Transcript Snow Formation - Atmospheric Sciences at UNBC

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.
1
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.
2
• 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.
3
4
Source: Gray and Male (1981)
• 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
5
6
Source: Cotton (1990)
• 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.
7
•
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 impede storm motion and
increase duration of snowfall events.
8
9
Source: Gray and Male (1981)
• 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.
10
• 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.)
11
Source: Stull (2000)
12
•
1)
2)
•
•
There are two types of ice nucleation:
homogeneous nucleation
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).
13
• 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
14
15
Source: Rogers and Yau (1987)
16
Source: Gray and Male (1981)
• 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.
17
Source: Stull (2000)
18
• 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
19
through gravitational acceleration.
• 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) plates
2) needles
3) sheaths
4) columns
5) dendrites
20
Source: Stull (2000)
21
22
Source: Rogers and Yau (1987)
23
Source: Gray and Male (1981)
Source:
Libbrecht
24
2007
Source:
Libbrecht
2007
25
Source:
Libbrecht
26
2007
Source:
Libbrecht
27
2007
Source:
Libbrecht
2007
28
Source:
Libbrecht
2007
29
Source:
Libbrecht
30
2007
Source:
Libbrecht
31
2007
Source: Bentley and Humphreys (1931)
32
Source: Bentley and Humphreys (1931)
33
Source: Bentley and Humphreys (1931)
34
Source: Bentley and Humphreys (1931)
35
• 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
36
• 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.
37
• 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.
38
• 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.
39
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
40
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.
41
• 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.
42
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.
43
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.
44
45
• 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.
46
• 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.
47
• 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.
48
• 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.
49
• 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.
50
• 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.
51