Shear Lines, Frontsand

Areas of Confluence/Diffluence

Mike Davison

HPC International Desks

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.

Wind Confluence and Diffluence in the Caribbean

Wind Confluence and Diffluence in South America

Wind Divergence

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

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

Directional Confluence Directional Diffluence

Speed Diffluence/Confluence

Speed Diffluence

Speed Confluence

A

B

Convergent or Divergent?

A

Directional Confluence

Speed Diffluence

Convergent or Divergent?

Convergent or Divergent?

A

Directional Diffluence

Speed Confluence

Convergent or Divergent?

Evaluation of Shear Lines

Front/Shear Line Evaluation

Front and Shear Line

Front and Shear Line

Front and Shear Line

Front and Shear Line

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.

Example of “Ideal” Conditions for Heavy Rainfall

Example Front/Shear Line Caribbean

Example Front/Shear Line South America

Identify Features

Satellite Interpretation

Analysis

Analysis