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Surface faceting and near crust faceting Learning Outcomes Understand and recognize surface faceting. Understand and recognize near-crust faceting. American.

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Presentation on theme: "Surface faceting and near crust faceting Learning Outcomes Understand and recognize surface faceting. Understand and recognize near-crust faceting. American."— Presentation transcript:

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

2 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. Surface faceting and near crust faceting

3 What is a Snowpack?

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

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

7 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. Surface faceting and near crust faceting Vapor Pressure

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: 1)Conduction through the ice skeleton 2)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 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 Convection = transfer of energy where the substance (molecules )mixes (liquids and gases) - Sensible Heat or Latent Heat Fluxes Radiation Radiation = transfer of energy by electromagnetic waves, the only energy type requiring no medium (shortwave and long wave) Advection Advection = transfer of energy by mass transfer (eg. rain on snow, avalanches, etc.)

11 Heat Definitions Latent Heat 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 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

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

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

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

17 A B T0CT0C Initial Stratigraphy Resultant Stratigraphy Depth below surface (cm) Long Wave Short Wave L0L0 S1S1 S0S0 Snow Temp.

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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 

21 Surface faceting example 0ºTº C vs TG w TG -1º -21º  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. 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…..

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.

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

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

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

28 3 TYPES OF NSF RADIATION RECRYSTALLIZATION MELT LAYER RECRYSTALLIZATION DIURNAL RECRYSTALLIZATION

29 Why Surface Facets are Important

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

31 DIURANAL RECRYSTALLIZATION 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 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

32 RADIATION RECRYSTALLIZATION DAY ~ 3-5cm warm cold SWinSWout SWabsorbed LWout { Snow cover

33 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. 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. RADIATION RECRYSTALLIZATION

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

35 MELT-LAYER RECRYSTALLIZATION cold Sun or Rain or wet snow Saturated snow surface Warm melt layer New snow New snow cold weather SNOW Followed by …..

36 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 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 MELT LAYER RECRYSTALLIZATION

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 0ºTº C vs TG w TG -1º -21º 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. T 10 – T gnd HS/10 = cTG What is the temperature gradient? remember we are interested in degrees/10 cm? Near-crust faceting

40 0ºTº C vs TG w TG -1º -21º  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. T 10 – T gnd HS/10 = cTG Near-crust faceting

41 0ºTº C vs TG w TG -1º -21º 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. Near-crust faceting

42 Tº C  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 Near-crust faceting

43 Tº C 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 Near-crust faceting

44 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. crust hi water vapor Near-crust faceting

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

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

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

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

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

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

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

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

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

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

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

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

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 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. Depth Hoar - facets A depth hoar grain, photograph using polarized light.

59 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.

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


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