Download Topographically Induced Circulations and Precipitation

Topographically Induced
Circulations and
Precipitation
Michael Schlifke
William(Kyle) Corteaux
ESC452
Overview
Topographic lifting/upflow
Downslope flow
Gap Winds
Lee troughs
Cold air damming
Convergence zones
Thermal Contrasts
Questions/comments 
Topographic Lifting
When this type of lifting occurs the land forces the air
to move to a higher elevation.
Air is forced up and over a topographical barrier
such as a hill or mountain.
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The windward side will be cloudy and wet as air ascends
The leeward side will be warmer and drier as the air
descends
Other terms that mean the same as topographic
lifting are orographic lifting, upslope flow, and forced
land lifting.
Topographic
lift(continued)
Air blowing against a range of
hills or mountains is forced
upward, reaches a region of
lower pressure, expands and
cools. Condensation will occur
when the dewpoint is reached.
The type of cloud formed will
depend on the moisture content
and on the stability of the air.
The slope and height of the
terrain and the strength of the
wind component that produces
the upslope flow also have an
effect.
Topographic
lifting(continued)
Examples of where this type of lifting occurs are of
the west side of the Cascades and Sierra Nevada
range in the western U.S.
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places on the sea-facing side of these coastal mountains
see over 100 inches of precipitation per year.
This phenomenon is also seen on the Tug-Hill
Plateau.
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Located just east of Lake Ontario
Elevation changes from ~350 feet on the west side to over
2000 feet on the east side.
Tug-Hill Plateau(Con’t)
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This change in elevation allows for
enhanced lifting and instability, especially
during lake effect snow events
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The town of Montague located on Tug Hill owns the
single day New York State record of snowfall with 78
inches
Hooker, another town on Tug Hill, received an
extraordinary accumulation of snow in the winter of 197677, with a total accumulation of 466.9 inches
Annual snowfall increase by 8-12 inches per
100 foot increase in elevation.
Tug-Hill Plateau
This diagram
shows the added
enhancement as
the Tug-Hill
Plateau averages
over 200 inches
of snow annually.
Downslope Flow
Air that descends an elevated plain and
consequently warms and dries. Occurs when
prevailing winds are from the same direction
as the elevated terrain
Often produces fair weather conditions

As the air flows down the mountains, it warms at the rate of
about 5.5 degrees Fahrenheit per 1,000 feet of descent.
Also produces very dry conditions.
Downslope flow(con’t)
This phenomenon occurs
frequently off the Rocky
Mountains. The warm, dry
winds blowing down the
mountains often quickly
evaporate snow on the
ground, which is why local
forecasters refer to down
slope winds off the Rockies
as "Chinook" winds, which
literally means snow eater.
Chinook Winds
Chinook winds have been observed to elevate midwestern winter
temperatures, often from below −20°C (−4°F) to as high as 10°C
to 20°C (50°F to 68°F), for a few hours or days, at the end of
which, the temperatures plummet to their base levels.
One of the most dramatic examples of the chinook winds
occurred on January 15, 1972, in Loma, Montana. The
temperature rose from −47°C (−54°F) to 9°C (49°F); the greatest
temperature change ever recorded during a 24-hour period.
Winds speeds also increase dramatically as well as the warm dry
air accelerates down the mountain. Speeds of 40-60 mph can be
found in extreme cases.
Gap Winds
Occur when winds push through the terrain in spots,
rather than going over it
The terrain steers the wind flow to more or
less follow the gap axis and thus increases its
speed
Typically, the stronger the pressure gradient
and narrower the gap, the stronger the
resulting wind speed.

Stronger at exit region
Gap Winds(Con’t)
Gap winds typically blow over shallow depths of
several hundred to a few thousand feet deep
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can reach velocities of over 50 knots near the surface
particularly hazardous to boaters along the coastlines or
among islands, and to motorists winding through rugged
mountain terrain
Venturi Effect- The speedup of air through a constriction
due to the pressure rise on the upwind side and the pressure
drop on the downwind side as the air diverges to leave the
constriction. (example: nozzle of hose)
Gap Winds Diagram
Lee-Side Troughs
A pressure trough forms on the lee side of a mountain
range in situations where the wind is blowing with a
substantial component across the mountain range
Caused by the stretching of a column of air as it descends
on the lee-ward side of highly elevated terrain such as the
Rocky Mountains. This stretching causes a spin-up of
vorticity and a resulting upper level divergence
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often seen east of the Rocky Mountains
The presence of a lee-side trough can lead to the
initiation of cyclogenesis.
Lee-Side Trough
Cold Air Damming
Cold air damming occurs when a shallow cold
air mass is funneled across the earth's
surface due to topography.

Cold air being pushed southwards by a high pressure
system cannot press pass the Appalachian
Mountains. Therefore funnels southward (see
diagram)
 Often linked to heavy snow and wintry precipitation
in areas such as Virginia, and North Carolina.
Cold Air Damming(Con’t)
Convergence Zones
Caused by winds circulating around
mountains and converging on the other
side
Puget Sound ( Northwest U.S)

The Puget Sound Convergence Zone (PSCZ) works best
when there is a northwest flow after the passage of a storm
system
Convergence Zones
(Con’t)
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Those northwest winds will collide with the
Olympic Mountains. Part of the air flow will be
deflected east while the other part will be deflected
down the western side of the Olympics.
Each branch of the wind will collide with the
Cascades and be diverted towards the other
(convergence)
Convergence => rising motion=>convection/storm
development
Puget Sound
Froude Number (Fr)
Used to describe the flow pattern over an
obstacle

Air flow over a mountain range
Fr = (U/(gH)^1/2)
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U = representative speed(wind in this case)
g = gravity
H = height (mountain height in this case)
If Fr<<1,when the airflow is slow, the air will flow
around the mountain, not over.
If Fr>>1, the air flows over the mountain
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Topographic lift/precipitation.
Thermal Contrasts
Thermal contrasts caused by differential
heating can also cause topographic
circulations.

Sea/lake breezes
Lifting mechanism
 Can lead to deep moist convection.
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Dry(warm) soil vs. moist(cool) soil
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Boundary=>circulation=>=LL
convergence=>Upward vertical motion.
References
http://www.suite101.com/article.cfm/13646/107207
http://www.theweatherprediction.com/habyhints/335/
http://www.komotv.com/weather/faq/convergence_zone.asp
http://www.theweatherprediction.com/habyhints/129/
http://en.wikipedia.org/wiki/Froude_number