From AMS Glossary
A pattern in a Doppler velocity field representing horizontal divergence (or convergence) associated with atmospheric phenomena.
In a storm-relative reference frame, the idealized signature associated with flow diverging from a point source (or converging toward a sink) is symmetric about a line perpendicular to the radar viewing direction with marked radial shear across the core region between peak Doppler velocity values of opposite sign. The signature associated with a divergence line (or convergence line) depends on the Doppler radar viewing direction. When the radar viewing direction is perpendicular to the line, there is a band of radial shear across the line that corresponds to the full measure of divergence (convergence). When the viewing direction is parallel to the line, none of the divergence is sensed by the radar. At angles in between, a fraction of the divergence is sensed.