# Shear Lines, Fronts and Areas of Confluence/Diffluence - PowerPoint PPT Presentation

1 / 27

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).

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

Shear Lines, Fronts and Areas of Confluence/Diffluence

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

## Shear Lines, FrontsandAreas 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 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

• Confluenceis not equal toConvergence

• Diffluenceis not equal toDivergence

• ### 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 Confluence

B

A

Speed Diffluence

### Convergent or Divergent?

Directional Confluence

A

Speed Diffluence

### Convergent or Divergent?

Speed Confluence

A

Directional Diffluence

## 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.