Norwegian Cyclone ModelFall 2009Meteorology 311Mid-Latitude Cyclones• What?– Low pressure located between 30º and 60º latitude.• Impact?–Cause of most of the stormy weather in U.S., –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 (M)•Development (M)•Development (M)• Mature or Occluded (M)• DissipationInitial Setup• Front exists separating warm air to south and cold air to north.•The front is often •The front is often stationary.• Can be just about any boundary.– Differential heatingInitiation• Weak surface wave develops.• Usually the effect of some upper level 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).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.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.•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).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 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.• 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.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 CenturyIsthmus of TehuantepecStorm of the CenturyOther