The Dilemma of RESOLUTION: How Good is Good Enough: A Case Study from Greenland James L Fastook...

The Dilemma of RESOLUTION: How Good is Good Enough:

A Case Study from Greenland

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-0424589) 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-0424589) for the

excellent data product used here.

SeaRISESeaRISE

● 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 – http://websrv.cs.umt.edu/isis/index.php/SeaRISE_Assessment

● 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 – http://websrv.cs.umt.edu/isis/index.php/SeaRISE_Assessment

● 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

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 h3.

– Thinning therefore is proportional to h4.

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

– 1500 - 1800 m deep

– 16-20 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 (100)

– 10 m/a = 1 (101)

– 100 m/a = 2 (102)

– 1000 m/a = 3 (103)

– 10000 m/a = 4 (104)

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: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.

The Dilemma of RESOLUTION: How Good is Good Enough: A Case Study from Greenland James L Fastook...

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