Surface faceting and near crust faceting

1. American Institute for Avalanche Research & Education Level II Avalanche Course Surface faceting and near crust faceting • Learning Outcomes • Understand and recognize surface faceting. • Understand and recognize near-crust faceting. 

2. Surface faceting and near crust faceting Faceting occurs when there is a strong temperature gradient. While it is common for facets to occur at or near the bottom of a shallow snowpack, especially early in the season. Facets can develop in other parts of the snowpack, sometimes in very localized regions. Lecture covers: two circumstances where faceting occur: 1) on the surface of the snowpack, and 2) near crusts deeper in the snowpack where temperature gradients appear weak.

3. What is a Snowpack?

4. Air-ground-Atmosphere cartoon Air Colder Snow Less Water Vapor Ground Warmer More Water Vapor stored heat in the ground from summer warming and geothermal heat from the interior of the earth

5. Colder Air Less Water Vapor Snow More Water Vapor Ground Warmer Water Vapor in the Snowpack • mass exchange across pore spaces within a snowpack by sublimation, vapor diffusion, and redeposition • RH within the snowpack is always very close to 100% because pore spaces in snow are poorly ventilated • Water vapor then diffuses from warmer (higher vapor pressure)to colder (lower vapor pressure) areas

6. Why Temps are Important Temperature is only important because vapor pressure decreases nonlinearly with ice temperature !!! Slide 6

7. Surface faceting and near crust faceting Vapor Pressure In the snowpack, RH of the pore space is always near 100%. A delicate balance between under-and-super-saturated water vapor in the pore space drives processes such as sintering and depth hoar formation.

8. Energy Balance Schematic

9. Energy exchange at snow surface • Energy gained or lost at the snow surface is transferred within the snowpack by two primary mechanisms: • Conduction through the ice skeleton • Vapor diffusion through the pore spaces • Energy exchanges at snow-atmosphere interface are driven primarily by: • 1) Radiation • 2) Turbulent fluxes (sensible and latent energy exchanges)

10. Methods of Energy Transfer Conduction = transfer of energy in response to a temperature gradient, via molecule to molecule contact, where the substance itself does not mix (liquids and solids) Convection = transfer of energy where the substance (molecules )mixes (liquids and gases) - Sensible Heat or Latent Heat Fluxes Radiation = transfer of energy by electromagnetic waves, the only energy type requiring no medium (shortwave and long wave) Advection = transfer of energy by mass transfer (eg. rain on snow, avalanches, etc.)

11. Heat Definitions Latent Heat = the amount of heat energy released or absorbed when a substance changes phases (e.g., ice to vapor, or rain to ice) Sensible Heat = the heat that is transported to a body that has a temperature different than it’s surroundings (the heat difference you can “feel” or “sense”)

12. ADVECTION LATENT HEATEXCHANGES Advection and Latent Heat

13. Energy exchange at snow surface Turbulent fluxes, sensible and latent energy exchanges, can be large in magnitude, but usually have opposite signs - tend to cancel each other out Therefore, useful to concentrate on radiation

14. Define Radiation Terms • Longwave Radiation (LWR): heat you can’t see • Shortwave radiation: visible light

15. Albedo Reradiation Radiation Balance The balance between LWR and SWR radiation drives vapor transfer in the snowpack

16. -3 -1 -4 -2 0.0 T0C S1 S0 L0 Resultant Stratigraphy Initial Stratigraphy Short Wave Short Wave Long Wave A 0 1 B 2 Depth below surface (cm) 3 Snow Temp. 4

18. Energy Balance Summary • You should: • Know why energy exchange at the snow surface and within the snowpack is important • Understand what drives vapor movement in the snowpack • Understand radiation balance (SWR and LWR) • Understand why we measure snow temperatures • Understand the mechanisms for energy exchange

19. Surface faceting example • Here, the snowpack is deep (several metres) and uniform. • The temperatures from bottom to top are warm (0ºC at the ground to -1 ºC at the surface) and temperature gradients are very weak. • The dominant process will be rounding throughout the snowpack. -21º 301 300 -1º vs TG w TG 0º Tº C Now, a minor cold front moves through one afternoon and depositing a centimeter of snow on the surface of the pack.

20. Relative Humidity is….It changes when…..

21. Surface Hoar and Longwave Radiation the snow surface is constantly losing long-wave energy is very important. Main driver of near-surface faceted crystal formation.

22. Surface hoar or faceting LWR WARMER, HUMID AIR RH of air mass is often less than 100% COLD SURFACE RH at snow-air interface reaches 100% SNOW

23. Conditions that promote surface hoar growth • Clear skies • Calm winds • Sheltered terrain • Cooling air temperatures • High relative humidity • Proximity of water vapor sources

24. NEAR SURFACE FACETING

25. 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

26. Why Surface Facets are Important

27. Near-crust faceting In the wake of the cold front the skies clear, and nighttime temperatures drop to -21ºC. In this scenario, we have a 20ºC degree temperature difference between the bottom of the 1 cm layer of new snow and the top. -21º 301 300 -1º vs TG w TG 0º Tº C What is the temperature gradient? remember we are interested in degrees/10 cm? T10 – Tgnd = cTG HS/10 

28. Near-crust faceting • A 200ºC/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. • The new snow in this case will show faceted characteristics in a short period of time, sometimes as little as a few hours. • DF grains or rounded grains at or near the surface which are subjected to extreme temperature gradients will become faceted as well. -21º 301 300 -1º vs TG w TG 0º Tº C T10 – Tgnd = cTG HS/10 

29. Near-crust faceting When surface faceting is occurring, the surface of the spx will change texture and appearance. Surface crusts and even soft slabs can soften or disappear altogether if surface faceting persists. -21º 301 300 -1º vs TG w TG 0º Tº C

30. Near-crust faceting • When crusts form on the surface and are buried in the snowpack, sometimes facets form near the crust. • These facets may appear even if there was no sign of faceting while the crust was at the surface of the snowpack. • These facets generally develop some time after the crust is buried. crust Tº C

31. Near-crust faceting Faceting near buried crusts is most common when the crust is strong and form a layer that is a barrier to the movement of water vapor in the spx. Faceting can occur with weaker, more permeable crusts. Near-crust faceting can occur above and/or below the crust. Near-crust faceting is observed even when the temperature gradients in the area of the crust are weak. crust Tº C

32. Near-crust faceting • A crust acts as a vapor barrier or trap which inhibits or stops the flow of water vapor. • The crust itself is a good source of water vapor since it has a high water content due to its higher density. • These two factors create high concentrations of water vapor in the regions just below and above the crust. hi water vapor crust hi water vapor

33. Near-crust faceting How does near-crust faceting occur with weak temperature gradients? 1) The crust is denser than the surrounding snow. 2) The crust has different thermal conductivity than the surrounding snow. 3) The crust transmits heat at a rate that is different from the snow above and below it. hi water vapor Low TG crust hi water vapor

34. Near-crust faceting How does near-crust faceting occur with weak temperature gradients? 4) Since the crust is denser (a poorer insulator) it will conduct heat more readily. 5) The greater conductivity results in a lower than average temperature gradient in the crust. hi water vapor Low TG crust hi water vapor

35. Near-crust faceting If there is a weak temperature gradient in the snowpack as a whole, the gradient in the crust is even weaker. This creates an anomaly in the overall temperature gradient. hi water vapor Low TG crust hi water vapor

36. Near-crust faceting Example: dense rain crust formed of frozen water embedded in a snowpack. If we put a heat source at the base of crust: 1) heat will move from the source (the earth). 2) through the structure (the snowpack) and into the 3) atmosphere, where temperatures are colder. hi water vapor Low TG crust hi water vapor

37. Near-crust faceting The snow is a relatively porous material with lots of air in it. Snow is a poor conductor of heat. hi water vapor Low TG crust hi water vapor

38. Near-crust faceting The rain crust on the other hand, is a good conductor of heat: it is much denser and has far fewer pore spaces and air in it (like a sheet of steel). Heat will move through the snow at a different rate than it will move through the ice (slower in the snow and faster through the ice). This sets up the anomaly in the temperature gradient. hi water vapor Low TG crust hi water vapor

39. Measurements indicate a TG in the layers of snow above and below the crust that averages 0.5ºC/10cm. This is a weak temperature gradient and rounding will dominate. In the crust, the TG is only 0.1ºC /10cm; also a weak temperature gradient. Moving towards equilibrium will equalize the TG throughout the crust and the layers above and below Near-crust faceting hi water vapor Low TG crust hi water vapor

40. Near-crust faceting Changes of heat flow of heat through the varying materials will increase the gradient just above and below the crust. This creates a localized strong temperature gradient in the very region where there is lots of vapor available. Now faceting occurs very readily in those regions. hi water vapor Low TG crust hi water vapor

41. Near-crust faceting The localized strong temperature gradients may exist over only a few millimetres and are probably not measurable with the crude instruments used in avalanche work. Mini-TG is enough of a gradient to promote faceting in that small area w/I the crust. hi water vapor Low TG crust hi water vapor

42. Early in near-crust faceting, the facets form a distinct layer that is observable above and/or below the crust. As the process continues, the crust “erodes” and slowly breaks down. The crust metamorphoses into a crumbly layer of mixed grains including the type that made up the original crust, rounds from layers nearby, and faceted grains from the near crust faceting process. Near-crust faceting hi water vapor Low TG crust hi water vapor

43. Near-crust faceting In Colorado (continental climate): A near-crust faceting was observed in a fracture line profile where the snowpack was completely faceted over its entire depth. An old, weak sun crust that was almost completely eroded had notably larger facets just above and below the crust. hi water vapor Low TG crust hi water vapor

44. Near-crust faceting Near crust faceting created a very persistent problem in the Columbia Mountains of Western Canada during the 1996-1997 season. Facets that formed in conjunction with a November rain crust caused large avalanches for several months. hi water vapor Low TG crust hi water vapor

45. Discussion These examples, while extreme, indicate that near-crust faceting can be a significant factor in the metamorphism of the snowpack. Be aware of its potential and know what to look for.

46. Depth Hoar - facets Angular grains with poor sintering. Each different color is a different facet within the depth hoar grain. Each facet represents a wave of water vapor that depositied as a single unit onto the existing grain. A depth hoar grain, photograph using polarized light.

47. Water vapor is moving upwards, from the bottom of the image towards the top of the image. Hence the depth hoar grains are growing downwards and into the source of water vapor. As each wave of water vapor condenses on the depth hoar grain, the grain becomes larger. The result is an unstable grain that acts like a lever. Depth Hoar - facets Image is about 5 cm. Note that each grain is pointed towards the top of the image and widest towards the bottom of the image.

48. Depth Hoar - facets Another example of depth hoar. Again, the depth hoar grain is growing from the top of the screen towards the bottom of the screen.