GEOL 410 New material  Near-surface facets Photo: Ruby Mt. Helicopter Skiing

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

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 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

An example of why near-surface facets are important Terminology 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:

An example of why near-surface facets are important

Mechanisms of near-surface facet formation 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 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 Mechanisms of near-surface facet formation Type 1: Diurnal re-crystallization Perhaps the most widespread process that forms near-surface facets Type 1: Diurnal re-crystallization Perhaps the most widespread process that forms near-surface facets

Near-Surface Gradients from Radiation Balance Diurnal Changes Near-Surface Gradients from Radiation Balance Diurnal Changes Strong night followed by strong daytime

Conditions that Promote Near- Surface Gradients from Radiation Balance (Diurnal Changes) 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  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 OLD BIRKELAND,JOHNSON,SCHMIDT 24 HOURS OLD BIRKELAND,JOHNSON,SCHMIDT

DIURNAL RECRYSTALLIZATION DAY ~30cm Relatively cool warm SWinSWout SWabsorbed LWout NIGHT Relatively warm cold Fairly constant temperature (diurnal average) LWout { Snow cover

Diurnal Crystallization 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 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

Mechanisms of near-surface facet formation Mechanisms of near-surface facet formation Type 2: Radiation balance Or Radiation re-crystallization Type 2: Radiation balance Or Radiation re-crystallization

Near-Surface Gradients from Radiation Balance (Extensive LWR loss during the day) Near-Surface Gradients from Radiation Balance (Extensive LWR loss during the day)

0°Tº C vs TG w TG -1° -21° 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. 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. T 10 – T gnd HS/10 = cTG Near-Surface Gradients from Radiation Balance

0°Tº C vs TG w TG -1° -21°  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.  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. T 10 – T gnd 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 theOccurs in the upper few cm of the snowpack snowpack Southern aspectsSouthern aspects Clear sunny daysClear 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 freeze crustPRODUCT: faceted crystals often over a melt freeze crust Usually found at high altitudesUsually found at high altitudes Occurs in the upper few cm of theOccurs in the upper few cm of the snowpack snowpack Southern aspectsSouthern aspects Clear sunny daysClear 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 freeze crustPRODUCT: faceted crystals often over a melt freeze crust Conditions that promote faceting from extensive LWR loss during the day Conditions that promote faceting from extensive LWR loss during the day

BIRKELAND,JOHNSON,SCHMIDT 24 hours old

Mechanisms of near-surface facet formation Mechanisms of near-surface facet formation Type 3: 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 Sunny days Clear days Low-density new snow at surface Subfreezing conditions Warm precipitation events follow by cold ones

From Birkeland, 1998

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