Transcript Mountain_Met_280_Lecture_snow1

Snow and Avalanche
Mechanics
Avalanches and Snow Climate
Avalanches are falling masses of snow that can
contain rocks, soil, or ice (McClung 1993).
Avalanches affect people and damage property,
but these effects are fairly minimal when
compared to ‘real’ severe weather.
While avalanches are interesting and their
dynamics are still not well understood, there is
little justification to fund any major research
programs since their influence is limited in area
and affect only tourism, recreation, and some
transportation.
Avalanches and Snow Climate
Trend in avalanche fatalities in US is increasing.
This is due in part to increased recreation, i.e.,
backcountry skiing and snowmobile use.
Fatalities in Alps, on the other hand, are greater
due to the larger populations in those areas.
Avalanches can have a larger impact on
populations by destroying communication and
power transmission lines.
(McClung 1993
Avalanches and Snow Climate
Avalanches are formed by a combination of snow
layering and weather elements interacting with
the snowpack.
The most destructive avalanche cycles are
caused by direct loading of snowfall form
synoptic-scale weather systems.
(McClung 1993
Avalanches and Snow Climate
Snow Climates:
Maritime Snow Climate- characterized by
relatively heavy snowfall and relatively mild
temperatures.
Snow cover (depth) is deep, rain may also fall
anytime during winter.
Maritime snow covers are often very unstable
with rapidly fluctuating instability.
Ex: Cascades-US, Coastal Mtns British
Columbia, Western Norway.
(McClung 1993
Avalanches and Snow Climate
Snow Climates:
Continental Snow Climate- characterized by
low snowfall, cold temperatures, and locations
inland from coastal areas.
Snow covers are typically shallow and often
unstable due to persistent structural weaknesses.
Rocky Mtns-US, Brooks Range of AK, Pamirs of
Asia.
Failures in old snow is a distinguishing feature of
a continental snow climate.
Effects of wind on snow
The redistribution of snow by wind is a major feature of
mountain snowpacks and it is essential for avalanche
formation in some cases (McClung 1993).
Blowing snow is reserved to describe particles raised to a
height of about 2 m or more.
Blowing snow often obscures visibility.
Drifting snow (90% of transported snow) is used to
describe near-surface transport.
(McClung 1993
Effects of wind on snow
McClung 1993
Effects of wind on snow
The critical wind speed (threshold wind speed) at which
snow is picked up from the surface by turbulent eddies of
wind.
1. Threshold wind speed increases with increasing temperature and
humidity.
2. If original deposition occurs with wind, particles will be broken into
small pieces and will pack to a higher density to subsequently
increase threshold speed.
3. Threshold speed will increase with time since deposition (due to
bond formation between surface grains). Increase will slow with time
and is slower at colder temps.
4. Threshold speed will be much lower if there is a source of particles
such as new snowfall, a low strength layer at the surface or snow on
trees.
Effects of wind on snow
For loosed unbonded snow, the typical threshold wind
speed (at 10 m AGL) is 5 m/s. for a dense bonded snow
cover, winds greater than 25 m/s are needed to produce
blowing snow.
Blowing snow will occur with modest winds whenever snow
is falling.
(McClung 1993c
Effects of wind on snow
Three modes of transport for wind-redistributed snow.
1. Rolling involves the creeplike motion of dry particles
along the surface (depth 1 mm).
2. Saltation occurs as particles bounce along the surface
in a layer about 10 cm deep, dislodging other particles
as they impact the surface. Saltation is initiated with
winds of 5 – 10 m/s over cold loose snow.
3. Suspension is caused by turbulent eddies lifting
particles up to tens of meters above the surface.
(McClung 1993
Effects of wind on snow
(McClung 1993
Effects of wind on snow
In mountainous regions, snow redistribution is uneven
because it is strongly influenced by local topography,
including vegetation, rock outcrops, etc.
(McClung 1993
Lee Slope Deposition: Avalanche and Cornice
Formation
On the lee side of alpine ridge crests, where a sharp
change in slope angle occurs, cornices and avalanches
deposits may form due to formation of eddies by flow
separation.
The windward slope angle is thought to be critical in
determining whether a cornice or snowdrift will form.
As snow is redistributed, the particles become broken and
abraded as they impact snow surface. Upon deposition,
become tightly packed and rapidly produce a slablike
texture as they bond together.
Lee zones usually collect greater amounts of snow then
nearby wind protected valleys locations.
(McClung 1993
Lee Slope Deposition: Avalanche and Cornice
Formation
Cornices usually form on ridge crests but they can form at
any place where a sharp change in slope angle occurs.
Threshold wind speed for cornice formation and growth is
about the same as for transport over loose cold snow (5
to 10 m/s).
For winds in excess of 25 m/s, studies have shown that
cornices can decrease in size due to windward scouring
of the root.
(McClung 1993
Lee Slope Deposition: Avalanche and Cornice
Formation
Lee Slope Deposition: Avalanche and Cornice
Formation
Cornices have three features important for avalanche
safety:
1. An overhanging cornice provides a quick assessment of
the prevailing wind direction in a mountain range from a
distance
2. The steep, lee area below the face of a cornice is itself a
prime area for unstable snow slabs to form.
3. The overhanging face of a cornice on a ridge crest can
and often does collapse.
Avalanches are often triggered by cornices collapsing.
Cornices may form on both sides of a ridge.
Cornice Formation
(McClung 1993
Photographer unkown
Heat Exchange at the Snow Surface
The exchange of heat between the snow surface and the
atmosphere is important for avalanche formation for both
wet and dry snow.
Heat exchange can alter surface snow to produce weak
snow there, which may fail or it may act as a future
failure layer when subsequently buried.
Heat can enter or leave the snowpack surface by
conduction, convection, or radiation.
Heat may be transferred to and from the snowpack by
turbulent exchange-sensible heat.
If air is warmer than snowpack, heat is added to the
snowpack.
Heat Exchange at the Snow Surface
If surface is warmer than air, heat is lost from the snowpack.
Warm air flowing over a snowpack can result in significant
surface warming (foehn wind).
Heat may also flow to and from the snow surface by
condensation resulting from diffusion of water vapor.
Direction of heat flow is from regions of high water vapor
concentration to regions of low concentration.
Since saturated warm air can hole more H2O than saturated
cold air, flux of heat (and H20) is from regions of high
temperature to low temperature.
(McClung 1993)
Heat Exchange at the Snow Surface
Surface Hoar Formation
Surface hoar forms when relatively moist air over a cold
snow surface becomes oversaturated with respect to the
snow surface causing a flux of water vapor, which
condenses on the surface.
The result is feathery crystals (ice/solid equivalent of dew)
varying in thickness from 1 mm to several cm.
Once buried, by new snow, results in a weak layer.
Surface hoar forms at night when the snow surface
generally cools and adjacent air becomes supersaturated.
Surface Hoar
Heat Exchange at the Snow Surface
Surface Hoar Formation: role of katabatic flows.
There has been a long research interest in determining the
role of katabatic flows on surface hoar formation (Colbeck
1988; Hachikubo and Akitaya 1997, 1998).
Hachikubo and Akitaya 1997 investigated the role of wind
speed on surface hoar formation.