Middle-Latitude Cyclones - I
RECAP: Types of Fronts
• Cold
fronts: cold, dry stable air is
replacing warm, moist unstable air.
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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.
Stationary Front
• Stationary front- a front which does not move or barely moves.
• Stationary
fronts behave like warm fronts, but are more
quiescent.
• Many
times the winds on both sides of a stationary front are
parallel to the front and have opposite direction.
• Typically
stationary fronts form when polar air masses are
modified significantly so as to lose their character (e.g., cold
fronts which stall).
• Typically
there is no strong precipitation associated with
stationary fronts (why? – no big contrast in the air mass
properties, no air uplifting and condensation).
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Occluded fronts.
Cold fronts move faster than warm fronts.
They can catch up and overtake their related
warm front. When they do, an occluded front
is formed.
Cold occlusion: very cold air behind, not so
cold air ahead of, the warm front
The upper warm front follows the surface
occluded front
Cold occlusion
Warm Occlusion
• Very
cold air ahead of, not so cold air
behind, the warm front
• The
cooler air from the cold front
cannot lift the very cold air ahead,
•
rides “piggyback”
The warm front aloft precedes the
surface occluded front
Weakening/Strengthening of the Front
• Frontolysis:
♦ The front weakens and dissipates
♦ Why?-the air masses start losing
their identities.
♦ The temperature (humidity)
contrast across the front is
decreasing.
♦ Typical for slow moving fronts
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Frontogenesis:
♦ The front intensifies.
♦ Why? – The temperature
(humidity) contrast across the
front is increasing.
♦ Example: cP air mass moves
over warm ocean water.
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Weather Map
Shown: surface-pressure systems, air masses, fronts,
isobars, winds and air flow (large arrows)
Green-shaded area: precipitation
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Weather Map
The example from Chapter 1: Fig. 1.14.
Sample weather maps
Polar Front Theory
• Atheir
model of how mid-latitudes storms develop:
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.
Vilhelm
Bjerknes
Jacob
Bjerknes
+ Halvor Solberg
+ Tor Bergeron
The life of a mid-latitude (wave) cyclone
The life of a mid-latitude (wave) cyclone
• A:on the
polar front is stationary: the winds are in opposite directions
the two sides of the front. This creates a cyclonic wind shear.
• B:alongA local
perturbation: a region of low pressure appears somewhere
the front. The front then breaks in two fronts:
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♦ 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.
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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.
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The life of a mid-latitude (wave) cyclone
• D:front.
Mature cyclone (initial occlusion). Cold front closes in on the warm
Most intense stage of the storm. Clouds cover a large area.
• E:together.
Advanced occlusion. Triple point: where all three fronts come
The center of the storm gradually dissipates:
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♦ 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
Dying out
E: Advanced
occlusion
C: Open wave
Just forming
B: Frontal wave
Cyclogenesis
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• Some regions have greater
Any development or strengthening
of a mid-latitude cyclone
propensity for cyclogenesis:
♦ Gulf of Mexico
♦ Eastern slopes of Rockies
and Sierra Nevada
Lee-side lows
♦ Atlantic ocean east of
Carolinas
Nor’easters
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Lee cyclogenesis
Famous nor’easters:
the Great Blizzard of 2006
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• Developed an “eye”
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Began on Feb 11 2006
All-time record snowfall
(27 in) in New York City.
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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)