple mass-balance climate-initiated retreat (simply warming temperature) produces different results. The abrupt retreat episode occurring in the Jakobshavn Canyon clears 350 km^2 in under 5 years, with velocities approaching 10 km/yr. an event commensurate with the observed retreat in the last decade.