GEOL 410
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GEOL 410. New material Near-surface facets. Photo: Ruby Mt. Helicopter Skiing. What type of temperature gradient is required?. For near-surface faceting to occur?. Photo: Ruby Mt. Helicopter Skiing. For near-surface faceting to occur we need what?. Photo: Ruby Mt. Helicopter Skiing.

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GEOL 410

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Geol 410

GEOL 410

  • New material

  • Near-surface facets

Photo: Ruby Mt. Helicopter Skiing


Geol 410

What type of temperature

gradient is required?

For near-surface faceting to occur?

Photo: Ruby Mt. Helicopter Skiing


Geol 410

For near-surface faceting to

occur we need what?

Photo: Ruby Mt. Helicopter Skiing


Near surface facetted grains

Near-surface facetted grains

  • Snow formed by near-surface vapor pressure gradients caused by strong temp gradients

  • Usually form within 15cm of the surface

  • The weakest grains form near top of layer


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An example of why near-surface facets are importantTerminology and predominant processes associated with the formation of weak layers of near-surface faceted crystals in the mountain snowpack. Birkeland, K., 1998, Arctic and Alpine Research, 30:193-199


An example of why near surface facets are important

An example of why near-surface facets are important


Geol 410

Mechanisms of near-surface

facet formation

  • Near-surface gradients from radiation balance

  • Three types of near-surface processes currently identified – have been studied over the past 25 years


Geol 410

Mechanisms of near-surface

facet formation

Type 1: Diurnal re-crystallization

Perhaps the most widespread process that forms near-surface facets


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  • Near-Surface Gradients from Radiation Balance

  • Diurnal Changes

Strong –TG @ night followed by strong +TG @ daytime


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Conditions that Promote Near-Surface Gradients from Radiation Balance

(Diurnal Changes)

  • Clear cold nights following relatively warm

  • days

  • The cold nights promote the faceting

  • process

  • Faceted crystals may get a lot larger if

  • conditions persist for several days

  • PRODUCT: bi-directional faceted crystals


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14 HOURS OLD

24 HOURS OLD

BIRKELAND,JOHNSON,SCHMIDT

BIRKELAND,JOHNSON,SCHMIDT


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NIGHT

DAY

LWout

LWout

SWin

SWout

warm

cold

SWabsorbed

~30cm

Relatively cool

Relatively warm

Fairly constant temperature (diurnal average)

DIURNAL RECRYSTALLIZATION

{

Snow cover


Geol 410

Temperatures below 0.3 m of spx change little

Snow surface cools and warms daily; deep pack constant temp

Strong temperature gradients (>200°C/m)

Temperature gradient positive during day

Temperature gradient negative during the night

Facets may be bi-directional grow toward the warm surface in the colder areas vapor flux and heat transfer from the warm area to the cold condensation on a colder crystal growth toward vapor source

Optimum conditions:

Clear cold nights warmer sub-freezing days.

Those clear sky nights when surface hoar does not form

Persistent atmospheric high pressure ridge

Diurnal Crystallization


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Mechanisms of near-surface

facet formation

Type 2: Radiation balance

Or Radiation re-crystallization


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Near-Surface Gradients from Radiation Balance

(Extensive LWR loss during the day)


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Near-Surface Gradients from Radiation Balance

In the wake of the cold front the skies clear, and nighttime temperatures drop to -21°C.

In this scenario, we have a 20°C degree temperature difference between the bottom of the 1 cm layer of new snow and the top.

-21°

301 300

-1°

vs TG

w TG

Tº C

T10 – Tgnd

= cTG

HS/10 


Geol 410

Near-Surface Gradients from Radiation Balance

  • A 200°C/10 cm gradient in a 1 cm layer on the surface of the snow.

  • This is a very strong gradient and faceting will occur very quickly.

  • DF grains or rounded grains at or near the surface which are subjected to extreme temperature gradients will become faceted as well.

-21°

301 300

-1°

vs TG

w TG

Tº C

T10 – Tgnd

= cTG

HS/10 


Geol 410

Conditions that promote faceting from

extensive LWR loss during the day

  • Usually found at high altitudes

  • Occurs in the upper few cm of the

  • snowpack

  • Southern aspects

  • Clear sunny days

  • Short wave radiation absorbed (may melt,

  • certainly warms)

  • Creates a strong TG in upper few cm

  • PRODUCT: faceted crystals often over a melt freeze crust


24 hours old

24 hours old

BIRKELAND,JOHNSON,SCHMIDT


Geol 410

Mechanisms of near-surface

facet formation

Type 3: Dry snow over wet snow faceting


Geol 410

Dry snow over wet snow faceting


Dry snow over wet snow faceting

Dry snow over wet snow faceting

  • Looks sparkly, loose, granular, small-medium sized

  • Feels like a granular weak layer in the snow

  • Grows as a result of strong TG between a buried warm/wet old surface and a cold, dry layer of new snow layer on top

  • Distributed by aspect and altitude

  • Persistence ranges from days to months


Conditions that promote dry snow over wet snow faceting

Conditions that promotedry snow over wet snow faceting

  • Sunny days

  • Clear days

  • Low-density new snow at surface

  • Subfreezing conditions

  • Warm precipitation events follow by cold ones


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From Birkeland, 1998


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NSF Wrap Up

  • Faceting occurs when?

  • Facets occur where?

  • Is faceting good or bad?


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