Download Figure 22 (cover)

Lecture 18
Lake Effect
Storms
Homework Due Friday, December 12, 2014
TYU Ch 13: 2 ,4, ,6, 7,18 ; TYPSS 3
TYU Ch 16: 1, 2, 3, 7, 11 ; TYPSS 2
Extra Credit, (Not to be covered in Class):
TYU Ch 17: 1, 3, 5, 13
TYU Ch 15: 1, 2, 3, 4, 5, 6, 7
Snow climatology highly influenced by lake effects
13.1
Why Lake Effect?
• In Fall season, lakes are warm, advancing polar air
comes across with cold air and strong winds
– Thermal contrast, together with strong winds across water
whips up strong surface fluxes of heat and moisture
– Near surface warming gives rise to static instability, i.e.
near surface warm, moist air underlies cold advancing air
above
– Strong unstable overturning results, producing
progressively deeper clouds as air traverses lake
– If clouds and their precipitation process is sufficiently
mature as air approaches lee shore, precipitation reaches
surface on lee shore
– When advancing polar air is sufficiently cold, precipitation
will be snow
The wind and temperature structure on the back
side of a polar cyclone sets up the situation for
lake effect snow
Cold, northerly wind along long
axis of Lake Michigan
Air Moving off the lake shore in Sheboygan, Wi
Lidar Observation of Steam Fog
Just 5 km across this domain!
Lidar Observation of Steam Fog
‘(5 km across this domain)
Lidar Observation of Steam Fog
Vertical cross section (RHI scan)
Notice lake effect plumes rising into 500 m deep boundary
layer coming off of land
Lake Effect Storm Types
•
Wind/Shear Parallel Bands (a)
–
–
•
Shore Parallel Bands (a and b)
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–
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•
Wind move across short axis of lake
Convective overturning forming rolls nearly parallel to wind
Wind blows along long axis of lake
Mesoscale land breeze circulation develops, producing band of upward
motion in center of lake, parallel to long axis of lake
Deep overturning compared to wind parallel bands (about 4 km)
Heavy snow where mid lake band intersects shoreline
Mesoscale Vortex
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–
–
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–
Usually when a moderate cold wind blows along long axis of lake
Down middle or toward lee shore when wind is not exactly parallel
Converging flow may produce increasing horizontal surface wind shear that
becomes dynamically unstable and rolls up into a vortex or series of vortices
Vortex may also be influenced by movement of high vorticity at upper levels
These vortices are warm core, and may behave somewhat like small
hurricanes
13.3
Lake effect clouds become deeper and feature
and increasingly mature precipitation process as
flow traverses lake
13B
Notice smaller wind parallel bands…these are small
scale convective rolls resulting from unstable
overturning
13.4
Convective Rolls produce bands
13.10
13.5
Visible
Satellite
Loop
• Cloud rolls over
water
• Spectacular Cloud
streets over land
• Effect of lake
shoreline
• Gravity waves
perpendicular to
flow
1704 UTC - 1748UTC
13.13
Lake Erie Shore Parallel Band
December 24, 2001 Buffalo
13A
Lake Michigan Shore Parallel Band
Lake Michigan Shore Parallel Band
Lake Erie Lake Effect Vortex
December 24, 2001 Buffalo
13.14
Ice Cover Analysis
13.6a
13.6b
Water Temperature
Lake effect greater in early winter when
lakes are relatively warm and ice free
13.7
Longer wind Fetch will produce more mature clouds and
precipitation process and so more snow. Therefore more snow
13.9
downwind of where lake is wider