ensc 454/654: snow & ice week 2: snowfall formation & distribution 1

ENSC 454/654: Snow & Ice

• Week 2: Snowfall formation & distribution1

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Snow• Definition: Snow is defined as particles of ice

formed in a cloud that are large enough to fall toward the ground.

• Snow is therefore solid precipitation formed of white or translucent ice crystals, chiefly in complex hexagonal form and often agglomerated into snowflakes.

• It does not include other forms of solid precipitation such as ice pellets or hail.

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Snowfall

• Definition: Snowfall is falling snowflakes or snow crystals or also the accumulation of snow during a specified period of time.

• The formation of snow requires at least 3 conditions: 1) atmospheric moisture and 2) mechanisms to convert this water vapour into precipitation (i.e. vertical motion of air), 3) temperatures (T) below 0oC.

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• Moisture is supplied by evaporation from oceans, rivers, soils and plants, as well as sublimation from snow and ice.

• The atmosphere’s capacity to hold water decreases exponentially with temperature (Clausius-Clapeyron relation) such that there is potential for heavier snowfall when T nears the freezing point.

• The Pacific Ocean is the main source of atmospheric moisture for B.C.

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Clausius-Clapeyron Relationship

Stull (2000)

6Source: Gray and Male (1981)

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• Vertical motion yields adiabatic expansion and thus cooling of air that may lead to the condensation of water vapour into clouds.

• Four main types of atmospheric processes lead to vertical motion of air:

1) horizontal convergence

2) orographic lift

3) convective lift

4) frontal lift

8Source: Cotton (1990)

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• The intensity of the precipitation and hence snowfall accumulation depends on the rate of vertical motion.

• For B.C. fronts associated with synoptic weather systems and enhanced lifting along the Western Cordillera are the two main mechanisms for vertical motion that yield snowfall.

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• There are 3 main stages in the life cycle of a low pressure system (mid-latitude cyclone):

1) formation of disturbance on a front (formation stage or cyclogenesis)

2) increase in amplitude (developing stage)3) occlusion (mature stage or cyclolysis)

• Orographic upglide increases the rates of condensation and precipitation. Mountains also retard storm motion and increase duration of snowfall events.

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• Summary: The formation of snow depends on many variables including ambient air temperature, moisture and motion.

• Supercooled water (liquid water at T < 0oC) and ice nuclei (particles that serve as nuclei for condensation or deposition and ice crystal formation) must also be present.

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• Different types of snow (rimed crystals, graupel, snow crystals or snowflakes) arise depending on environmental conditions in which they form.

• Ice nuclei from 0.01 to 1 μm are abundant in the atmosphere and consist of aerosols of natural and anthropogenic sources (dust, clay-silicate particles, organic matter from vegetation, pollutants, etc.)

14Source: Stull (2000)

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• There are two types of ice nucleation:

1) homogeneous nucleation

2) heterogeneous nucleation

• Homogeneous nucleation is the formation of pure ice crystals without a “foreign” agent; it occurs mainly at T <

-40oC.

• Heterogeneous nucleation involves freezing of water onto a “host particle” (ice nucleus).

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• There are 4 ice nucleation mechanisms:

1) heterogeneous deposition

2) condensation followed by freezing

3) contact

4) immersion

• Once formed, an ice crystal grows by several mechanisms:

1) deposition

2) riming

3) aggregation

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Source: Rogers and Yau (1987)

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• An ice crystal embedded in a cloud of water droplets will grow at the expense of the droplets because the vapour pressure at the ice surface is less than at the water surface.

• At temperatures conducive for the formation of snow, a cloud may be just slightly super-saturated with respect to water but 10 to 20% supersaturated with respect to ice.

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Source: Stull (2000)

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• Supersaturation is air with relative humidity greater than 100%, i.e. having more water vapour than is needed to produce saturation with respect to a plane surface of pure water.

• This yields a net transfer of water vapour from the cloud droplets to the ice surface, leading to preferential growth of ice crystals over raindrops (the Bergeron process).

• Once the snow crystals become large enough, they accelerate downward through gravitational acceleration.

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• The basic habit or shape of an ice crystal is determined by the temperature at which it grows whereas its growth rate and secondary crystal features are determined by super-saturation.

• Ice crystal habits occur in many different shapes:

1) plates2) needles3) sheaths4) columns5) dendrites

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Source: Stull (2000)

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Source: Rogers and Yau (1987)

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Source: Gray and Male (1981)

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Source: Libbrecht 2007

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Source: B

entley and Hum

phreys (1931)

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Source: B

entley and Hum

phreys (1931)

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Source: B

entley and Hum

phreys (1931)

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Source: B

entley and Hum

phreys (1931)

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• The rate of mass (m) growth of an ice crystal by water vapour diffusion over time (t) is:

• dm/dt = 4πCDFAC(ρ∞ - ρ0), where:• C = shape factor• D = water vapour diffusivity in air• F = ventilation factor • AC = function of crystal size• ρ∞ = water vapour density (wvd) at large

distance from the ice crystal• ρ0 = wvd at the crystal surface

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• This equation is only valid if crystal diameters are less than a few 100 microns, after which growth by collision with cloud droplets becomes dominant.

• Riming is a growth process by accretion of cloud droplets that collide and adhere to a snow crystal.

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• The collision efficiency is defined as the ratio of the number of droplets that actually impact the crystal to the number of droplets that are swept out by it inside a column of air enclosed by the crystal’s cross sectional area and fall distance.

• Aggregation is the adhesion of two or more snow crystals after their collision.

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• This process is most effective at T ~ 0oC when snow becomes “sticky”. Aggregation also depends on distance covered during snowfall.

• At T > 0oC, snow will melt rapidly and fall as rain if the freezing level isotherm is high enough above ground.

• Through deposition, aggregation and riming, a 1 mm diameter snowflake can grow to 10 mm in about 20 min. and if melted form a raindrop of about 1 mm in diameter.

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References

• Cotton, W.R., 1990: Storms, Aster Press.• Bentley, W. A. and W. J. Humphreys, 1931:

Snow Crystals, Dover Press.• Gray, D. M. and D. H. Male, 1981: Handbook of

Snow, The Blackburn Press.• Rogers, R. R. and M. K. Yau, 1987: A Short

Course in Cloud Physics, Pergamon Press.• Stull, R. B., 2000: Meteorology for Scientists and

Engineers, Brooks/Cole

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Physical Characteristics of Snow Crystals

• Atmospheric ice particles are classified based on the different shapes and growth processes of the crystal types: plate, stellar crystal, column, needle, spatial dendrite, capped column, irregular crystal, graupel, ice pellet and hail.

• Within each category the precipitation type may be distinguished according to: broken crystals, rimed particles, clusters, wet or melted, and the maximum particle dimension.

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• Individual snow crystals observed at the earth's surface range in maximum dimension from about 50 μm to 5 mm.

• Particle densities range from approximately 100 to 700 kg m-3.

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Physical Characteristics of Snowflakes

• Snowflakes may consist of two to several hundred snow crystals joined together.

• Generally, for snowflakes to form, a myriad of crystals should be moving at different velocities at air temperatures slightly lower than 0oC.

• Because they have abundant radiating arms, dendritic crystals tend to aggregate more readily than other types and are often found as the constituent crystals in snowflakes.

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Areal Distribution of Snowfall

• The following atmospheric conditions are important in determining the occurrence of significant amounts of snowfall:

1) Sufficient moisture and active nuclei at a temperature suitable for the formation and growth of ice crystals,

2) Sufficient depth of cloud to permit growth of snow crystals by aggregation or accretion,

3) Temperatures below 0oC in most of the layer through which the snow falls, and

4) Sufficient moisture and nuclei to replace losses caused by precipitation.

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Source: Gray and Male (1981)

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Source: Ahrens et al. (2012)

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• The two areas of relatively heavy snowfall in North America are on the east and west coasts of the continent.

• Certain areas of western British Columbia, the Yukon and Alaska adjacent to the mountain ranges which parallel the Pacific Coast, receive seasonal values exceeding 400 cm.

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• However, amounts are highly variable; e.g., along the southern coast of B.C., near sea level, the air temperatures are normally above freezing so that most of the winter precipitation is rain, and the seasonal average snowfall is less than 60 cm.

• Snowfalls are also relatively light in areas to the lee of the mountains.

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• Widespread heavy snowfall also occurs in eastern Canada throughout central Ontario, southern Quebec, much of the Atlantic provinces, Labrador, and the east coast of Baffin Island where the seasonal amounts range from 250 to 400 cm.

• These parts of Canada lie on or near several principal tracks of transient low pressure systems which are frequently vigorous and well developed. Varying amounts of moisture are supplied to these areas from the Pacific and Atlantic Oceans and the Gulf of Mexico.

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• In addition, the Great Lakes serve as an important moisture source for local precipitation, e.g., average seasonal snowfalls greater than 250 cm occur southeast of Lake Huron.

• Snowfall amounts decrease rapidly in the southward direction from the eastern Ontario-northern New England area to the southeastern United States. This is mostly a result of increasing temperatures, as opposed to decreasing precipitation.

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• Over the prairie provinces of Alberta, Saskatchewan, and Manitoba the seasonal snowfall is considerably lower than in the eastern or western regions of Canada, averaging between 75 and 140 cm.

• The small amounts of snowfall over these regions can be attributed, in part, to the infrequent occurrence of vigorous weather systems. Also, the relatively flat terrain is not conducive to snowfall formation since the Pacific air moving inland subsides because of the downward slope in topography from the Rocky Mountains.

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• The western half of the Arctic Islands receives less snow (< 80 cm) than most other parts of Canada.

• Although this area experiences long winters; it is remote from major moisture sources; the extremely low temperatures over the region reduce the moisture holding capacity of the air to extremely low values thereby reducing snowfall amounts.

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