Norwegian Cyclone Model Fall 2011 Meteorology 311Mid-Latitude Cyclones • What? – Low pressure located between 30º and 60º latitude. • Impact? – Cause of most of the stormy weather in U.S., especially during the winter season. • Why? – Crucial for predicting significant weather phenomena such as blizzards, flooding, rains, and severe weather.Stages of Development • Initial Setup • Initiation* • Development* • Mature or Occluded* • DissipationInitial Setup • Front exists separating warm air to south and cold air to north. • The front is often stationary. • Can be just about any boundary. – Differential heatingInitiation • Weak surface wave develops. • Usually the effect of some upper level forcing. – PVA from short-wave. – Jet streakInitiation cont. • Closed circulation around low is able to produce temperature advections. • Precipitation begins to develop with the heaviest occurrence along the front.Development • Both warm and cold front become organized. • Low pressure becomes stronger (surface pressures fall in the low). • Winds intensify around the low. • Temperature advections become large.Temperature Advections • Warm air advection: upward motion – Lowers surface pressure. • Creates surface forcing in addition to the upper level forcing. • Systems can intensify rapidly.Temperature advections cont. • Low-level temperature advections also alter the upper-level fields. • Warm advection: Upper level heights increase. • Cold advection: Upper level heights decrease. • Advections intensify the upper level disturbance or trough that initially cause the disturbance. • As trough deepens, forcing from PVA increases. – Positive feedback.Development • Both warm and cold front become organized. • Low pressure becomes stronger (surface pressures fall in the low). • Winds intensify around the low. • Temperature advections become large.Mature or Occluded • Cold front “catches up” with warm front. • Occlusion forms. • Temperature advections are intense at low levels.Mature cont. • Warm sector gets cut-off from the low. • Low no longer has a supply of warm, moist air. • Begins to dissipate.Mature cont. • Heights fall so much aloft due to cold air advection that an upper-level low develops. • Low becomes closed aloft (complete circulation around it).Mature cont. • Vorticity isopleths tend to become parallel to the flow (course notes). • Upper level forcing weakens and eventually dissipates. • Lows become “stacked”. • System no longer intensifies.Dissipation • System begins to dissipate. • A new low may form at the triple point. • Triple point: point where cold, warm, and occluded point come together.“Textbook” case“Textbook” case“Textbook” caseMarch “super-storm” of 1993 • More than a foot of snow from Alabama to Maine. • Tornados ripped through Florida. • Hurricane force winds and barometric pressure followed by record cold over eastern United States.Storm of CenturyStorm of the Century Isthmus of TehuantepecStorm of the CenturyOther
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