Middle-Latitude Cyclones - II
Review and Outline
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polar front model (Norwegian model) of a developing midlatitude cyclonic storm represents a simplified but useful model of
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how an ideal storm progresses through the stages of birth, maturity
and dissipation.
Cyclogenesis, lee-side lows, northeasters, bombs.
For a surface mid-latitude cyclonic storm to form, there must be an
area of upper-level divergence above the surface low. For the
surface storm to intensify, this region of upper level divergence
must be greater than surface convergence.
When the polar-front jet stream develops into a looping wave, it
provides an area of upper-level divergence for the development of
surface mid-latitude cyclonic storms.
The curving nature of the polar-front jet stream tends to direct
surface mid-latitude cyclonic storms northeastward and surface
anticyclones southeastward.
Skip the sections on “Vorticity, Divergence, and Developing MidLatitude Cyclones” and “Polar Lows”
The life of a mid-latitude (wave) cyclone
Satellite image from Chapter 1: Fig. 1.13.
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What happens with the high and low
pressure centers over time?
The development of a cyclonic wave is determined by the
motion of the air aloft:
♦ the air flow aloft is parallel to the isobars
♦ the air flow aloft is neither into L nor away from H.
Air flow at the surface is across the isobars.
♦ air convergence near low pressure centers Where does it go?
does it
♦ air divergence near high pressure centers Where
come from?
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Vertical structure of deep dynamic lows
The air cannot disappear or come out of nowhere: the
surface flow is related to the vertical flow and
consequently to the air flow aloft.
The surface winds are coupled to the winds aloft
What type of upper-level air flow would favor the
formation and development of a surface wave cyclone?
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Favorable conditions for a strong
midlatitude storm
Air convergence at the
surface is aligned with air
divergence aloft.
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Air divergence at the
surface is aligned with
area of convergence aloft
aloft
Divergence
Convergence
L
H
Convergence
Divergence
surface
• stronger (weaker) than the surface convergence.
Surface L intensifies (weakens) if the divergence aloft is
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Surface H intensifies (weakens) if the convergence aloft is
stronger (weaker) than the surface divergence.
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But …
We know that at upper levels the winds are blowing
along the isobars.
The air aloft is NOT converging into nor it is
diverging away from pressure centers (L or H).
What is the right pattern of the upper level winds
that will result into a strong storm?
Is it this one?
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does NOT support the storm.
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Low over Low,
High over High.
No convergence or
divergence aloft.
The surface Low will
fill up and disappear
The surface High will be
depleted and disappear
Such a configuration
Similarly, Low over High, High over Low won’t work.
Surface and upper-level pressure centers cannot be
aligned above each other.
Is it this one?
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constant wind speed
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• This configuration will
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Perfect geostrophic
flow – parallel isobars,
Again, no divergence or
convergence (i.e. piling
up or removal of air).
NOT support a strong
storm at the surface
Conclusion: the isobars
must curve, creating
regions where they
spread apart and/or get
closer together.
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500 mbar pressure map
The air converges as it flows toward the L trough.
The air diverges as it flows away from the L trough.
Upper Level Waves
• Long waves (Planetary, Rossby waves)
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♦ Typically the longwaves have 3-6
wavelengths around the Earth.
♦ The result from the uneven heating
of the Earth and the rotation of the
planet.
♦ Long waves move very slowly or are
stationary.
♦ Often created by mountain ranges
Shorter waves
♦ Imbedded in the long waves
♦ Short waves travel faster
♦ They can intensify the troughs of the
long waves.
The Vertical Structure
of a Middle-latitude
Storm
• The surface L is aligned
with the diverging part of
the jet stream flow.
• The surface H is aligned
with the converging part
of the jet stream flow.
• H and L centers are not
aligned! Upper-level
centers are generally
shifted to the west
Another way of looking at it.
Another way of looking at it
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• C) storm dies out: Low over Low.
B) storm intensifies: convergence over surface H,
divergence over surface L.
Summary of the Processes in a Cyclonic Wave