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GEOL 410

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

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

  3. For near-surface faceting to occur we need what? Photo: Ruby Mt. Helicopter Skiing

  4. 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

  5. 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

  6. An example of why near-surface facets are important

  7. 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

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

  9. Near-Surface Gradients from Radiation Balance • Diurnal Changes Strong –TG @ night followed by strong +TG @ daytime

  10. 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

  11. 14 HOURS OLD 24 HOURS OLD BIRKELAND,JOHNSON,SCHMIDT BIRKELAND,JOHNSON,SCHMIDT

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

  13. 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

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

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

  16. 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 0° Tº C T10 – Tgnd = cTG HS/10 

  17. 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 0° Tº C T10 – Tgnd = cTG HS/10 

  18. 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

  19. 24 hours old BIRKELAND,JOHNSON,SCHMIDT

  20. Mechanisms of near-surface facet formation Type 3: Dry snow over wet snow faceting

  21. Dry snow over wet snow faceting

  22. 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

  23. 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

  24. From Birkeland, 1998

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

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