CHARLOTTE, NC (WBTV) - Early Monday evening a storm rolled through the Huntersville area and knocked down a tree. Not exactly a major headline, but because this happened close to our Charlotte Terminal Doppler Radar, it serves as a good example of how radar can detect low-level weather events and how meteorologists interpret the radar imagery.
Because the weather conditions lately have been so hot and dry near the surface, any storms that form deposit their rainfall into this parched air. As the rain falls from above, the rain evaporates very quickly and efficiently which causes a rapid cooling of that column. Thus, in addition to all the air being dragged downward already from the falling rain, the cold air being created also starts sinking very rapidly.
Hot air rises, cold air sinks. This can create strong downdrafts, sometimes reaching 100mph or more. These are known as Microbursts. As the air approaches the ground, it spreads outward, creating potentially damaging winds in all directions near the base of the storm.
Microbursts are typically no larger than 2.5 miles across, lasting only a couple minutes. See Sample 1 for a diagram.
The terminal Doppler radar was erected specifically to detect events like this to help make aviation safer in and around the Charlotte Douglas International Airport.
In the second picture, Sample 2, you can view the storm report. Also note the radar imagery. This is a velocity image from the terminal Doppler radar. Notice the warm colors and the cool colors. Proceed to Sample 3.
Radars can only detect particles moving toward or away from the radar (along the radial if you prefer). The radars cannot detect motion moving across the radial (tangentially to the radar beam). But this is still enough information to solve the puzzle.
The warm colors represent air motion away from the radar, cool colors depict wind blowing toward the radar. Note the radar location which is the gray dot between I-77 and Shuffletown.
I marked the velocity maxima with a 96 mph outbound wind and a 56 mph inbound wind. But wait, the storm itself was moving away at 20 mph, so if you subtract out the storm motion, you'll see that in both directions the storm was generating 76 mph winds, and it's probably safe to say that wind was also blowing to the right and left (or NW and SE if you prefer). See Sample 4.
Now you can see how Huntersville was affected by the outflow of this storm. It's lucky there wasn't more damage. Keep in mind these winds were occurring about 300 feet off the ground based on the beam elevation there. That might not seem like much, but it's above all the trees and houses which provide friction and slow the winds down somewhat in that lowest 50 – 100 feet off the ground.