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Shear Lines, Fronts and Areas of Confluence/Diffluence. Mike Davison HPC International Desks. Shearlines vs. Fronts. Shear Lines: are associated with wind shifts (direction and speed).

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shear lines fronts and areas of confluence diffluence

Shear Lines, FrontsandAreas of Confluence/Diffluence

Mike Davison

HPC International Desks

shearlines vs fronts
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.
divergence of the wind
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
    • Confluenceis not equal toConvergence
    • Diffluenceis not equal toDivergence
divergence cont
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
example directional diffluence confluence
Example Directional Diffluence/Confluence

Directional Confluence

Directional Diffluence

speed diffluence confluence
Speed Diffluence/Confluence

Speed Confluence

B

A

Speed Diffluence

convergent or divergent
Convergent or Divergent?

Directional Confluence

A

Speed Diffluence

convergent or divergent12
Convergent or Divergent?

Speed Confluence

A

Directional Diffluence

forecast hints
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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