Shear Line Radar Signatures

Detection of shear lines is based on the Doppler velocity rather than the reflectivity. However, accurate measurement of the Doppler velocity requires sufficient returned power. The radar reflectivity associated with shear lines can be from precipitation or clear air echoes. Because clear air echoes tend to be weak, the most efficient detection requires that the radar scan slowly for sufficient averaging.

The climatology of shear lines for a particular location dictates the scan strategy. If shear lines tend to be associated with weak returns, a slow scan rate is necessary for reliable detection. The trade-off between reliable detection of weak echo features and the need for rapid updates to detect phenomena such as microbursts is a decision that you must make, based on your site and application.

Shear lines are characterized by both vorticity and convergence (negative divergence). The vorticity associated with cold fronts in the Northern hemisphere is typically positive. The vorticity associated with thunderstorm gust fronts is usually, but not always, positive. Shear lines are not characterized by positive divergence, as are microbursts.

Both azimuthal shear (partial vorticity) and radial shear (partial divergence) typically show a signature for a shear line.