Weather radar now and days has become universal. Whether it be used on our phones, computers, or seen on television, doppler radar information can be retrieved anywhere. Though, I have often wondered how many people out there truly know what they are looking at. High radar signature values have been misinterpreted time and time again, and a key radar interface that is I feel is most often misinterpreted is the Base Velocity and Storm Relative Velocity interfaces that are available for consumers to use. Here are two radar images that I saved while tracking a tornadic supercell near Fayetteville, Tennessee on April 28th, 2014:
These radar images are two of the same scans, but on different panels. Both of these radar images are at a 0.5° tilt, which is the lowest scanning tilt that a WSR-88D (Weather Surveillance Radar – 1988 Doppler) can operate on. Note that WSR-88D radars have different horizontal tilt angles, of which 0.5° is the lowest. Other scan tilts are 1.5°, being the second lowest, followed by tilts approximately at 2.4° and 3.4°, being the second highest and highest scans, respectively. I’m bringing the angle tilts up due to the fact that the distance and height must always be taken into account when diagnosing radial velocity images, as well as the other doppler radar products. The doppler radar that retrieved these scans were the KHTX WSR-88D in Huntsville, Alabama. The lowest scan tilt was used in these images because the core of the storm was roughly 25 miles (40 kilometers) away from the location of the doppler radar. This means that the beam height was about 1,100ft above the ground, therefore detecting the low-level rotation within the supercell. Doppler radars cannot detect tornadoes. However, they can detect the rotation itself and in some extreme cases, tornado debris signatures (debris ball). So by using these two panels, which one works best for detecting rotation within thunderstorms?
Base Velocity panels scan and detect storm motion that propagates toward and away from the radar. The negative values that are detected is motion moving toward the radar (green) and the positive values that are detected is motion moving away from the radar (red). This goes for both Base Velocity and Storm Relative Velocity. The Base Velocity panels can detect not only the storm motion, but motions relative to the storm such as rotation, etc.
Storm Relative Velocity panels scan storms with the storm motion subtracted out. So for example, if a storm is propagating at a speed of 50MPH and has a 120MPH shear couplet, it will only detect the 120MPH shear couplet. This can be seen on the radar images at the top of the page. Storm Relative Velocity can deliver a more prominent verification of rotation within thunderstorms and can be deterministic for the issuance of tornado warnings by NWS personnel.
Cross-comparing both Base Velocity and Storm Relative Velocity can lead toward heightened confidence in determining whether or not a thunderstorm has rotation that could be capable of producing a tornado. In case you are wondering how to determine radar beam height and distance, use this simple equation:
height = (distance x tan(angle))
Plugging in the numbers of the distance of the storm you are tracking, as well as the scanning angle, can give you an idea of the height the beam is located. If a beam height is 10,000ft above the ground and you are seeing a strong rotation couplet, it is only detecting the mid-level rotation of the storm and not a tornado. Now, the question that everyone is probably itching to find out: Was there a tornado on the ground at the time those images scanned on April 28th, 2014? The answer: Yes. Dual-Polarized radar on the Correlation Coefficient panel detected debris being hurdled thousands of feet into the air and was also confirmed by storm spotters and law enforcement.
Not to digress from the original purpose of the blog, being the discussion between BV and SRV, but I felt it was necessary to show the Dual-Polarized CC panel to insert a point. There have been numerous other instances where there have been intense rotation couplets on radar, but no tornado. Here is another brief case of a tornadic supercell moving near Lubbock, Texas, on June 7th, 2014. Here is the image:
Take notice to the intense gate-to-gate wind shear couplet, which was about 120-130MPH wind shear detected on the KLBB WSR-88D in Lubbock, Texas. This supercell was about 55 miles northwest of the radar position, so the wind shear detected in this scan was nearly 4,000ft above the ground. Eventually, about 10-15 minutes after these scans, the storm did go on to produce a brief tornado. But I consider this another instance where you can have an intense shear couplet, yet have no tornado.