Accumulation II: Deposition, Wind Transport, and Metamorphism€¦ · Accumulation II: Deposition,...
Transcript of Accumulation II: Deposition, Wind Transport, and Metamorphism€¦ · Accumulation II: Deposition,...
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AccumulationII:Deposition,WindTransport,andMetamorphism
ESS431:PrinciplesofGlaciologyESS505:TheCryosphere
Wednesday,10/09– KnutChristianson
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Today’s Questions -
• How does wind affect the distribution, surface morphology, and density of snow?
• What is the typical thermal structure of the snowpack? What is Fourier’s Law and what information does it provide about depth-temperature profiles in the cryosphere?
• How does the temperature and crystal structure of ice control snow metamorphism?
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Snow Distribution
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Air Flow over RidgesAir moving over ridge moves faster than air over flat terrain because the same flux of air must be transported through a narrower �window� in elevation range
(Analogous to faster river flow where channel narrows)
Avalanche Handbook
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Mountain Terrain and Snow
Snow transport depends on wind speed
• Accelerating air can pick up more snow
• Decelerating air drops some transported snow
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Mountain Wind and Snow
Avalanche Handbook
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Cornice Structure
Avalanche Handbook
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Cornice on a ridge crest
Avalanche Handbook
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Modes of Snow Transport
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Rolling Snow
Grains move along the surface in a layer ~ 1mm thick.
• Rolling crystals can account for 10% of mass transport when both rolling and saltation are active.
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Saltating Snow(Latin saltare, to jump)
Grains are kicked into the air when bombarded by other crystals
• Need some airborne particles to get started (e.g. dust, precipitating snow)
• Can get started at w ~ 5 m s-1 in cold loose snow• Saltating grains concentrated in ~0.1 m above
surface.
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Traverse from Dome C to Dumont d�Urville, February 2004
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Suspended SnowHorizontal wind over rough surface can cause eddies.
• Turbulent eddies can pick up snow.• Snow crystals typically fall at 0.2 –2.0 m s-1
• Upward eddy speed must exceed rate of fall.• Saltation ® suspension when w >15 m s-1
• Most mass is transported within ~1 m of snow surface•�blowing snow� vs. �drifting snow�
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Consequences of Snow Transport
Snow can be redistributed on the ground.
Surface morphology can be altered1) Dunes2) Sastrugi3) Wind crust/ wind packing
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Dunes
Saltating snow forms moving dunes
http://www.icehouse.net/john_benham/black&white-11.htm
Migrating dunes Niwot Ridge CO
Video:
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Sastrugi• Dunes can sinter and stabilize when wind drops• Saltating snow can be agent of erosion• Subsequent wind can erode sintered dunes
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Crusts•Saltation can break crystals into fine fragments•Surface snow is porous and permeable•Wind drives fragments into holes between grains•Often denser and less permeable than snow below.•Result is called �wind slab�
https://www.flickr.com/photos/somenice/3404055374/
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Traverse from Dome C to Dumont d�Urville, February 2004
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Consequences of Snow Transport
Snow can be redistributed on the ground.
Surface morphology can be altered1) Dunes2) Sastrugi3) Wind crust/ wind packing
Implications• Hard to measure precipitation• Sublimation losses can be severe during transport
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Snow Metamorphism
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Heat Flow and Temperature in Snow2. Thermal conductivity• Snow is a mixture of air and ice• Which has higher thermal conductivity?
Kice ~ 2 W m-1 deg-1
Kair ~ 0.024 W m-1 deg-1
• How does heat move through snowpack?Fourier’s Law: What is Keff?
• Snow is supported by forces between touching grains. How does the structure of contacts affect heat flow?
Q = −KeffdTdz
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Wet Snow• Temperature gradients and freezing-point
differences are created by curvature differences.
• Heat flows from concave to convex surfaces• Heat is used for phase changes.
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Temperatures in Snow• air is cold, ground is warm
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Vapor-Pressure Gradients• Vapor-pressure gradients created by temperature gradients•Vapor diffuses from higher to lower pressure• Refreezes in faceted forms and hoar
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Equilibrium-form Metamorphism(�Destructive�Metamorphism)
• Ice-air surface area represents stored energy (broken bonds)
• New snow has high surface-to-volume ratio (small grains, complex shapes)
• Old snow with large rounded grains represents a lower-energy state
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Mass Transfer Reduces
Surface Area
C.F. Raymond
Result is equilibrium forms
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Destructive Metamorphism of a Snow Crystal
E. LaChapelle, Field Guide to Snow Crystals
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Early Destructive Metamorphism
LaChapelle, Field Guide to Snow Crystals
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Ongoing Destructive
Metamorphism
LaChapelle, Field Guide to Snow Crystals
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�Old Snow�: the equilibrium form.
The end result of destructive
metamorphism.
LaChapelle,Field Guide to Snow Crystals
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Destructive Metamorphism
•Scan Field Guide Fig. 35
McClung and Schaerer, Avalanche Handbook
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Kinetic-Growth Metamorphism [�Constructive�Metamorphism]
vapor-pressure gradients created by temperature gradients are more important than
vapor-pressure gradients from grain curvature(e.g., when dT/dz > 10 deg m-1)
• Formation of frost and hoar• Complex shapes increase surface area
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LaChapelle 1969
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LaChapelle 1969
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WAIS Divide Ice-Core Drill Arch and Hoar
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WAIS Divide 2013/14 - Ice Core Storage
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WAIS Divide 2013/14 - Ice Core Storage
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WAIS Divide 2013/14 - Ice Core Storage
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LaChapelle 1969
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LaChapelle, 1969:Field Guide to Snow Crystals
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Summary
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Deposited Snow
Snow Stratigaphy• Layers with differing density and texture
(grain size, shape, bonding)• Interfaces between layers
Causes of Stratigaphy• Variations in snowfall• Wind action• Metamorphic processes acting on surface
and interior of snow pack
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Physical properties of snow1. Density (water is 1000 kg m-3 or 1 tonne m-3)
Type of Snow Density (kg m-3)Dry new snow 50 – 70Damp new snow 100 – 200Settled snow 200 – 300�Depth hoar� 100 – 300Cold wind-packed snow 300 – 400Melting snow 300 – 550�Firn� (survived 1 year) 550 – 830Glacier ice (bubbles occluded) 830 – 917
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Wind Transport
Wind must exceed a threshold speed wc to move snow.
wc depends on snow characteristics• Warmer snow ® larger wc• Higher humidity ® larger wc• Older snow ® larger wc
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Threshold Wind Speed
•Warmer snow ® stickier snow • Higher humidity ® stickier snow • Older snow ® larger, heavier,
better-bonded grains
Wind speed varies with height.Standard to measure wc at 10 m above surface:• Loose unbonded snow wc ~ 5 m s-1 (~10 kt) • Dense bonded snow wc ~ 25 m s-1
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Example Exercise
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Latent Heat of Fusion: 334 J/gHail Size: 10^-3 gHeat Capacity of Ice: 2.1 J/(gK)Hail Temperature: -10 CAir Temperature: -20 C
What is the mass of rime required to warm the hail grain to the freezing point?
When the surface of hail reaches the melting point, it no longer traps air within the ice. What conditions are required to form hail like the hail stone pictured?
(Hint: Conductive cooling plays a role)