middle-latitude cyclones - imatchev/met1010/notes/chapter12a.pdf · the life of a mid-latitude...
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Middle-Latitude Cyclones - I
RECAP: Types of Fronts
• Cold fronts: cold, dry stable air is replacing warm, moist unstable air. Moves fast, showers along the leading edge (squall line).
• Warm fonts: warm, moist unstable air is replacing cold dry stable air. Overrunning: warm air rides up and over the cold air, widespread cloudiness, light-to-moderate precipitation well ahead of the front
• Stationary fronts: essentially no movement, winds blow parallel to the front, in opposite directions on both sides
• Occluded fronts: when a cold front catches up with a warm front (more later)
• The symbols on a map are in the direction of the air mass motion.
Formation of an occluded front
Cold occlusion
Weather Map•Shown: surface-pressure systems, air masses, fronts,
isobars, winds and air flow (large arrows)
•Green-shaded area: precipitation
Weather Map•The example from Chapter 1: Fig. 1.14.
Sample weather maps
Polar Front Theory• A model of how mid-latitudes storms develop:
their birth, growth, and decay.
• The model connects the storms with the dynamics of the polar front: the transition zone between the cold air in the polar cell and the warmer air at middle-latitude (Ferrel cell).
• The polar front is a region of air conversions at the surface, upward motion, and divergence aloft. This results in low surface pressure.
VilhelmBjerknes
JacobBjerknes
+ Halvor Solberg+ Tor Bergeron
The life of a mid-latitude (wave) cyclone
The life of a mid-latitude (wave) cyclone
• A: the polar front is stationary: the winds are in opposite directions on the two sides of the front. This creates a cyclonic wind shear.
• B: A local perturbation: a region of low pressure appears somewhere along the front. The front then breaks in two fronts: ♦ warm (moving northward - why? Coriolis force) ♦ cold (moving southward - why? Coriolis force)♦ Central pressure: the junction of the two fronts
• A frontal wave is formed.♦ The winds aloft set the general direction of motion (black arrow)♦ The wave starts moving to the east and gradually becomes
• C: Open wave. The cold front moves faster than the warm front -> polar front bends. Warm sector between the two fronts. The central pressure keeps dropping: isobars now encircle it.
Weather patterns around a cyclonic wave
•South of the wave:♦ First a warm front
•Warmer air advancing•Wide band of precipitation.•Starts with snow first.•Then rain and drizzle.
♦ Cold front•Cold air advancing•Sharp drop of pressure•Strong precipitation at the front.•Then dry, cold, clear weather.
•North of the wave: some clouds associated with the low pressure center but no strong precipitation because there is no warm moist air around.
The life of a mid-latitude (wave) cyclone• D: Mature cyclone (initial occlusion). Cold front closes in on the warm
front. Most intense stage of the storm. Clouds cover a large area.
• E: Advanced occlusion. Triple point: where all three fronts come together. The center of the storm gradually dissipates:♦ Cold air on both sides of the occluded front♦ Warm sector far removed – the rising warm and moist air provides
energy for the storm (kinetic energy, latent heat of condensation)
• F: Cut-off cyclone plus a stationary front once again.
Family of cyclones
Just formingB: Frontal wave
C: Open wave
Dying outE: Advanced
occlusion
Cyclogenesis
•Any development or strengthening of a mid-latitude cyclone
•Some regions have greater propensity for cyclogenesis:♦ Gulf of Mexico♦ Eastern slopes of Rockies
and Sierra Nevada•Lee-side lows
♦ Atlantic ocean east of Carolinas•Nor’easters
Lee cyclogenesis
Famous nor’easters: the Great Blizzard of 2006
•Began on Feb 11 2006
•Developed an “eye”
•All-time record snowfall (27 in) in New York City.
Where do mid-latitude cyclones form?
•Typical paths of winter mid-latitude (anti)cyclones:♦ Lows: towards the east-northeast♦ Highs: towards the east-southeast
•Explosive cyclogenesis (bomb): when the central pressure drops very rapidly (more than 24 mb in 24 hours)