1 / 25

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.

viet
Download Presentation

GEOL 410

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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?

More Related