Glaciers Flashcards
3 ways of measuring glaciers
glaciological, geodetic, gravimetric
Three approaches to calculating past
contributions of glaciers & ice caps to
sea level
Statistical area-weighted
extrapolations of locally- or
regionally-derived glaciological &
geodetic data over most glaciers on
earth
Geodetic approach only, using repeat
DEMs from ASTER satellite
Gravimetric approach using GRACE
data
DEM stands for
direct elevation model
4 essential ingredients of a model
- A spatial domain to run your model. Do you want to model conditions at
a single point? Or investigate two-dimensional spatial patterns? - A Mathematical description of the system – either empirical (i.e., derived
from data), or based on physical principles (e.g., Newton’s Laws). - Suitable inputs. For glacier mass balance, we need meteorological data
(e.g., precipitation & temperature) to calculate accumulation & ablation.
Real-world observations to calibrate &
validate the mode
3 main options for spatial domain of glacier model
0 dimensional applications that treat glaciers as a single bulk entity
1 dimensional applications that split glaciers into a series of bulk elevation bands
2 dimensional applications that are fully spatially distributed
equation for SMB
Bn = c + a + R
where c is accumulation, a is ablation (defined negatively), R is refreezing
three main types of glacier mass balance model
- Degree Day (also known as temperature index) models
- Energy Balance (also known as physically-based) models
- Hybrid (also known as enhanced degree day / temperature index) models
+ves and -ives of degree day model
Advantages
- Quick & easy
- Only requires air temperature as input - easy to apply in data sparse,
remote regions - Good for predicting melt in future if future air temperatures modelled
well
Disadvantages
* Model is empirical - extrapolating over space & time is uncertain
* Not physically-based - offers limited insight into processes
* Limited information on spatial patterns across glacier, or temporal
information at sub-seasonal scales.
+ves and -ves of energy balance model
Advantages
- Physically-based – offers good insight into the processes occurring
- Good for investigating spatial patterns, and short, sub-daily, time periods
- Does not depend on empirical relations, so easy to apply to other
glaciers & different time periods
Disadvantages
- Computationally expensive
- Requires many meteorological inputs so application often limited to
locations with lots of observations - Arguably less useful for future predictions where some of the climate
drivers, e.g. cloudiness, humidity, wind speeds are less well predicted
than, e.g. air temperature
Advantages & Disadvantages of Hybrid Models
- Account for spatial patterns
- Variant 1 requires no extra field measurements; Variant 2 requires Gs measurements
- Depends on empirical relations, so not easy to apply to other glaciers & time periods
- More computationally expensive than classic DD method but less than EB approach
- Variant 1 could be useful for future predictions
outline the maths behind c (accumulation)
Precipitation usually treated as
linear function of elevation
* Air temperature threshold (e.g. 1oC)
used to distinguish between snow or
rain
* Works well on a glacier-average
scale over whole summers
* Works less well at smaller spatial
and temporal scales
* Does not account for local
topographic features or snow
redistribution by wind.
outline the traditional approach to monitoring meteorological inputs to glaciers
Automatic weather station (AWS) on
or near glacier.
* Meteorological variables then
extrapolated over the DEM.
* e.g. temperature assuming a
standard atmospheric lapse rate (6.5
˚C per km).
* e.g. precipitation assuming
regionally measured gradient.
* BUT… most glaciers are in remote
locations & so it is often difficult to get
local measurements.
outline the new approach ~(last 20 years ) to monitoring meteorological inputs to glaciers
Climate reanalysis (e.g. ERA-40, ERA
Interim, ERA 20C, JRA55, NOAA 20CR).
* Produced by ‘reanalysing’ observations
using a weather/climate model.
* Global fields of meteorological variables
(e.g., temperature, precipitation) on a
moderate resolution grid (typically 1
degree lat./long.). So we can model any
glacier!
* BUT… reanalyses are produced using
models so are subject to uncertainty (e.g.
biases)
outline some parameters that must be estimated that are used in a model
- DDFs for snow and ice (for a DD model) * snow and ice albedo (for an EB model) * temperature lapse rate
- precipitation gradient
- threshold temperature for rain / snow
what is calibration
we adjust uncertain model parameters so model output agrees
well with real-world observations
what is validation
we test our calibrated model to see how well it performs against real-world observations.
what do we do if we do not have have measurements of parameters
we have to
calibrate (or ‘tune’ or ‘optimize’) them.
what does DD model stand for
degree day
what is DDF stand for
degree day factor
outline the work of Orleans and Fortune 1992
- Applied energy balance model to 12 glaciers around the world
- Examined sensitivity to a 1 ̊C increase in air temperature.
Key conclusion:
Maritime glaciers are more sensitive to air temperature changes than continental glaciers..
why are Maritime glaciers are more sensitive to air temperature changes than continental glaciers..
relationship between temperature & melt is exponential. So perturbing temperature has greater impact in warmer maritime regions.
- More precipitation will fall as rain in warmer maritime regions compared to sub- zero continental regions
- A positive feedback loop: melting lowers albedo, which increases melting
what is perturbing temperature
term used to describe a departure from the regular flow of atmospheric currents
outline hock et al 2007
compared 5 models of varying complexity ( 3 DD and 2 EB).
Applies to Storglaciaren, calibrated using ERA 40 reanalysis
used regional climate model output to predict MB up to 2100
how are models used to calculate global glacier MB
we first calibrate for a few glaciers where measurements have been made.
outline why glacier hydrology is important
- Glaciers are a natural reservoir, storing water in high precipitation, cold years; releasing it in low precipitation, warm years.
- Glacier hydrology modulates effects of surface melting on stream runoff.
- Controls quantity and quality (sediment, chemistry) of water in glacier-fed streams.
- May increase risk of flooding – ‘jökulhlaups’, Glacier Outburst Floods (GLOFs).
- Implications for water resource management.
- Controls spatial & temporal distribution of water pressure beneath glaciers, & therefore glacier movement (sliding/sediment deformation).
3 types of hydrology
supraglacials, englacial and subglacial hydrology
what is a moulin
an erosional feature which occurs on the surface of a glacier. formed by erosion by meltwater, creating circular inlet down that meltwater can enter the body of the glacier
what is a crevasse and how does it form
deep crack that forms due to movement and resulting stress of the moving ice
what does TDR stand for
time domain reflectrometry
how does TDR work
probes are installed in snow in the accumulation area
they measure two way travel time through the snow of EM waves - this time is affected by the water content of snow
when EM pulse encounters a change in material properties (such as boundary between ice and water) part of the signal reflects back to surface.
time delay is measured to determine distance to and nature of material change
where does the majority of englacial water flow
in small pipes or larger conduits fed by crevasses / moulins
what is glacio speleology
study of caves/cave like structures within glaciers
3 ways englacial passages form and cite
- incision of surface streams or base of crevasse followed by roof closure
- hydrologically driven ice fracturing
- exploitation of pre existing permeable structures within the ice
gulley et al 2009
outline shreves theory and the equation
theoretical direction of water flow calculated by considering hydraulic potential throughout ice
this is the sum of
gravitational potential (height above sea level) + pressure potential (overburden pressure).
what is a conduit
a passageway within which water flows
if snowpack is isothermal, what does this mean
it is uniform, at 0 degrees C
how does snow melt
conduction, infiltration of water, refreezing and release of latent heat
what does K equal in shreeves theory
a fraction where 1 is when max pressure potential and 0 atmospheric pressure
when does subglacial drainage occur
wherever ice at a glacier bed reaches the pressure melting point
what is the pressure melting point
is when ice melts at a given pressure.
list some ways we learn of subglacial drainage characteristics
radio-echo sounding
use of artificial tracers
monitoring
manipulation of conditions via boreholes
monitoring runoff properties
4 main sources of subglacial water
surface, englacial and basal melt,
subglacial meltwater
what determines the relative importance of the sources of subglacial meltwater
climatic regime at ice surface,
temperature of glacier ice
ice flow dynamics
nature of glacier bed
why might basal Melt be the dominant source in antarctica
the surface temps never reach above the freezing point, but the ice temps reach pressure melting point near the glacier bed.
two main types of subglacial drainage system
distributed or slow
channelised or fast
outline the different steady state discharge and effective pressure relationships
smaller fluxes can be stable in cavities, as it rises the effective pressure drops.
inverse relationship between discharge and effective pressure
more water going through increases water pressure - this causes cavities to become unstable, behave like channels, meaning water pressure drops, and effective pressure rises
what is the effective pressure
ice overburden pressure - water pressure
what is the relationship between steady state discharge and effective pressure
direct- as discharge increases the water pressure decreases as channels widen, leading to greater effective pressure
outline what happens to cavity system when water discharge is increased
- Water pressure will rise, due to piling water from above
- this will increase sliding which increases cavity XSA
- increase in WP means effective pressure falls, lowering the creep closure rate
- however on net cavity has enlarged less than discharge did
- as discharge is velocity times XSA and discharge went up more, velocity must have too
- this raises water pressure
list the % of air in each layer of sintering
snow - 90%
coarse grained snow - 50%
firn - 20-30%
glacial ice - 20% as bubbles
define sintering
the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction.
2 types of ice
massive ice and glacial ice
3 types of ductile deformation
elastic, viscous, plastic
what is deformation
how a material responds to an applied stress
what is elastic deformation
a stretch goes back to where it came from
what is viscous deformation
permanent deformation, not going to snap back, will continue oozing out
speed is proportional to the stress being given
plastic deformation
elastic at first until a critical stress is met, then permanent movement
what type of deformation is ice
between viscous and plastic- viscoplastic
how is deformation measured
in terms of strains
how is the measure of stress
measured in pressure.
Newtons/Metres2 = pascals
how is strained measured
how much change since original position
(end position - start) / start
it is a proportion, a unitless ratio
why is strain meaningless in viscous flows
strain keeps increasing as stress is applied
what unit is strain rate
per years
two types of deformation
pure shear and simple shear
outline pure shear
an object that squishes in one direction, onto the sides
extension and compression
outline simple shear
parallel surfaces moving past each other
material deforms in a way that layers slide over each other. this results in a change in angles between lines that were originally perpendicular
what is the stress/strain relationship known as
glens flow law
outline the stress/strain relationship
logarithmic scales, it is not linear
exponential scale
strain rate = ice hardness*stress^usually 3
why is the exponent usually 3 glens flow law
when we plot it is the most fitting value for the viscoplastic movement , but some think it should 4.
works most of the time with n=3
how does ice hardness change with temperature and give example
increases significantly with fall in temp
ice hardness increase by factor of 1000 from 0 to -55 degrees
other factors affecting ice hardness
water content
impurities
ice at 0 degrees is how much viscous than water?
10^15 times (massive number)
how does strain rate change through time and name phases
primary - initially drops under applied stress, due to stiffening, as ice grains redistribute
secondary - then softens due to recrystallisation and rotation of crystals
tertiary - reaches a steady state
briefly distinguish stress and strain
stress is a measure of how hard a material is being compressed, stretched or twisted as the rest of applied forced
strain measures the amount of deformation that occurs as the result of stress
outline example of stress and strain in relation to toothpaste
toothpaste comes out due to deformation (strain) resulting from an higher pressure on the surface than the nozzle (stress)
define force
the physical influences which change the state of motion of a mass
mass times acceleration
define stress
force per unit area
unit for stress
pascal (newton/metres squared)
two components of stress on a surface
stress acting at right angles to the surface (normal stress)
acting parallel to the surface (shear stress)
what are the two equal and opposing tractions on normal stress
either pressing together across the surface compressive stress
or pulling away from it tensile stress
how do the tractions work in shear stress
parallel but act in opposite directions
at the base of a glacier what is the normal stress
mostly due to weight of overlying ice
define traction
force per unit area on a surface of a specified orientation - a measure of force intensity
define surface stress
a pair of equal and opposite tractions acting across a surface of specified orientation
define shear stress
a pair of tractions acting parallel to a surface
define normal stress
a pair of tractions acting at right angles to a surface
two basic types of strain
elastic (recoverable)
permanent (irrecoverable)
two forms of permanent deformation
brittle failure - where the material breaks along a fracture
ductile deformation - where material undergoes flow or creep
what is it known as when a material undergoes a change in volume due to deformation
dilation
why is transformation of now to ice dilation
the accumulation area of a glacier reduces in volume while increasing in density
why is a glacier mostly constant volume strain
it is essentially incompressible. after snow is condensed to ice
how is strain measured
comparing the shape and size before and after deformation.
two fundamental types of strain
pure shear
simple shear
outline pure shear
flattening or stretching of a material under compressive and tensile deviatoric stresses
two ways of measuring strain
strain rate - amount per unit of time
cumulative strain - net amount that takes place in a given time interval
what is rheology
the way in which strain rate varies with applied stress for a given material
what is yield strength
the value of the applied stress at the onset of permanent deformation, measured in Pascals
what does it mean if yield strength = 0
permanent deformation will happen at any stress, no matter howsmall
what is the yield strength of ice
0
the yield strength of subglacial sediments can be understood as the sum of which two properties…:
cohesion, friction
what does glens flow law calculate
how much deformation do you get if you increase the force on ice
glens flow law equation
E (strain) = A * T^3 (ice hardness*stress cubed)
main control on ice hardness
temp
why if we increase stress a little does strain increase a lot
strain is hardness * stress cubed
how is shear stress calcualted
ice density x gravity x thickness of ice x sin of ice gradient
weight x sin of ice surface gradient
why is the ice surface gradient important in shear stress
if it was flat, there would just be normal stress crushing the ice down
as it increases shear stress increases
how much of the weight of the ice that is generated through shear stress rather than normal stress depends on that gradient
why study glacial erosion?
key component of landscape evolution - mountains, valleys, fjords
sediment flux to oceans and global cycles
palaeoclimate insights from landslides
hazards
