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

13.1 Snow climatology highly influenced by lake effects

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) – Wind move across short axis of lake – Convective overturning forming rolls nearly parallel to wind Shore Parallel Bands (a and b) – 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 – 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

13.4 Notice smaller wind parallel bands…these are small scale convective rolls resulting from unstable overturning

13.10 Convective Rolls produce bands

13.5

1704 UTC UTC Cloud rolls over water Spectacular Cloud streets over land Effect of lake shoreline Gravity waves perpendicular to flow Visible Satellite Loop

13.13

Lake Erie Shore Parallel Band December 24, 2001 Buffalo

13A

Lake Michigan Shore Parallel Band

Lake Erie Lake Effect Vortex December 24, 2001 Buffalo

13.14

13.6a Ice Cover Analysis

13.6b Water Temperature

13.7 Lake effect greater in early winter when lakes are relatively warm and ice free

13.9 Longer wind Fetch will produce more mature clouds and precipitation process and so more snow. Therefore more snow downwind of where lake is wider