Snow fundamentals Roman Juras [email protected] Why do we care about snow? Glaciology Meteorology and climatology Sport Snow science Ecology Hydrology Natural hazard History René Descartes Wilson Bentley The first Ukichiro Nakaya First descriptions of The first photographs snow crystals artificial crystals of the snow in a book: Les Johannes Kepler in laboratory crystals Météores First attempts (1930) (1931) to describe (1637) snow crystals (1611) 3 What is snow? More points of view Monomineralic rock (Schneebeli, 2018) „Snow is a highly porous, sintered material made up of a continuous ice structure and a continuously connected pore space, forming together the snow microstructure.“ (Fierz et al. 2009) ICE + AIR + Water Vapour + (Liquid water + Impurities) „All three phases of water can coexist in snow on the ground“ (Fierz et al. 2009) • Snow is part of the cryosphere – water is represented mostly in a solid form • Most of the snow/ice covered area is located in the Northern Hemisphere 3,8 mil km2 (Aug) – 46,5 (Jan) mil km2 (area of Europe ~ 10,2 mil km2) • Snowmelt contributes approx. by 50 % to annual runoff. Mountainous regions can reach 85 % (i.e. Colorado river). En.wikipedia.org Origin of a snowflake Snow flake consists of one, but more often of multiple ice crystals Supercooled water droplets in clouds is still liquid even in temperature around -40°C. Snow crystals grow in supersaturated environment. Snow crystals start to grow around nuclei (dust particles, droplets, another snow crystals) – process of nucleation See Libbrecht (2005) Snow flake shape – Nakaya Diagram Libbrecht 2005, original Nakaya 1954 Why is snow flake hexagonal shape? Crystallography of snow Water molecules are grouped (chained) by hydrogen bonds – hexagonal shape Snowpack metamorphism Snow flake X snow on ground (snowpack) Snow cover consists of snow grains, which start to change immediately after the deposition Snow metamorphism is driven by more factors: temperature gradient, water vapour pressure grad., shape of snow grains, pressure of the snowpack 11 itself Phases of snow metamorphism Dry snow Wet snow 12 Dry snow metamorphism Destructive changes Starts immediately after snow deposition Flat snow flake is rounding. Current ratio area/volume is large and ineffective. Sphere has the best ratio area/volume. Rounded and well connected grains are created. Water vapour pressure is greater over convex shape -› mass is rather tranfered to the concave areas. This transport is driven by vapour pressure grad., temperature does not change (Equi-temp. metamor.) No liquid water occurs – T < 0°C. 13 Dry snow metamorphism Constructive changes Driven by temperature gradient Vapour transport between grains from warmer areas to the colder. Crystals are only little connected. Facets and depth hoar. Layers of such crystals are very unstable – critical for avalanche triggering This process prevails during long lasting freezing periods – high temperature gradient. 14 Wet snow metamorphism Snow melt Snow grain size is effected by the repeating process of melt and freeze Larger snow grains grows, because smaller grains disappear Liquid water provides heat for melt Snow cover is being ripened and homogenized (grain shape and temperature) Rounded snow grains prevail Great pressure on the bottom of snowpack can cause creation of ice – 15 principle of glacier ice development Snow grains classification - examples Rounded grains Faceted particles Depth hoar Firn Slush Fierz et al. 2009 – The international classification for seasonal snow on the ground Snow cover properties Temperature Temperature gradient is one of the most Isothermal important snowpaCK characteristic T = 0°C Adapted from presentation of Florent Domine (2018) Bulk density - ρ Snow type Density [kg m-3] New snow (dry) 10 - 70 New snow (moist) 100 - 200 Older snow (settled) 200 – 300 Depth hoar 200 - 300 Wind packed snow 350 - 400 Firn 400 - 650 Very wet snow (slush) 700 - 800 Glacier ice 850 - 910 Water (4°C) 1000 Bulk density is specific quantity for any granular/porous material (powder, soil, gravel, snow, …) Snow water equivalent - SWE Depth of liquid water vertical column theoreticaly obtained by complete melting of the snowpack Fundamental variable describing liquid water supply in the snowpack on the watershed SWE = (ρsnow/ρwater) x snow depth Snow water equivalent [mm] Hardness Resistence to penetration of an object to the snowpack Measured by RAM sonde 푛ℎ 푚 R = 1 + 푚 + 푚 푧 1 2 R – resistence [N], m1 weight of the RAM (hammer) m2 weight of the sonde, n – number of beats, z – depth of penetration, h – height of hammer fall. Grain size Grain type Grain size [mm] Very fine < 0,2 Fine 0,2 – 0,5 Medium 0,5 – 1 Coarse 1 – 2 Very coarse 2 – 5 Extreme coarse > 5 Albedo Ratio between reflected (Sr) and received (Si) radiation A = Sr/Si SSA – specific surface area Another way how to look at the grain size and grain shape in one variable 퐴 -3 푆푆퐴 = , A – Surface area, V – Volume, 휌푐푒 = 917 kg.m 휌푖푐푒∙푉 For spherical particles: 3 푆푆퐴 = , R – sphere radius 휌푖푐푒∙푅 SSA varies between > 160 m2.kg-1 (fresh snow) and < 2 m2.kg-1 (refrozen snow) Adapted from poster of Florent Domine (2018) Snow with higher SSA (fresh snow, small grain) scatters light more = higher albedo Snow profile - traditional Sněhový profil Snow profile by NIR photography (Near infrared) Wavelengths ≈ 870 – 940 nm Adapted from Matzl & Schneebeli 2006 Porosity - Ф Volume of the air pores related to the volume of the snow sample Ф = (ρice – ρsnow )/ ρice We can also estimate porosity as Ф ≈ 1/ρsnow Very important factor for surviving in under snow Snow type Porosity [%] New snow 67 - 99 Old snow 35 – 78 Firn 8 – 56 Glacier ice 0 - 8 Liquid water content Liquid water is presented, when snow temperature reaches 0°C Hygroscopic/residual water – irreducible, held by grain surface. Not moving water Capillary water – held by surface pressure of capillary forces. This water is moving in the pores, but do not leave the snowpack Gravitational water – moving water, driven by gravity Liquid water holding capacity – how much liquid water can be held against gravity – cca 6 % (depents on snow type) Liquid water content in snow Category Description Vol. (%) Dry Temperature < 0°C, grains are 0% only little adhesive Moist Temperature ≈ 0°C, Grains are < 3% adhesive (can make snow ball). Water is not visible by 10x magnification. Wet Temperature = 0°C. Water can be 3 – 8% recognised by 10x magnification. Pendular regime Very vet Temperature = 0°C. Water can 8 – 15% released by pressing the snow. Funicular regime Slush Temperature = 0°C. All pores are > 15% filled by water, almost no air bubbles left. Lot of gravitational water Liquid water content in snow Slush 3 – 8% Vol. 8 – 15% Vol. > 15% Vol. Liquid water flow in the snowpack Understanding of flow characteristic enable us to better estimate runoff volume and its timing in the watershed Pores cover Matrix flow Preferential flow Flow direction Vertical flow Horizontal (lateral) flow Capillary action Preferential Matrix flow flow Rainfall simulations – flow paths identification Snow distribution over the globe Sturm et al. 1995 References Fierz, C., Armstrong, R. L., Durand, Y., Etchevers, P., Greene, E., Mcclung, D. M., … Sokratov, S. A. (2009). The International Classification for Seasonal Snow on the Ground. International Classification (1st ed., Vol. 83). Paris: IACS. Libbrecht, K. G. (2005). The physics of snow crystals. Reports on Progress in Physics, 68(4), 855–895. https://doi.org/10.1088/0034- 4885/68/4/R03 Matzl, M., & Schneebeli, M. (2006). Measuring specific surface area of snow by near-infrared photography. Journal of Glaciology, 52(179), 558–564. https://doi.org/10.3189/172756506781828412 Schneebeli, M. (2018). Quantifying snow: traditional and emerging methods. Presentation at SSWS, Col de Lautaret Domine, F. (2018). Impact of climate of the snowpack structure : the example of the Arctic. Presentation at SSWS, Col de Lautaret.