Middle-Latitude Cyclones - II

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Middle-Latitude Cyclones - II
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Review and Outline

• The polar front model (Norwegian model) of a
  developing mid-latitude cyclonic storm represents
  a simplified but useful model of 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 Mid-Latitude Cyclones and Polar
  Lows

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The life of a mid-latitude (wave) cyclone
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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
• air divergence near high pressure centers

Where does it go?
Where does it 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.
• Air divergence at the
• surface is aligned with
• area of convergence aloft
• Surface L intensifies (weakens) if the
  divergence aloft is stronger (weaker) than the
  surface convergence.
• 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?

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Is it this one?

• 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
• does NOT support the storm.
• Similarly, Low over High, High over Low wont
  work.
• Surface and upper-level pressure centers cannot
  be aligned above each other.

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Is it this one?

• Perfect geostrophic flow parallel isobars,
  constant wind speed
• Again, no divergence or convergence (i.e. piling
  up or removal of air).
• This configuration will 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.

Convergence
Divergence
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Upper Level Waves

• Long waves (Planetary, Rossby waves)
• 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.
  

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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

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Another way of looking at it.
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Another way of looking at it

• B) storm intensifies convergence over surface H,
  divergence over surface L.
• C) storm dies out Low over Low.

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Summary of the Processes in a Cyclonic Wave