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Surface faceting and near crust faceting

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1 Surface faceting and near crust faceting
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? Get them thinking of a snowpack as a blanket. This blanket is an ice skeleton with pores. The pores are filled mostly with air and water vapor, and sometime liquid water. Many researchers believe that all snow that falls from the sky sublimates and redeposits at some point….this means there is now snow in a snowpack that exists as it did when it fell out of the sky. All snow has gone from solid to vapor phase , and back to solid again at some point, sometime becoming a liquid as well.

4 Air-ground-Atmosphere cartoon
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 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 Snow Ground Air Colder Warmer More Water Vapor Less Water Vapor

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 and Latent Heat
LATENT HEAT EXCHANGES ADVECTION Advection could also include things like avalanches.

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 This slide may not be needed
This slide may not be needed. It is included here to illustrate that turbulent fluxes tend to cancel each other out while radiation and melt energy does not.

15 Define Radiation Terms
Longwave Radiation (LWR): heat you can’t see Shortwave radiation: visible light LWR =Heat radiation with wavelengths greater 4 µm. SWR= visible and near-visible portions of the electromagnetic spectrum (roughly 0.4 to 4.0 µm in wavelength)

16 Radiation Balance Reradiation Albedo The balance between LWR and SWR radiation drives vapor transfer in the snowpack Discuss SWR in and out and exiting LWR---- start them thinking of yellow and blue arrows The balance between LWR and SWR radiation drives vapor transfer in the snowpack

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

18 LWR is lost from the snowpack on clear nights where exitance is not inhibited by cloud cover. No net loss when there are clouds

19 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

20 Some examples of where going over energy balance is helpful…….
Ground Heat Transfer Mass Change Net Radiation Transfers Latent Heat Transfer Sensible Heat Transfer S

21 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 -1º vs TG w TG Tº C Now, a minor cold front moves through one afternoon and depositing a centimeter of snow on the surface of the pack.

22 Relative Humidity is…. It changes when…..
Review the concept of relative humidty and put in the context of surface hoar.

23 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. A big reason for the formation of surface hoar is that the snow temp at night is lower than the atmospheric temp because of long-wave radiation loss. Thus the RH at snow surface is 100% (dew point) even when atm above has RH less than 100%.

24 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

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

26 NEAR SURFACE FACETING Coming after L2 TG / Meta Discussions

27 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

MELT LAYER RECRYSTALLIZATION Three types currently identified – have been studied/ID’d over the past 25 years DIURNAL RECRYSTALLIZATION

29 Why Surface Facets are Important
Why should we be concerned – because of the above

~30cm Relatively cool warm SWin SWout SWabsorbed LWout NIGHT Relatively warm cold Fairly constant temperature (diurnal average) LWout { Derived from K. Birkeland paper During the day yellow and blue add such that the snow surface is warmed. At night, the yellow arrows go away, but the surface cools due to LWR loss. Snow cover

Clear cold nights following relatively warm days The cold nights crank up the faceting process Faceted crystals may get a lot larger if conditions persist for several days PRODUCT: bi-directional faceted crystals

From K. Birkeland paper The blue arrow is bigger than yellow arrows, such that the snow surface cools due to LWR loss while a few cm below the surface is warmed due to absorption of incoming SWR. cold SWabsorbed ~ 3-5cm Snow cover warm

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.

34 Near crust Faceting Dry snow over wet snow Facets at interface

Sun or Rain or wet snow Saturated snow surface New snow cold weather Followed by ….. cold New snow Warm melt layer K. Birkeland paper Again think about latent heat exchanges here. SNOW SNOW

Occurs in the upper few cm of the snowpack Melt of snow surface/near surface due to solar radiation (short wave) or rain New cold snow falls Strong temperature gradient between 0 o C layer cold snow ( oC/m) PRODUCT: Faceted crystals above the new ice crust

37 Conditions that promote near-surface faceting
Sunny days Clear days Low-density new snow at surface Subfreezing conditions

38 Again think latent heat…..
snow on crusts or wet snow Rain or wet snow on snow events (heat record) Avalanche debris

39 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 -1º vs TG w TG Tº C What is the temperature gradient? remember we are interested in degrees/10 cm? T10 – Tgnd = cTG HS/10 

40 Near-crust faceting T10 – Tgnd = cTG HS/10
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 -1º vs TG w TG Tº C T10 – Tgnd = cTG HS/10 

41 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 -1º vs TG w TG Tº C

42 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

43 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

44 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

45 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

46 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

47 This creates an anomaly in the overall temperature gradient.
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

48 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

49 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

50 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

51 Near-crust faceting 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 hi water vapor Low TG crust hi water vapor

52 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

53 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

54 Near-crust faceting 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. hi water vapor Low TG crust hi water vapor

55 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

56 Near-crust faceting Near crust faceting created a very persistent problem in the Columbia Mountains of Western Canada during the 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

57 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.

58 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.

59 The result is an unstable grain that acts like a lever.
Depth Hoar - facets 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. 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.

60 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.

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