GEOL 410

New material Near-surface facets

Photo: Ruby Mt. Helicopter Skiing

What type of temperaturegradient is required?

For near-surface faceting

to occur?Photo: Ruby Mt. Helicopter Skiing

For near-surface faceting tooccur we need what?

Photo: Ruby Mt. Helicopter Skiing

Near-surface facetted grainsNear-surface facetted grainsSnow 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

An example of why near-surface facets An example of why near-surface facets are importantare important

Terminology and predominant processes associated with the Terminology and predominant processes associated with the formation of weak layers of near-surface faceted crystals in the formation of weak layers of near-surface faceted crystals in the mountain snowpack. mountain snowpack.

Birkeland, K., 1998, Arctic and Alpine Research, 30:193-199Birkeland, K., 1998, Arctic and Alpine Research, 30:193-199

An example of why near-surface An example of why near-surface facets are importantfacets are important

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

Mechanisms of near-surface facet formation

Type 1: Diurnal re-crystallization

Perhaps the most widespread process that forms near-surface facets

Near-Surface Gradients from Radiation Balance

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

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

14 HOURS OLD14 HOURS OLD

BIRKELAND,JOHNSON,SCHMIDT

24 HOURS OLD24 HOURS OLD

BIRKELAND,JOHNSON,SCHMIDT

DIURNAL RECRYSTALLIZATIONDIURNAL RECRYSTALLIZATION

DAY

~30cm

Relatively cool

warm

SWin SWout

SWabsorbed

LWout

NIGHT

Relatively warm

cold

Fairly constant temperature (diurnal average)

LWout

{Snow cover

Diurnal CrystallizationTemperatures below 0.3 m of spx change littleSnow surface cools and warms daily; deep pack constant tempStrong temperature gradients (>200°C/m)Temperature gradient positive during dayTemperature 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 formPersistent atmospheric high pressure ridge

Mechanisms of near-surface facet formation

Type 2: Radiation balance

Or Radiation re-crystallizationOr Radiation re-crystallization

Near-Surface Gradients from Radiation Balance

(Extensive LWR loss during the day)

0° Tº C

vs TG

w TG

-1°

-21°301 300

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.

T10 – Tgnd

HS/10 = cTG

Near-Surface Gradients from Radiation Balance

0° Tº C

vs TG

w TG

-1°

-21°301 300

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.T10 – Tgnd

HS/10 = cTG

Near-Surface Gradients from Radiation Balance

•Usually found at high altitudesUsually found at high altitudes•Occurs in the upper few cm of the Occurs in the upper few cm of the snowpack snowpack •Southern aspects Southern aspects •Clear sunny days Clear sunny days •Short wave radiation absorbed (may melt,Short wave radiation absorbed (may melt, certainly warms) certainly warms) • Creates a strong TG in upper few cm Creates a strong TG in upper few cm •PRODUCT: faceted crystals often over a melt PRODUCT: faceted crystals often over a melt freeze crustfreeze crust

Conditions that promote faceting from extensive LWR loss during the day

BIRKELAND,JOHNSON,SCHMIDT

24 hours old24 hours old

Mechanisms of near-surface facet formation

Type 3: Dry snow over wet snow facetingType 3: Dry snow over wet snow faceting

Dry snow over wet snow Dry snow over wet snow facetingfaceting

Dry snow over wet snow Dry snow over wet snow facetingfaceting

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 promoteConditions that promote dry dry snow over wet snow facetingsnow over wet snow faceting

Sunny daysClear daysLow-density new snow at surfaceSubfreezing conditionsWarm precipitation events follow by

cold ones

From Birkeland, 1998

NSF Wrap Up• Faceting occurs when?• Facets occur where?• Is faceting good or bad?