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ENSC 454/654: Snow and Ice Week 3 – Part 2: Snowcover formation and distribution 1 Snow Class Distributions Source: Sturm et al. (1995) 2 Snowcover formation and distribution • Snowcover forms the net accumulation of snow on the ground resulting from precipitation deposited as snowfall, ice pellets, hoar frost and glaze ice, and water from rainfall, much of which subsequently has frozen, and contaminants (such as woody debris, algae, pollutants, etc). • Its structure and dimensions are complex and highly variable both in space and time. 3 Snowcover variability depends on many different factors: 1) the variability of the “parent” weather (in particular, winds, temperature and moisture of the air during precipitation and immediately after deposition); 2) the nature and frequency of the parent storms; 4 3) the weather conditions during periods between storms This is when radiative exchanges may alter the structure, density and optical properties of the snow and wind action may promote scour and redeposition as well as modification of snow density and crystalline structure; 5 4) the processes of metamorphism and ablation can alter the physical properties of the snowcover so that it hardly resembles the freshly-fallen snow; 5) surface topography, physiography and vegetation also affect snow conditions. Influenced by both accumulation and ablation, snowcover is therefore the product of complex factors that affect its accumulation and ablation. 6 Spatial scales of snowcover variability 1) Macroscale (regional scale) 2) Mesoscale (local scale) 3) Microscale 7 Macroscale Variabilty • Areas up to 106 km2 with characteristic linear distances of 104 to 105 m depending on latitude, elevation and orography. • Here the dynamic meteorological effects such as standing atmospheric waves, the directional flow of wind around barriers and lake effects are important. 8 Mesoscale Variability • Characteristic linear distances of 102 to 103 m. • Here redistribution along meso-relief features may occur because of wind or avalanches and deposition and accumulation may be related to the elevation, slope and aspect of the terrain and to the canopy and crop density, tree species or crop type, height, extent and completeness of the vegetative cover. 9 Microscale Variability • Characteristic distances of 10 to 102 m. • Major differences occur at these scales and the accumulation patterns result from numerous interactions, but primarily between surface roughness and transport phenomena. 10 Factors Controlling Snowcover Distribution and Characteristics • Snow accumulation and ablation are controlled primarily by atmospheric conditions and land surface “state”. • The governing atmospheric processes are precipitation, deposition, condensation, turbulent transfer of heat and moisture, radiative exchange and air movement. 11 Environmental Controls on Snow Atmospheric Air Temperature Precipitation Winds Humidity Radiation Turbulent Fluxes (Sensible/Latent) Land Surface Physiography Latitude/Longitude Elevation Slope Aspect Vegetation Soil Temperature Roughness 12 Temperature • The temperature at the time of snowfall, controls the dryness, hardness and crystalline form of the new snow and thereby its erodability by wind. • The importance of temperature is apparent on mountain slopes, where snow depth increases (often linearly) with elevation. 13 • Wet snow, which is heavy and generally not susceptible to movement by wind action, falls when air temperatures are near the melting point; this often occurs when air flows off large bodies of water. • Within continental interiors where colder temperatures often prevail the snowfall is usually relatively dry and light. 14 Winds • The roughness of the land surface affects the structure of wind and hence its velocity distribution. • Because of the frictional drag exerted on the air by the earth's surface, the wind flow near the ground is normally turbulent and snowcover patterns reflect a resulting turbulent structure. 15 • Wind moves snow crystals, changing their physical shape and properties, and redepositing them either into drifts of greater density than the parent material. • For example, Church (1941) found that fresh snow with densities of 36 and 56 kg m-3 increased in density to 176 kg m-3 within 24 hours after being subjected to wind action. 16 • Although initiated by wind action this timedensification of snow is also influenced by condensation, melting, and other processes (see Table 5.1). • Wind transports loose snow causing erosion of the snowcover, packing it into windslabs and crust, and forming drifts and banks. 17 18 Source: Gray and Male (1981) 19 Source: http://www.avalanche.org/~uac/encyclopedia/wind_slab.htm 20 21 • A loose snowcover composed of dry crystals, 1-2 mm in diameter, is readily picked up even by light winds (~ 10 km h-1) • Erosion (mass divergence) prevails at locations where the wind accelerates (at the crest of a ridge). • Deposition (mass convergence) from a fully-laden air stream occurs where the wind velocity decreases (along the edges 22 of forests and cities). • The rate of transport is greatest over flat, extensive open areas, free of obstructions to the airflow, and is least in areas such as cities and forests having great resistance to flow. • Snow mass transport rates in the highly exposed Arctic Coast and tundra regions are substantially greater than those in more sheltered regions (Table 5.2). 23 24 Source: Gray and Male (1981) Source: Déry et al. (2010) 25 • Drifts are deepest where a long upstream fetch covered with loose snow has sustained strong winds from one direction. • The drifts are less pronounced when the winds change direction, especially at low speeds. • Very slight perturbations in the airflow, such as produced by patches of grass, ploughed soil, or fences, may induce drift formation. 26 • In areas with no major change in land use, and where the wind distributions are repeated seasonally, the drifts tend to form in approximately the same shapes and locations from year-to-year. • The largest drifts are caused by major wind storms such as blizzards which may have speeds exceeding 40 km h-1. • Most snow is transported by saltation and turbulent diffusion (suspension). 27 • Saltation is the dominant wind-transport process at low wind speeds (U10 < 36 km h-1) whereas suspension dominates mass transport rates at higher wind speeds. • An important aspect to consider in the redistribution of snowcover by wind is the mass change of a snow crystal, while it is being transported, resulting from its exchange of vapour with the surrounding air (“blowing snow sublimation”). 28 Source: Déry and Taylor (1996) 29 Source: Liston and Sturm (1998) 30 31 Source: Jones et al. (2001) Photographic evidence of blowing snow in the Cariboo Mountains 32 30 June 2007 AWS 16 September 2008 33 Interaction in a Forest Environment • Maximum accumulations of snow often occur at the edges of a forest as a result of snow being blown in from adjacent areas, but depend very highly on the porosity of the stand borders. • Within the stand accumulations may not be uniform, however, generally the snowcover distribution is more uniform within hardwoods than within coniferous forests. • Further, most studies have reported that more snow is found within forest openings than within the stand. 34 Energy and Moisture Transfer • During the winter months energy and moisture transfers to and from the snowcover are significant in changing its state. • Prior to the period of continuous snowmelt the radiative fluxes are dominant in determining changes in depth and density. 35 • The underlying surface, the physical properties of the snowcover and trees, buildings, roads or other features, and activities which interrupt the snowcover or alter its optical properties, affect the net radiative flux to the snow. • Such factors, therefore, influence how the snowcover is modified by the different radiative fluxes to change its erodability, mass and state. 36 • A property of the snowcover surface that directly affects the solar energy absorbed by the snow is its albedo (Table 5.4). • The spatial changes in albedo of a snowcover relate to the snow depth (“masking depth”), which is a regional characteristic. • Heat and mass transfers from the air and ground lead to changes in the crystal structure within the snowcover and to loss 37 of mass as melt or water vapour. 38 Source: Gray and Male (1981) Physiography • Landform and the juxtaposition of surfaces with different thermal and roughness properties are major factors governing snowcover properties. • Winter snowcover reaches the greatest depths in snowbelt areas to the lee of open water areas, and on windward slopes through orographic enhancement of precipitation. • Shallow depths occur on sheltered slopes, particularly those with sunny exposures and at lower elevations where melt losses are more probable. 39 • The usual wind patterns and slides occurring in rugged terrain may result in extremely varied depths. • The physiographic features that relate to snowcover variations are elevation, slope, aspect, roughness and the optical and thermal properties of the underlying materials. 40 Elevation • Normally, in mountainous regions elevation is presumed to be the most important factor affecting snowcover distribution. • Often a linear association between snow accumulation and elevation can be found within a given elevation interval at a specific location. • The increases observed with elevation reflect the combined influence of slope and elevation on the efficiency of the precipitation mechanism. 41 Source: Slaymaker and Kelly (2007) 42 Source: Slaymaker and Kelly (2007) 43 Slope • Mathematically, the orographic precipitation rate is predominantly related to terrain slope and windflow rather than elevation. • If the air is saturated, the rate at which precipitation is produced is directly proportional to the ascent rate of the air mass and, over upsloping terrain this rate is directly proportional to the product of the wind speed and the slope angle. 44 • Even where orography is the principal lifting mechanism and snowfall may be expected to increase with elevation, the depth of accumulation or deposition may not exhibit this trend. • Besides the many factors affecting distribution, winds of high speed and long duration at the higher elevations are more frequent causing transport and redistribution. 45 • In areas topographically-similar to the Prairies, where snow is primarily due to frontal activity and the exposed snowcover is subjected to high wind shear forces, slope and aspect are important terrain variables affecting the snowcover distribution. 46 Aspect • The importance of aspect on accumulation is shown by the large differences between snowcover amounts found on windward and leeward slopes of coastal mountain ranges. • In these regions the major influences of aspect contributing to these differences are assumed to be related to: the directional flow of snowfallproducing air masses; the frequency of snowfall; and the energy exchange processes influencing snowmelt and ablation. 47 48 Source: Déry et al. (2004) Source: Déry et al. (2004) 49 Source: Déry et al. (2004) 50