Week 5: Glacier and ice sheet modelling Flashcards

1
Q

Why do we model glaciers and ice sheets?

A

Predict future change

Reconstruct past change

Understand their controls

Glacier response to climate change

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2
Q

Predicting future change with models

A

SL rise contribution

Local change e.g. glaciers as a hazard

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3
Q

Reconstructing past change with models

A

Fluctuations of pale ice masses

Insight into longer term behaviour

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4
Q

Understanding glacier controls with models

A

Processes that influence change and feedbacks between

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5
Q

Glacier responses to climate change with models

A

Glacier length/thickness/flow speeds

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6
Q

Model building blocks

A

Accumulation
Ablation
Ice flow law

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7
Q

Ice flow law =

A

how ice gets from one point to another in a landscape

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8
Q

What do models allow us to do?

A
  1. Calculate ice surface (how it evolves/length evolves i.e. thicker = longer, thinner = shorter) within spatial framework
  2. Quantify ice flow; ice deformation/basal sliding
  3. Apply climate forcing; net mass balance
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9
Q

Flow line model

A

1 dimensional
Discrete points along flow
Grid size (often regular)
Very simple

Good for valley glaciers/constrained ice streams

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10
Q

Representation of space (2D/3D)

A

Grid in x, y (+z if 3D) direction

2D = plan view

3D = plan view but also divided vertically

More complicated/computationally expensive = lower resolution

Good for ice sheets/caps

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11
Q

Choice of model dimension and flow physics depends on:

A

Location of interest

Scientific question - prediction/process study

Resources: computational/data

Process understanding

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12
Q

Flow physics =

A

flow approximation/processes:

SIA (shallow ice approximation)

SSA (shallow shelf approximation)

1st order

2nd order

Full equations

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13
Q

Valley glacier model choice

A

All SIA

1/2/3D

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14
Q

Ice shelf model choice

A

All SSA

2/3D

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15
Q

Ice streams and marine terminating glaciers model choice

A

SSA to full equations

2/3D

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16
Q

Ice sheets model choice

A

SIA to full equations

3D

17
Q

Ice thickness:

A

Add: flow in / accumulation
Take: flow out / ablation

18
Q

Thickness change =

A

mass balance - change in ice flux along flow

Conservation of mass (continuity equation)

dH/dt = b - (1/w x d(qw)dx)

19
Q

Mass balance (q) =

A

m3/yr

Udef (sliding due to deformation) x H

Can incorporate Glen’s Flow law

20
Q

Assumptions in mass balance calculation

A

No basal sliding i.e. Ubasal = 0

No longitudinal stresses = shallow ice approximation (SIA)

21
Q

Glenn 1955 deforming ice…

A

Colder = harder to deform up to -20/30’C

A = how quickly ice deforms for given ice T (constant)

22
Q

How can you calculate mass balance? (b)

A
  1. Increases linearly with surface elevation S (b = x, S = y)
  2. Use separate model e.g. energy balance model (EBM), positive degree day models (PDD)
    - require field data for validation/calibration
  3. Create ‘scenarios’ for future mass balance
    - IPCC
    - CO2 levels det T/acc/abl
  4. Use past measurements e.g. proxies
  5. Solve the equation
    - discretisation
    - solved for each grid point = dH/dt
    - solve again and again to investigate time evolution
    (lines spaced equally w.r.t time so further apart = faster glacier advance)
23
Q

Inputs of glacier model

A

Run time/time step/grid size

Geometry (on grid): bed topography, reference width/thickness, ice surface

Model parameters: values for (1) net mass balance (2) flow-law values

24
Q

Values for (1) net mass balance

A

ELA

b slope

25
Values for (2) flow-law
A (ice softness) n (usually 3) N.B. A varies with ice T, may relate to ice T and ice thickness Ice = cold at top then warmer deeper
26
Outputs generated by glacier model
Glacier length and volume with time Ice thickness/surface elevation Net mass balance Flux, velocity
27
How can you validate/verify a model?
Observed values of: - surface velocities - surface elevations - trimlines - length record - moraines - terminus - sea level record
28
Trimline =
line that ice surface has reached to in past
29
Hysterisis =
value lags behind changes in the effect causing it
30
What is elevation-mass balance feedback?
If ice surface isn't high enough ice can't grow (e.g. after deglaciation) e.g. Greenland ice sheet of today couldn't form under modern conditions, it must have formed under a cooler past climate
31
Is Greenland or Antarctica more sensitive to climate according to the Java Ice Sheet Model (J-ISM)?
Greenland
32
Case studies
READ IN NOTES
33
Modelling conclusions
Can tell us about past/present/future glacier behaviour Range of processes (flow/basal sliding/ice shelves/hydrology/ocean warming) Model must have an appropriate spatial framework (spatial grid resolution/dimension) Needs to be given a forcing e.g. climate/mass balance Needs data for input/tuning and validation