Radar Pinwheels and Wind Shear
July 04, 2008
The map of Oklahoma shown above for 11:26 a.m. Central Daylight Time June 5, 2005 was cobbled together from Doppler radar base velocities. A tip-off of the severe-weather fireworks that would be featured later in the day is contained in the 4 pinwheels on this map. The pinwheel colors are indicative of motion in the atmosphere: the warm colors of red, orange and yellow indicate particles moving away from the radar. It is as if the particles are "showing their taillights" as they scoot away from the radar. Cool colors of blue and green show inbound particles, moving toward the radar. Each of the four stations shows the same pattern: roughly half of the scope shows the red-orange-yellow of outbound particles, while roughly the other half shows the blue-green of inbound particles. What is more important are the gray shades in an S-shape. At these locations, airborne particles are neither moving toward nor away from the radar: in other words, they are flying directly across the radar beam, from greens to reds, in fact.
These S-shapes are the tip-off for strong directional wind shear in the lower part of the atmosphere. So, how does an S-shape indicate directional shear? And what is directional shear, anyway? Let's answer the second question first: directional wind shear is a lot like it sounds: the changing of wind direction with height. And it's important to severe weather because highly-sheared environments support rotating, tilted updrafts. Rotating, tilted updrafts tend to live longer and have the capacity to produce higher-end severe weather, like very large hail and, occasionally, tornadoes.
Now back to that first question: how does the S indicate directional shear? It's due to the fact that radar beams get higher off the ground the further they get from the radar. This is because the beam is slightly elevated from horizontal to begin with and because the earth's curvature actually helps "bend" its surface away from the radar beam. Anyway, what all this means is: the further an image is from the source of the radar, the higher it is off the ground. In the context of the "pinwheel" image, it means that near-surface winds are out of the south, blowing from greens to reds across the radar. However, higher up in the atmosphere and further away from the radar the gray lines are oriented more northwest-southeast, indicating southwesterly winds.