Convergence, Divergence

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Convergence, Divergence Vorticity
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DEFINITIONS

• Convergence - Its the increase of mass within a
  given layer of the atmosphere. The wind must be
  such as to result in a net inflow of air into
  that layer. We generally associate convergence
  with low pressure areas where converging winds
  result in an increase in mass into the low and
  force upward vertical motion. Two types
  horizontal or vertical depending upon the axis of
  flow.

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DEFINITIONS CONT.

• Divergence - This is the decrease of mass within
  a given layer of the atmosphere. The wind must
  be such as to result in a net flow of air outward
  from a layer. We generally associate divergence
  with high pressure areas where the flow of air is
  outward from the center, causing downward
  vertical motion. Two types horizontal or
  vertical depending upon the axis of flow.

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DEFINITIONS CONT.

• Vorticity- The measure of an air parcels
  rotation about an axis which is
  perpendicular to the earths surface. A parcel
  of air has vorticity when the parcel spins as it
  moves along its path.

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CONVERGENCE DIVERGENCE

• SIMPLE MOTION
• TYPES OF
• SPEED
• DIRECTIONAL
• COMBINATION OF SPEED DIRECTION
• COMPLEX MOTION
• TYPES OF
• CHIMNEY EFFECT
• LEVEL OF NON-DIVERGENCE
• ISOPYCNIC LEVEL

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SIMPLE MOTION

• In a field of parallel wind flow, if wind speed
  decreases downstream (producing a net inflow of
  air for this layer) convergence is taking place.
  (SPEED CONVERGENCE)
• In a field of parallel wind flow, if wind speed
  increases downstream (producing a net outflow of
  air for this layer) divergence is taking place.
  (SPEED DIVERGENCE)

Net speed decrease
Net speed increase
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SIMPLE MOTION
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SIMPLE MOTION

• In an area of uniform wind speed and the wind
  field converges based on direction (producing a
  net inflow of air for this layer) convergence is
  taking place. (CONFLUENCE)

• In an area of uniform wind speed and the wind
  field diverges based
• on direction (producing a net outflow of air
  for this layer) divergence
• is taking place. (DIFFLUENCE)

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SIMPLE MOTION
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SIMPLE MOTION (Cont.)

• Normally, convergence and divergence components
  are combined. The fact that directional
  divergence or convergence exists does not
  necessarily indicate divergence or convergence.
  You must also consider speed divergence or
  convergence.

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SIMPLE MOTION (Cont.)

• There are other cases where it is difficult
  to tell whether divergence or convergence is
  taking place, such as when wind speed decreases
  downstream and there is directional divergence
  there, and vice versa. You must evaluate the
  factor and decide what is the predominant
  condition.

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SIMPLE MOTION (Cont.)
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SIMPLE MOTION (Cont.)
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COMPLEX MOTION
This section concentrates on convergence and
divergence with migratory pressure systems and
the factors associated with each throughout the
entire column of atmosphere.
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COMPLEX MOTION (Cont.)
The Chimney Effect

• First Converging air flow causes a pile-up of
  air
• with nowhere to go but up.
• As the air rises, this causes lower surface
  pressure.
• Pressure decreasing at the surface favors
  increased
• inflow and convergence.
• Unless some process removes this pile-up of
  air,
• it will back up and the surface low will
  fill.

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COMPLEX MOTION (Cont.)
The Chimney Effect Continued

• If a jet maximum or diffluent flow exists aloft,
  this
• will remove this rising air.
• Once started this circulation process of
  divergence
• aloft and convergence below will perpetuate
  itself.

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COMPLEX MOTION (Cont.)
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COMPLEX MOTION (Cont.)
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COMPLEX MOTION (Cont.)
The Level of Non-Divergence

• In the atmosphere, there is an inflection point
• at which convergence turns into divergence,
  and
• vice versa.
• At this point convergence and divergence equal
• zero, vertical velocity is at a maximum at
  this
• point (The level of non-divergence).
• This normally exists between 600 MB and the
• 500 MB isoheights.

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COMPLEX MOTION (Cont.)
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COMPLEX MOTION (Cont.)
The Isopycnic Level

• At an altitude of roughly 8 kilometers (in
• standard atmosphere) density remains nearly
• constant.
• This level is called the isopycnic level and
• it exists at roughly the 350 MB isoheight.
• Maintaining constant density requires
• temperature changes to balance pressure
  changes.

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COMPLEX MOTION (Cont.)
The Isopycnic Level Continued

• Example If pressure increases, so must
• temperature, and vice versa.
• Changes below the isopycnic level must be
• balanced by changes above it.
• Thus in the travel of migratory highs and
• lows, two vertical circulations exist. One
• below the isopycnic level and one above.
  

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COMPLEX MOTION (Cont.)
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The IMPORTANCE OF CONVERGENCE DIVERGENCE

• Convergence and divergence have pronounced
  effects upon the weather occurring in the
  atmosphere. Vertical motion, whether up or down,
  is an important mechanism in the production of
  the earths weather. For example extensive
  precipitation and cloudiness associated with
  extra-tropical cyclones, and nearly cloud free
  skies and stable weather associated with
  anticyclones.

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VORTICITY TERMS
Eight important terms

• Shear Vorticity The rate of change of wind
  speed
• perpendicular to the wind direction. Wind
  speed
• change with no change in direction.

• Curvature Vorticity The amount of parcel
  vorticity
• caused by movement along a curved
  streamline. As
• curves tighten,vorticity increases.

• Relative Vorticity The sum of shear vorticity
  and
• curvature vorticity.

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VORTICITY TERMS
More of the eight important terms

• Earths Vorticity Created by the rotation of
  the
• earth. Greatest at the poles and weakest
  near the
• equator.

• Absolute Vorticity Total vorticity or the sum
  of
• relative vorticity and the earths vorticity.

• Positive Vorticity Cyclonic turning of a
  parcel.

• Negative Vorticity Anti-cyclonic turning of a
  parcel.

• Inflection Point Specific point where contour
• curvature changes from cyclonic to
  anti-cyclonic.
• curvature vorticity equals zero at this
  point.

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VORTICITY GRAPHICS
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USING VORTICITY
When using vorticity in forecasting you must be
able to do three things

• Determine if vorticity is increasing or
  decreasing
• upstream.

• Determine the type of vorticity advection,
  whether
• positive or negative.

• Determine maximum and minimum centers on
• vorticity charts.

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VORTICITY GRAPHICS