The image shows a 4-panel display of reflectivity (upper left), storm-relative velocity (upper right), and spectrum width data (bottom 2 panels) from the NWS Louisville radar on April 24, 2010.
Spectrum width (SW) depicts a measure of the variability of the radial velocity estimates due to the presence of wind shear, turbulence, and/or the quality of velocity samples. It is used to estimate turbulence associated with boundaries, thunderstorms, mesocyclones in supercells, mesovortices in convective lines, etc. Low values (gray shades at left) indicate smooth or uniform flow, while high values (red, pink, and white colors) denote turbulent or variable flow.
In this case, a bow echo was moving east over central Kentucky. Velocity (upper right) showed a low-level boundary (red-green interface) with mesovortices (red-green couplets) over southwest Larue County (center of image). Spectrum width clearly identified the boundary location ("S" shaped line), while the mesovortices showed up as small clusters of high SW values (especially lower left panel). SW is another tool available to NWS forecasters to enhance the severe weather warning decision process.