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GEU 0047: Meteorology
Lecture 12
Mid-latitude Cyclones
Vorticity
Winds Aloft
Divergence Aloft and Cyclogenesis
No Upper
Divergence
Upper Divergence
forms due to the trough
No Lower
Divergence
No Lower
Divergence
Initially
Trough Approaches
from the West
Divergence Aloft and
Cyclogenesis
The divergence aloft
initiates an upward vertical
motion beneath the upper
level divergence.
No Lower
Divergence
Trough Approaches
from the West
Divergence Aloft and
Cyclogenesis
Surface convergence develops
in response to the rising motion.
Divergence Aloft and
Cyclogenesis
L
If more mass is being removed
from the column by the upper
level divergence than is replaced
by surface convergence, then a
low pressure center at the surface
will either develop or deepen.
Cyclogenesis!!!!!
Formation of
Cyclones
• Cyclogenesis -- The development or
strengthening of a region of low pressure -a cyclone.
• Conditions:
– Cyclonic flow must be established at the
surface. Cyclonic flow is CCW in the
Northern Hemisphere.
– Inward flow of air at the surface must be
supported by outward flow of air aloft. This
is the “upper level support.”
Ocean Wave
Development
Undisturbed
Water
Friction causes waves to develop
at the surface of the water.
Wave Development
If the wind is strong enough, the wave
can crest and “break.”
Wave Cyclone Development
Now, consider
the atmosphere:
H
cP
H
mT
Wave Cyclone Development
The cyclone may deepen.
cP
The warm air moves
northward and the cold air
moves southward. If the
pressure falls, the cyclone
deepens or gets more
intense.
L
mT
Cyclone Fronts
Warm Front
Cold Front
Cyclone Stages
Wave Cyclone Development
cP
The cyclone matures.
A cloud formation in the
shape of a “comma” is
characteristic of a well
developed wave cyclone.
This characteristic shape is
often used to find storms
over the oceans using satellite
imagery.
Cloud
“Shield”
L
mT
Wave Cyclone “Comma”
Satellite Image
Warm Front
L
Dry Slot
Comma
Cloud
Cold Front
© 1997 Prentice-Hall, Inc. -- From: Moran and Morgan, Meteorology
Wave Cyclone Development
The cyclone occludes.
The cold front overtakes the
warm front and lifts the warm
air off the ground.
L
The cold air now surrounds the
cP
cyclone and, since the sloping
temperature gradient (front) is gone,
the pressure gradient weakens
and the low slowly dies.
mT
Extratropical Cyclone Surface Map
© 1997 Prentice-Hall, Inc. -- From: Moran and Morgan, Meteorology
Cyclone Evolution
Occlusion
Occlusion
Occlusion
Weather Associated With A Wave
Cyclone
1. Northwest Sector
Usually cold with strong and
gusty winds out of the northwest.
Precipitation light to moderate,
with heavier precipitation closer
to the low.
If cold enough, snowfall can be
significant.
4
1
L
3
2
Weather Associated With A Wave
Cyclone
3. Warm Front (South Side)
Usually fair and warm.
Later in the day, showers and
thunderstorms may develop.
Some thunderstorms may be
heavy but most are of the
thundershower type.
4
1
L
3
2
Weather Associated With A Wave
Cyclone
4. Warm Front (North side)
Mostly cloudy and cold.
Precipitation often heavy and
occasionally substantial.
Winds often out of the east or
northeast.
Depending on the temperature
sounding, potential for sleet and
freezing rain.
4
1
L
3
2
Cyclone Weather Table
I
II
III
IV
V
VI
VII
VIII
Cyclone Weather Table
I
In between cyclones and anticyclones or High pressure approaching.
Stable Air, Clearing conditions.
Clear Skies likely for many days.
Cyclone Weather Table
II
The passage of a cold front.
The approach of an anticyclone.
Cyclone passes West to East, NorthEast of your location.
Cyclone Weather Table
III
Cyclone passes from directly North of you, and moves off to the East.
Cyclone Weather Table
IV
The approach of an anticyclone.
Cyclone approaching your location from the SouthWest to the North.
Cyclone Weather Table
V
The approach of an anticyclone.
Cyclone sliding from NorthWest to SouthEast of your location.
Cyclone Weather Table
VI
Cyclone approaching from NorthWest of your location.
Cyclone Weather Table
VII
Cyclone sliding from North to South, West of your location.
Cyclone Weather Table
VIII
Cyclone approaching from North of your location.
Cyclone Weather
Cyclone Weather Table
I
II
III
IV
V
VI
VII
VIII
Idealized sketch of the thermally direct circulation in the zonal (X-Z) plane for a
developing baroclinic wave in mid-latitude.
Solid lines: pressure surfaces
Dashed lines: vertical tilt of the trough and ridge axes
Heavy lines: position of tropopause
Arrows: flows