Transcript Meteorology 1014

Lake-Effect Snow (LES)
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Overview of the Lake-Effect
Process
Occurs to the lee of the Great Lakes
during the cool season
 Polar/arctic air travels across a lake,
picks up heat and moisture, and is
destabilized
 Cloud formation is enhanced by thermal
and frictional convergence and upslope
along lee shore

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Lake-Effect Snow Storms
Intense, highly localized snow storms that
form near major bodies of water
 Usually take the shape of narrow bands
downwind of the shore
 Can produce tens of inches of snow in a
single day
 Require a specific set of conditions
involving the atmosphere and land & water
surface

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Lake Effect Snow from space
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SeaWifs
Nov 30, 2004
Lake Effect Snow from space.
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A Lake-Effect Snow Storm on Radar
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A Lake-Effect Snow Storm on Radar
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Geographic Preferences
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Geographic Preferences
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Geographic Preferences
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Great Lakes Snowfall Climatology
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Zooming In – The
Average Annual Snowfall
(inches) Over the Eastern
Great Lakes
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Record Event
37.9 inches at the
Buffalo Airport in 24 h
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The Lake-Effect “Season”
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Basic Concepts of Formation
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Basic Concepts of Formation
The atmosphere upwind of the
lake is characterized by a very
strong temperature inversion, with
arctic air near the ground. Air is
blowing from the land toward the
water.
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Basic Concepts of Formation
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Basic Concepts of Formation
The warm water provides thermal
energy and moisture to the
overlying cold air – remember
that thermal energy transport
is from warm to cold. The warm
air rises to form clouds. Note that
it also raises the height of the
capping inversion.
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Basic Concepts of Formation
Note how the inversion has risen in altitude and the
lower-levels of the atmosphere have moistened.
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Basic Concepts of Formation
The rising air condenses to form
precipitation, and snow falls
downwind of the shore line. The
greater the air-water temperature
contrast, the heavier the snowfall
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Formation of Bands
Looking down the wind direction, from west to
east, the clouds tend to form into bands,
usually oriented parallel to the long axis of the lake
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Formation of Bands
Note the rising and sinking motion
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Formation of Bands
Note the rising and sinking motion
Clouds are suppressed in between bands
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Formation of Bands
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Ingredient #1 for Formation

Sufficient temperature difference between
the lake surface and overlying air
– Represents a measure of instability, similar to
the lifted index in the context of thunderstorms
– At least 13ºC difference between water and
850 mb surface
– This is approximately the dry adiabatic lapse
rate between 1000 mb (surface) and 850 mb
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The Temperature Difference
on a Thermodynamic Diagram
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Water Temperatures are Available
http://coastwatch.glerl.noaa.gov/cwdata/lct/glsea.png
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The State of the Water and Land
is Critical
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Ingredient #2 for Formation
 Sufficiently
deep cold air mass at
the surface
– One of the most important aspects
when considering intensity
– Inversion heights < 3000 ft preclude
heavy lake-effect snows
– Inversion heights > 7500 ft strongly
support heavy lake-effect snows
– In some cases, an inversion may not
be present or obvious
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Basic Concepts of Formation
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Ingredient #3 for Formation

Directional wind shear
– Small amount of directional wind
change with height (< 30 degrees)
below the inversion favors horizontal
roll convection
– Highly sheared environments (> 60
degrees) disrupt and diminish the
efficiency of rolls, leading only to
flurries
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Ingredient #4 for Formation

Adequate Fetch
– Fetch is the distance traveled by air
over water
– Long fetch promotes more heating of
the air and a higher inversion
– A minimum fetch of 100 miles is
needed for significant lake-effect
snow
– Flow over multiple lakes can help
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Demonstration of Fetch
~70 miles
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Favorable Fetches for LE Snow
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‘Preconditioning’ by upwind lakes
Lake Nipigon
SeaWiFS: Dec 5, 2000
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‘Preconditioning’ by upwind lakes
Lake Nipigon
MODIS: Dec 16, 2009
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Ingredient #5 for Formation

Sufficiently moist upstream air
– RH > 70% below the inversion favors
heavy lake-effect snow
– RH < 50% usually means little snow
– Often upstream RH is the factor that
kills potentially heavy lake-effect
events
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Orographic Lift Can Make a
HUGE Difference!
Lake Superior surface: 600 feet
Brockway Mountain: 1330 feet
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Effect of Orography
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Shoreline Orientation Can
Make a HUGE Difference!
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Shoreline Orientation Can
Make a HUGE Difference!
Change in
surface
friction as air
passes
from land to
water causes
convergence
in the
region shown
by a “+”
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Shoreline Orientation Can
Make a HUGE Difference!
First band
forms in the
convergence
region. Note
divergence
“-” nearby
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Shoreline Orientation Can
Make a HUGE Difference!
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This Theory in Action
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This Theory in Action
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Optimal snow growth T/RH
• Dendrites are the largest (lowest density) crystals and grow quickly
• 850 mb temperatures of -10ºC or lower needed for heavy lake-effect snow
http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm
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Cyclonic circulation
Cyclonic curvature (height contours curve to left downstream)
Cyclonic flow at ‘subgeostrophic’ wind speeds (e.g., through a
low pressure trough) increases convergence and leads to
heavier snowfall – check upper air charts (e.g., 850 mb) 53
If Atmosphere is Sufficiently
Unstable, Thundersnowstorms
Can Form
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Summary – setup for LE snow
Instability (dT from lake surface to 850 mb)
 Fetch
 Upstream moisture
 Preconditioning by upwind lakes
 Synoptic forcing (low pressure systems)
 Topography (lifting)
 Height of temperature inversion
 Low wind shear
 Snow/ice cover upwind
 Geometry of upwind lake shore

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