Meteorology 1014

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Lake-Effect Snow (LES)
1
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

2
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

3
Lake Effect Snow from space
4
SeaWifs
Nov 30, 2004
Lake Effect Snow from space.
5
A Lake-Effect Snow Storm on Radar
6
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
11
Zooming In – The
Average Annual Snowfall
(inches) Over the Eastern
Great Lakes
12
Record Event
37.9 inches at the
Buffalo Airport in 24 h
13
The Lake-Effect “Season”
15
Basic Concepts of Formation
16
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
19
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.
20
Basic Concepts of Formation
Note how the inversion has risen in altitude and the
lower-levels of the atmosphere have moistened.
22
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
24
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
1
2
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Formation of Bands
Note the rising and sinking motion
27
Formation of Bands
Note the rising and sinking motion
Clouds are suppressed in between bands
28
Formation of Bands
29
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
30
The Temperature Difference
on a Thermodynamic Diagram
31
Water Temperatures are Available
http://coastwatch.glerl.noaa.gov/cwdata/lct/glsea.png
32
The State of the Water and Land
is Critical
33
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
34
Basic Concepts of Formation
36
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
37
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
38
Demonstration of Fetch
~70 miles
39
Favorable Fetches for LE Snow
40
‘Preconditioning’ by upwind lakes
Lake Nipigon
SeaWiFS: Dec 5, 2000
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‘Preconditioning’ by upwind lakes
Lake Nipigon
MODIS: Dec 16, 2009
42
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
43
Orographic Lift Can Make a
HUGE Difference!
Lake Superior surface: 600 feet
Brockway Mountain: 1330 feet
44
Effect of Orography
45
Shoreline Orientation Can
Make a HUGE Difference!
46
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 “+”
47
Shoreline Orientation Can
Make a HUGE Difference!
First band
forms in the
convergence
region. Note
divergence
“-” nearby
48
Shoreline Orientation Can
Make a HUGE Difference!
49
This Theory in Action
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This Theory in Action
51
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
52
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
54
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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