Shear Lines, Fronts and Areas of ConfluenceDiffluence

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Shear Lines, FrontsandAreas of
Confluence/Diffluence

• Mike Davison
• HPC International Desks

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Shearlines vs. Fronts

• Shear Lines are associated with wind shifts
  (direction and speed).
• A line or narrow zone across which there is an
  abrupt change in the horizontal wind component
  parallel to this line
• A line of maximum horizontal wind shear.
• An area of directional wind confluence along the
  tail end of a surface front.
• Lacks the baroclinicity/density discontinuity of
  surface fronts
• Fronts The interface or transition zone between
  two air masses of different density.
• Density depends on temperature and moisture
  content
• Fronts either lie along shear lines or can lag
  behind them.

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Wind Confluence and Diffluence in the Caribbean
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Wind Confluence and Diffluence in South America
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Wind Divergence
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Divergence of the Wind

• We can express the divergence equation in a
  simplified form, with two terms
• Direction
• Speed
• The direction and speed terms, in-turn, can be
  expressed as directional/speed diffluence and
  confluence
• Confluence is not equal to Convergence
• Diffluence is not equal to Divergence

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Divergence (Cont)

• Divergence/convergence calculations need to take
  into account the direction and speed terms.
• This is done through objective analysis
• Streamline analysis is a subjective technique,
  and it only shows directional diffluence and
  confluence.
• It does not show convergence/divergence

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Example Directional Diffluence/Confluence
Directional Confluence
Directional Diffluence
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Speed Diffluence/Confluence
Speed Confluence
B
A
Speed Diffluence
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Convergent or Divergent?
Directional Confluence
A
Speed Diffluence
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Convergent or Divergent?
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Convergent or Divergent?
Speed Confluence
A
Directional Diffluence
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Convergent or Divergent?
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Evaluation of Shear Lines
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Front/Shear Line Evaluation
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Front and Shear Line
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Front and Shear Line
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Front and Shear Line
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Front and Shear Line
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Forecast Hints

• During the Winter Months
• Convective activity over the Gulf of Mexico and
  gulf coast of Mexico tends to be capped below 500
  hPa (20,000 ft)
• Evaluate upper dynamics for CONVERGENCE or
  DIVERGENCE before forecasting thunderstorms.
• If models forecast post frontal northerlies of
  25Kt or greater, and dew point temperature of 20C
  (68F) or greater over southern Mexico.
• Forecast 12 hrs rainfall accumulation of at least
  2-4 inches.

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Example of Ideal Conditions for Heavy Rainfall
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Example Front/Shear Line Caribbean
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Example Front/Shear Line South America
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Identify Features
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Satellite Interpretation
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Analysis
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Analysis