NWS Doppler radar senses atmospheric motion based on cloud and precipitation movement toward (green) and away from (red) the radar (radial velocity). Two types of velocity exist: base (ground-relative) velocity, and storm-relative velocity map (SRM). SRM equals storm motion minus base velocity, and shows winds "felt" by entities (e.g., thunderstorms) as they move through their environment. For more information, consult "Overview of the WSR-88D Radar System." SRM detects rotation in severe storms, and patterns of convergence and divergence.
The image here shows KLVX 0.5 degree SRM on April 24, 2010 of a bow echo over central Kentucky. The radar is at Ft. Knox (top of image). The interface between green inbound and red outbound colors in Hardin and Hart counties represents the leading edge (gust front) of the convective system, and where severe weather can occur. In this event, wind damage occurred over parts of Hart County where broad counterclockwise (cyclonic) motion was indicated.