In WSR-88D, the D stands for Doppler. The designation expands to Weather Surveillance Radar, 1988, Doppler, the technical name for the radar used in the U.S. NEXRAD network.
Doppler capability allows the radar to detect motion toward or away from the radar site by measuring frequency changes in returned signals. That information provides velocity data, helping meteorologists identify wind patterns, storm rotation, gust fronts, and other atmospheric motion that ordinary reflectivity imagery cannot show by itself.
The national system replaced older radar networks such as WSR-57 and WSR-74, which did not use Doppler technology. Development was influenced by the Joint Doppler Operational Project, formed in 1976 to evaluate whether Doppler radar could improve severe-storm detection. The first operational WSR-88D installation entered service in Sterling, Virginia, in 1992.
NEXRAD means Next-Generation Radar, but that is the network name rather than the expansion of the letter D. “Dual” refers to a separate capability found in modernized systems, such as dual-polarization radar.