What’s Happening Today?
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National Weather
Satellite
Atmospheric circulation
Radar
Temperature/Wind Profile
http://www.atmos.illinois.edu/weather/tree/viewer.pl?launch/sfcslp
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Module 3: Winter & Spring Storms
• Goals. Understand:
– how large-scale weather systems are organized
– that atmosphere constantly striving to maintain balance between
forces both in the vertical and horizontal directions
• However, solar heating tends to create unbalanced
conditions
• Nearly all severe weather is a reflection of the atmosphere’s
response to restore balance
Today:
– Terrain effects on weather
– Weather and the Great Salt Lake
• Review: April 2
• Exam: APRIL 7
Recognizing terrain’s role in the forecast process
• Planetary scale
– mean ridge and trough positions
• Synoptic scale
– cyclogenesis and anticyclogenesis
– fronts
• Mesoscale
– Dynamically & thermally driven
circulations
– Orographic precipitation processes
• Local scale
– Impacts of local surface
inhomogeneities
Why is Terrain So Important?
If the earth were greatly reduced in size while maintaining its shape, it
would be smoother than a billiard ball. (Earth radius = 6371 km;
Everest = 8.850 km)
However, the atmosphere is also shallow (scale height ~8.5 km) so
mountains are a significant fraction of atmosphere’s depth
And:
Stability gives the atmosphere a resistance to vertical displacements
The lower atmosphere can be rich in water vapor so that slight
ascent brings the air to saturation
Example: flow around a 500-m mountain (<< 8.5 km) might lead to
1) broad horizontal excursions, 2) downslope windstorm on lee
side, and 3) torrential orographic rain on windward side.
Schematic cross-section of prevailing southerly synoptic flow, northerly surface flow down
The gully, and easterly flow likely drainage flow from Flint Hills. Numbers identify the
Sonic anemometers on the E-W transect. E is to the right and N into the paper.
What is a mountain?
• Definition is subjective
– Roderick Peattie. Mountain Geography (1936)
Mountains are 1) impressive, 2) enter into the
imagination of people living in their shadow, and 3)
have individuality.
• Traditional definition: elevation increase above
surroundings > 300 m MSL
• Objective definitions are difficult:
– Elevation (insufficient criterion, e.g., Great Plains)
– Local relief (Grand Canyon?, incised into plateau)
– Steepness of slope
– The amount of land in slopes
Mountains of the western US
Whiteman (2000)
Western
U.S.
Terrain
(high- dark;
low-light)
Roughness
(dark)
Review:Effect on Precipitation
Average
Annual
Precipitation
http://www.prism.oregonstate.edu/normals/
Western
U.S.
Terrain
(high- dark;
low-light)
How does orography affect fronts?
• Movement
– Low-level blocking and channeling of the winds may retard or
accelerate a front, resulting in a distortion of its “shape”
• Creation or destruction of the front
• Terrain may interact with flows and contribute to their
formation or destruction
• In some cases, entire lower portion of a front may not be able
to cross a mountain ridge or range, leaving only an upperlevel front
Flow splitting around an isolated
mountain range
Convergence zones often form on the back side of
isolated barriers (Ex: Puget Sound convergence zone)
Whiteman (2000)
Terrain channeling
Steenburgh and Blazek (2001)
• Terrain-parallel jet may develop in post-frontal environment
• Contributes to development of frontal nose
Frontal movement up and over a
mountain barrier
Whiteman (2000)
Tax Day Storm:
April 15, 2002
Maximum Temperature: Monday. April 15. 2002
Tax-Day Storm (15 April 2002):
Todd Foisy. April 15, 2002
• Extensive damage ($4M+) from high winds >
35 m / s
• Record lowest SLP (982mb) at Salt Lake City
(SLC)
• Ushered in an extended period of cold/wet
weather
• 5-10 year event
• Max temperature change with cold front 16 C /
hr
• Prefrontal blowing dust visibility < 1 km, closed
roads,
• Rained mud, brownish/orange-colored snow
(J. Shafer)
Bagley. Salt Lake Tribune
Building blocks for orographic storms
• Large-scale weather (e.g., cyclones and
fronts)
– Determines the airmass characteristics, including
wind speed, wind direction, stability, and humidity
• Dynamics of air motion over and around the
mountains
– Determines depth and intensity of the orographic
ascent
• Cloud and precipitation microphysics
– Determines if condensation will lead to
precipitation
Drainage Basins and Lakes
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Gunnison Bay
Bear River
Bay
SPRR causeway
Great Salt Lake
Utah
Fremont Is.
Gilbert Bay
Farmington
Antelope Is.Bay
Salt Lake
Valley
Motivation for Studying the Great Salt Lake
• Level, salinity, & temperature of the lake respond to
regional weather and climate
• Ecological impacts (brine shrimp cysts, bird populations)
• Lake contributes to the development of lake
breezes/fronts and lake-effect snow storms
• Need for weather information over the lake for
protection of nearby population as well as boaters on the
lake
• As climate changes, how will lake, lake ecosystems, and
nearby weather evolve?
• Health effects of mercury and other pollutations
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…And we have our own Pirates!
• https://www.youtube.com/watch?v=NNdxBLn
O3-k
• The Salty Adventures of Jim and Barbados, a
10-episode webseries
• https://www.youtube.com/playlist?list=PLxjka
n5ClxJwRrJ8q6rCHZOGLtVJI18CI
• http://www.chickenwingmedia.com/
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Lake Level- Net Difference between Basin Precipitation & Evaporation
Annual Snow Total
OSU/WRCC
Photo: D. Judd
LEVEL (m)
Lake Level
NOW
YEAR
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Monthly Departure From Normal (meters)
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20 March 2015
http://www.sltrib.com/home/2107813-155/great-salt-lake-at-near-record-low
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Gunnison Bay
Bear River
Bay
SPRR causeway
Great Salt Lake
Utah
Fremont Is.
Gilbert Bay
Farmington
Antelope Is.Bay
Salt Lake
Valley
Variations in Lake Level
$80 million
1987-1989
2004
2.73 million acre-feet
Hat Island
D. Judd
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Weather/lake interactions
As lake level decreases…
• Salinity increases
• Length of the lake remains relatively
unchanged but width decreases significantly
• Lake temperature responds faster to
atmospheric forcing
All of these factors potentially influence
characteristics of lake breezes and lake effect
snowstorms
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Thermally-Driven Flows
• Complex interactions between winds generated
by temperature contrasts between mountains
and valleys and between lake surface and
surrounding desert soils
• Nighttime: down valley (towards lake) since lake
and valleys relatively warm
• Daytime: up valley (away from lake) since lake
and valleys relatively cool
• Thermally-driven flows controlled by larger-scale
atmospheric conditions
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Night:
2ºC
Day:
25ºC
Night:
10ºC
Day:
15ºC
12 – 15º C
October 17 2000
Ingredients Required for Lake-Effect Snowstorm
Despite its relatively small size, the Great Salt Lake produces
snowbands similar to those observed over the Great Lakes
– Lift: A mechanism to lift air parcels must be present (e.g., winds from
opposing sides of the lake converging in the center of the lake)
– Instability: Lake must be much warmer than air at mountain crest level
(by 17oC or more). Likely when cold northerly/northwesterly winds blow
across the lake.
– Moisture: There must be sufficient moisture in the air approaching the
lake for clouds to form when the air is lifted. Evaporation from the lake is
insufficient to generate appreciable snowfall
Steenburgh et al. (2000), Steenburgh (2003)
Lake effect storms likely contribute only 5% of the snowfall in the
Cottonwood Canyons…
Satellite Image of Lake-Effect Snowbands
https://www.youtube.com/watch?v=TWwhNp
uSKLE
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Wind parallel ba
Shore parallel
When/Where around the Great Lakes?
Ingredients Required for Lake-Effect
Snowstorm
Despite its small size, the Great Salt Lake produces
snowbands similar to those observed over the Great
Lakes
– Lift: A mechanism to lift air parcels must be present (e.g., winds from
opposing sides of the lake converging in the center of the lake)
– Instability: Lake must be very much warmer than air at mountain crest level
(by 17oC or more). Likely when cold northerly/northwesterly winds blow
across the lake.
Lake temperature of 7oC and 700 mb temperature of -10oC
– Moisture: There must be sufficient moisture in the air approaching the lake
for clouds to form when the air is lifted. Evaporation from the lake is
insufficient to generate appreciable snowfall
Lake Effect Snow Bands
Solitary midlake band
Alta
SLC
SLC
Alta
0
0.05
0.1
0.15
Precip rate (SWE/h)
0.2
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Great Salt Lake-Effect Snow Influenced
by Mountains and Lake…
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Summary
• The Great Salt Lake responds to regional weather and climate
• Changes in lake state in turn affect local weather and climate
• Future climate of region and future states of lake remain
unclear:
– Warmer temperature leading to changes in seasonality of
snow melt and mix of rain vs. snow
– Global GCMs and downscaling approaches provide mixed
signals for precipitation in region
• Terminal lakes around the world are sensitive
to ongoing and future land use policies
Summary
Lake effect snowstorm needs LIM (lift, instability,
moisture)
For good Salt Lake/Cottonwood lake-effect storm:
– winds out of the northwest along the primary lake axis
– Relatively warm lake temperature relative to
temperature at mountain crest
– Enough moisture flowing from upstream of the lake
over the lake (in other words, the lake is not the
moisture source)
Lake effect snowstorms don’t explain the “greatest
snow on earth” but may enhance Cottonwood
snow totals during some storms
Impact of lake-effect snowstorms on snow totals
in Cottonwood Canyons is perhaps 5-10% of
total for season
Summary
• Mountains provide a means to lift air leading to
the formation of clouds and precipitation
• Mountains distort the progression of fronts
causing them to be slowed down in some areas
and sped up in others
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