Doppler velocity products indicate storm motion toward or away from the radar. The velocity products in RadarScope use the Doppler effect to determine how fast the particles in the air are moving relative to the radar itself. Negative values (green in RadarScope) indicate motion toward the radar, while positive values (red in RadarScope) indicate motion away from the radar. They can be difficult to interpret without training and experience, but Doppler velocity products can be used to detect the overall movement of a storm as well as relative motion within the storm itself, such as rotation. Note that the radar can only detect the component of the velocity vector along the radar beam, so this isn’t a full picture of the wind field. But it gives you a fairly good idea which way a storm is heading.
The super-res velocity product has a resolution of 0.25 kilometers by 0.5 degrees. It provides the highest-resolution reflectivity available from NEXRAD radars to a distance of 230 kilometers from the radar. Super-res reflectivity is available at four different tilts or beam angles, with tilt 1 being the lowest to ground level.
Base velocity is the standard velocity product distributed in the NEXRAD Level III product suite. It’s the same data as super-res reflectivity but at a lower resolution and larger coverage area.
Storm Relative Velocity
Storm relative velocity is base velocity with the average storm motion subtracted out. When storms are moving quickly, this makes it easier to spot green/red velocity couplets that are indicative of rotation and which might be masked out by the motion of the storm. As with base velocity, green is motion towards the radar and red indicates motion away.
It’s also worth noting that the above rotation images are ideal cases. We aren’t always lucky enough to get such prominent radar signatures from tornadoes. The radar isn’t looking at ground level, so it can’t actually see the tornado itself. It’s seeing rotation higher up in the storm covering an area that is several miles wide. The height and width of the radar beam increases with its distance from the radar. So the farther away a storm is from the radar, the higher up the radar is seeing and the wider the beam, making it is less likely to detect the rotation associated with a tornado.
You can learn more about storm relative velocity on this National Weather Service page.
Learn more about Super-Res Storm Relative Velocity in RadarScope.
Super-Res Spectrum Width
Within any volume that is sampled by a radar, there can be a wide range of motions being observed. Spectrum width is a measure of that variation. Higher values of spectrum width correlate to a wider range of velocities being observed (turbulent flow; e.g. mesovortices); lower values indicate a narrower range (smooth flow; e.g. straight line winds). With proper interpretation, spectrum width can provide an indication of turbulence, which can be helpful in identifying conditions associated with severe thunderstorm activity.