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GEO 203 Notes – 11/30/10 (Lecture #17)(check lecture .ppt; not opened during lecture)Low pressure system—“comma” shape of clouds on sat/radCyclone: area of intense low pressure around which winds blow ccw in NH, cw in SHTropical cyclones: developed over subtropical/tropical water (aka hurricane, typhoon)Mid-latitude cyclones (aka extra tropical cyclones): developed in mid-latitudesPolar front theory for cyclone developmentCyclone begins with a stationary front (usually a segment of the polar front)Nascent stage of cyclone: frontal wave developedMature stage: fully developed open wave with cold and warm frontPartially occluded stage: cold front starts to overrun warm frontRemember: cold front moves faster than warm frontOccluded stage (advanced occlusion): more warm air is pushed aloft and size of warm air wedge at surface decreases significantly(Occluded front is purple)Dissipation stage: decay stage; warm air isolated aloft with cold air beneathLifetime: several days to a weekMoves thousands of km during life cycleRemember stages of mid-latitude cyclone for quiz/exam (graphic)Mid-latitude cyclones:Gulf of Alaska low: short, SEAlberta Clipper (low): long, EColorado low: E, then NE (affects MI the most)Gulf low: NE (over land or FL peninsula)Hatteras low: NE (out to Atlantic)Westerly flows over a major NS oriented mountain tend to curve cyclonically on the leeside, producing alee-side low. This process is called lee cyclogenesis.Why is it that some low pressure systems develop into huge cyclones, while others dissipate in a day or so?Low pressure system may develop into a cyclone (the process is called cyclogenesis) if the low pressure system intensifies (pressure drops in the center of the low).What causes surface low temperature to intensify?When upper air divergence is stronger than surface convergence (more air is taken out from the top than is brought in at the bottom), surface pressure drops, and the low intensifies (or deepens).Divergence aloft must be greater than convergence at surfaceAlthough surface conditions may influence the formation of mid-latitude cyclones, the real key for cyclogenesis is the presence of regions of strong upper-air divergence.Ahead of a troughRegions under the influence of strong warm advectionOn the left side of a jet streak exitDeepening of cyclones into explosive cyclogenesis is prohibited when low pressure aloft is directly abovethe surface low. This is because air is not taken out from above and as more air rises, pressure above surface low will increase, causing surface low to weaken or be “filled”Area of divergence ahead of trough, convergence behind trough = cyclogenesisWhen contours are evenly spaced, winds blow faster around high pressure than around low pressureAhead of ridge/behind trough: faster-moving air in ridge moves toward slower-moving air in trough, air piles up and convergesAhead of trough/behind ridge: slower-moving air in troughs moves toward faster-moving air in ride, air spreads apart and divergesMain thing to look at when looking for possible development: upper-level troughs/ridgesVertical structure of cyclone: upper-level low is tilted westward with height with respect to the surface lowUpper-level divergence initiates and maintains a surface low(in diagram) When air slows, dots converge; when winds aloft are faster, the dots are farther apartBaroclinic wave theory of cyclogenesisSeveral long-wave or Rossby waves encircling the globe at any given timeEmbedded in longwaves are shortwavesShortwaves move much faster than longwaves; deepen as they move around a trough of a longwave, weaken as they pass a ridge of a longwaveBarotropic when winds are parallel to isotherms; no temperature advectionBaroclinic when winds blow across isotherms; warm advection occurs ahead of trough with cold advection behindThis advection helps intensify surface low/highAdvection stops and upper air divergence weakens; surface front occludes and cyclone dissipatesRight side of jet streak entrance region: strong convergence occurs aloft, intensifying surface highLeft side of jet streak exit region: strong divergence occurs, intensifying surface lowStrong trough (divergence) aloft? Jet stream aloft? Warm-air advection aloft?Strong regions of divergence required for mid-latitude cyclogenesisVorticity used as a tool to locate strongest regions of


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MSU GEO 203 - Lecture #17

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