Jon Ahlquist 11/13/2006MET1010 Intro to the Atmosphere 1Chapter 12:Middle-Latitude CyclonesDefinitionExamples from historyPolar front theoryLife cycleTypical paths of lows and highsWhere and why storms startVertical structure of stormsConveyor belt modelNor’eastersPolar lowsMid-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 mid-latitude or tropical This 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)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.Development of concept of a front Basic concepts developed by Norwegian team, published in 1919 & 1922. Team members: Vilhelm Bjerknes (leader, physics professor) Jakob (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.) 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. 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.Life Cycle of Cyclone (fig. 12.1, p. 311)Low starts at kinkCold front movesfaster than warm frontReady to becomeoccludedMost intense. New stormmay form at “triple point”Separated from warmmoist air, low diesJon Ahlquist 11/13/2006MET1010 Intro to the Atmosphere 2Chain of Cyclones at Various Stages of Life Cycle (fig. 12.2, p. 312)Cyclone juststarting in leeof Rocky MtnsCyclone on East Coaststill developingMature cycloneover AtlanticTriple point:Place where3 fronts meet. New storm may form here. See column 1,p. 311.Typical Winter Path of Lows: Toward NE (fig. 12.5a, p. 313)Typical Winter Path of Highs: Toward SE (fig. 12.5b, p. 313)Where and Why Storms Start (p. 313) 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. 326Vertical Structure of Mid-latitude Cyclones (fig. 12.7, p. 315) As storm grows, surface pressure decreases, so air must be leaving column. For surface pressure to drop, air must be diverging (spreading out) at upper 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.30,000’18,000’Upper level disturbances: Long & Short Waves (fig. 12.9, p. 318) 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.Jon Ahlquist 11/13/2006MET1010 Intro to the Atmosphere 3Simplifications 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. 322Conveyor 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 frontFig. 12.13,p. 323Conveyor Belts in Relation to Features in Satellite Picture (Fig. 12. 14, p. 323)Northeasters = Nor’easters (p. 314) 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 eyeExample 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.Maps of Superstorm of March 1993Fig. 12.16, p. 324 Fig. 12.18, p.325Arrows denotesurface winds.mTmPcPJon Ahlquist 11/13/2006MET1010 Intro to the Atmosphere 4Color-enhanced infrared photo ofStorm of the Century (fig. 12.15, p. 324) Compare to surface weather map, fig. 12.16, p.324, for nearly the same time
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