UW-Madison ATMOCN 100 - Lake Effect Storms (6 pages)

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Lake Effect Storms

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Lake Effect Storms


A lecture about lake effect storms and what types there are.

Lecture number:
Lecture Note
University of Wisconsin, Madison
Atmocn 100 - Weather and Climate
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Atm Ocn 100 1st Edition Lecture 36 Outline of Last Lecture I Announcements II Weather of the day III Some Review IV Troughs Cyclones V Summary For Cyclone East of Rockies VI Cyclones Forming Along Gulf and East Coasts Outline of Current Lecture II Reminders III Weather of the day IV Lake Effect Storms V Snow climatology highly influenced by lake effects VI Why Lake Effect VII Lake Effect Storm Types VIII Different Animations of Lake Effect Storm Types Current Lecture Reminders Friday we will have a review session for the final next week There is a homework due Friday with extra credit associated with that Weather of the day The relative humidity plot shows the tropical bubble The blue area is a big elevated bubble of the tropics going around the earth And on the edges of the bubble the magenta colors are the jet streams They are occurring because of the energy from the bubble Last year we saw the polar vortex with strong persistent cold that was over Alaska was then went over us This is an El Nino year and the pattern is different We started off with the ridge but then we warmed up quite a bit There is a Jetstream coming into Baja peninsula This is associated with frequent precipitation near the Southwest coast like San Diego We are getting low amplitude waves moving across Lecture 18 Lake Effect Storms To get a band winds need to be moving parallel to the long axis of the lake The band forms from the contrast of the cold land and the water which if it isn t frozen has to be at 32 degrees Fahrenheit or warmer The water may have a temperature contrast of 30 to 40 degrees Fahrenheit and that thermal contrast produces a land breeze front And the thing you get in the Great Lakes is a land breeze front from both sides Snow climatology highly influenced by lake effects You get short parallel bands that are more intense If you get a cut off low pressure system at low levels with winds moving around cyclonically it may lead to heavy snow If they are cyclonically moving from west to southwest direction they could move parallel to Lake Erie for a long distance which could create a band that exits the coast and get high snowfall Short parallel bands tend to produce the strongest snowfall But wind parallel bands make longest duration snowfalls that are lighter Wind parallel bands if shorelines are far away Fetch How long the cold wind has moved across the lake and how much time it had to pick up moisture the faster it moves the greater the transfer of heat and moisture The faster the wind blows the faster they pick it up If you double the wind speed you pick up more moisture So if there is really strong wind that helps pick up more heat and moisture from the lake and at the same time it will reduce the time that the wind is over the water so it is a tradeoff Models sort this out because they both work to change things 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 are 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 backside of a polar cyclone sets up the situation for lake effect snow There is cold northerly wind along long axis of Lake Michigan Short parallel band and wind parallel bands that interact lee shore Lake Superior is mostly wind parallel bands but shoreline may have other things as well Southern shore will be more vulnerable have greatest fetch how long the cold wind has moved across the lake and how much time it had to pick up moisture heat and be vulnerable to wind parallel bands Air moving off the lakeshore in Sheboygan WI Can see condensation because of mixing The water is warm there is thermal air is overturning rising and sinking There is a pattern of rising steam and it forms cells where air is going up in the boundaries of the cell and sinking in the middle So there are lines that are forming between the cells There are like little tornados forming This lake is only 20 meters deep or so When you get further to the east that you see things Lidar Observation of Steam Fog just 5 km across this domain The Lidar is seeing steam in surface to the light It is looking horizontally at the steam You can animate the picture to see what it looks like Over time as layer moves off of the water it moistens up the layer and deepens the layer to the point it reaches the cloud base And then the cloud deepens to the east 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 of 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 Lake effect clouds become deeper and feature and increasingly mature precipitation process as flow traverses lake The clouds are barely enough to reach free convection Convective Rolls produce bands In between the rolls the air is sinking And on the other side the air is rising Where air is rising cloud streaks are produced Graph This graph shows the relationship between the average air temperature and average lake temperature There is a sweet spot in December January where water is warmer then the air where they will be lake effect storms Visible Satellite Loop Cloud rolls over water Spectacular Cloud streets over land Effect of lake shoreline Lake Erie Shore Parallel Band December 24 2001 Buffalo This band along Lake Erie

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