Jon Ahlquist Chapter 12 Middle Latitude Cyclones Definition Examples from history Polar front theory Life cycle Typical paths of lows and highs Where and why storms start Vertical structure of storms Conveyor belt model Nor easters Polar lows Examples of Important Mid latitude Storms in History not in book 1588 Spanish Armada defeated partly by English navy with superior armaments and tactics and later by bad storm 150 Spanish ships started only 65 returned March 1776 Storm caused British to cancel assault on US troops under Washington at Dorchester Heights 6 June 1944 D Day invasion of France in WW II Allied forecasters correctly forecasted break in storm allowing invasion when Germans thought storm would continue Polar Front Theory pp 310 312 Mid latitude storms develop where there is strong horizontal temperature contrast Most cyclones start on existing fronts because of large temperature contrast Mid latitude storms are strongest in winter when north south temperature contrasts are greatest Initial source of energy temperature contrast with warm air rising and cold air sinking Other sources release of latent heat during precipitation and air being pushed from high toward low pressure Cyclogenesis development or strengthening of a mid latitude cyclone MET1010 Intro to the Atmosphere 11 13 2006 Mid latitude Cyclones Cyclone low pressure area hundreds of miles in size with cyclonic circulation i e counterclockwise CCW in the Northern Hemisphere clockwise CW circulation in the Southern Hemisphere Synonym storm Either This mid latitude or tropical chapter focuses on mid latitude cyclones Older meaning of cyclone is tornado as in Iowa State Cyclones named in 1895 We are not using this sense here Anticyclone high pressure area with anti cyclonic circulation CW CCW in N S Hem Development of concept of a front Basic concepts developed by Norwegian team published in 1919 1922 Team members Vilhelm Bjerknes leader physics professor Jacob Jack Bjerknes son of Vilhelm and chief author 22 years old in 1919 30 years later he did fundamental El Nino research at UCLA Halvor Solberg Tor Bergeron Swedish pioneer of ice crystal process of rain formation p 193 of Ahrens Jakob Don t memorize their names but these guys had good ideas in several areas of meteorology Example of 80 20 rule 80 of ideas and productivity come from 20 of workers Life Cycle of Cyclone fig 12 1 p 311 Low starts at kink Cold front moves faster than warm front Separated from warm moist air low dies Ready to become Most intense New storm occluded may form at triple point 1 Jon Ahlquist 11 13 2006 Chain of Cyclones at Various Stages of Life Cycle fig 12 2 p 312 Typical Winter Path of Lows Toward NE fig 12 5a p 313 Mature cyclone over Atlantic Triple point Place where 3 fronts meet New storm may form here See column 1 p 311 Cyclone just starting in lee of Rocky Mtns Cyclone on East Coast still developing Typical Winter Path of Highs Toward SE fig 12 5b p 313 Where and Why Storms Start p 313 Vertical Structure of Mid latitude Cyclones fig 12 7 p 315 As storm grows surface pressure decreases so air must be leaving column 30 000 For surface pressure to drop air must be diverging spreading out at upper 18 000 levels faster than air is converging near the surface Surface high has low level divergence upper level convergence Low tilts NW toward cold high tilts SW toward warm Upper level disturbances Long Short Waves fig 12 9 p 318 MET1010 Intro to the Atmosphere Along East Coast and Gulf Coast in winter strong temperature contrast between cold land and warm water On east side of Rockies Columns of air are stretched vertically as they descend the mountains Like a spinning figure skater who extends her arms upward the column of air will spin faster Fig 12 19 p 326 Long wave disturbances in upper level conditions are nearly stationary Short wave disturbances move eastward more quickly at nearly the speed of the wind often triggering precipitation 2 Jon Ahlquist Simplifications for this course The preceding slide is all you need to know about the section on pp 316 318 titled Upper level Waves and Surface Storms You can skip the first part of the next section titled The Necessary Ingredients for a Developing Wave Cyclone pp 318 321 You can skip the special topic on p 321 Jet Streaks and Storms Resume on p 322 11 13 2006 Conveyor Belt Model pp 322 323 Air rises sinks in mid latitude storms on a slant conveyor belt Warm conveyor belt air in warm sector glides up warm front produces stratus clouds turns toward NE or E Cold conveyor belt Wind blows from east below northern part of warm conveyor belt toward surface low rises moves toward NE with jet Dry conveyor belt upper level dry air to W of storm sinks as it approaches cold front creating dry slot behind cold front Fig 12 13 p 323 Conveyor Belts in Relation to Features in Satellite Picture Fig 12 14 p 323 Northeasters Nor easters p 314 Example of Nor easter Superstorm of March 1993 The Storm of the Century References Meteorology Today our textbook pp 323 325 www comet ucar edu resources cases c1 11mar93 www weathermatrix net education blizzard93 Impact on Tallahassee on Saturday 13 Mar 1993 Snow Also lowest pressure ever recorded at Tallahassee National impact 270 killed 3 times combined death toll of 79 due to Hurricanes Hugo and Andrew 2 billion damage www noaanews noaa gov stories s334c htm Many records for low temperature and snowfall Every airport on east coast closed Only time one storm has ever done that MET1010 Intro to the Atmosphere Mid latitude cyclones that develop or intensify along the east coast Name refers to winds from the NE north of the warm front along the coast The nor easter in this map 11 Dec 1992 caused hundreds of millions of dollars of damage Sometimes have hurricane characteristics such as an eye Maps of Superstorm of March 1993 Fig 12 16 p 324 Arrows denote surface winds Fig 12 18 p 325 mP cP mT 3 Jon Ahlquist 11 13 2006 Color enhanced infrared photo of Storm of the Century fig 12 15 p 324 For more info search Web for Perfect Storm Compare to surface weather map fig 12 16 p 324 for nearly the same time Warm front is roughly along the east coast north of Florida Yellow area inland of warm front is light rain snow from nimbostratus Black region crossing Florida indicates high clouds along cold front thunderstorms intense precipitation Develop during winter Nov March over water poleward of polar front at arctic front where frigid air from land meets much warmer but
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