NWS Doppler Radar calculates atmospheric motion based on cloud and precipitation movements. Using the "Doppler effect," it senses targets moving toward and away from the radar parallel to radar beams (radial velocity). It cannot sense motion directed perpendicular to a beam since there is no phase shift in the Doppler effect for such movement. Two types of velocity are available: 1) base velocity which shows movement with respect to the ground (estimated actual environmental wind), and 2) storm-relative velocity map (SRM), which shows movement relative to a moving target, e.g., wind a thunderstorm "feels" as it moves through its environment. For more information, consult "Overview of the WSR-88D Radar System."
Above is base velocity data over Crawford County in southern Indiana on August 13, 2011 (west of Louisville). The blue line is the Ohio River. Green colors are winds directed toward the radar site at Ft. Knox, KY (black circle at lower right). The light blue color within the bright green and white colors in Crawford County represent estimated winds of 60-70 kts only a couple thousand feet off the ground. These intense winds are associated with a bow echo in reflectivity data, and where strong winds and/or wind damage would be located. As the winds moved east, wind damage occurred along its path in Crawford and Harrison counties in southern Indiana, then across Jefferson County (and Louisville), Kentucky where Bowman Field Airport (LOU) reported a 69 mph wind gust. More information on this August 13, 2011 event, including damage photos and radar imagery is available on our Science and Tech site.