On a WeatherUnderground map called a Wundermap, around the city of Joliet, Ill., I saw a peculiar radar signal. It was roughly circular in shape with two large gaps, more like inward facing parentheses. It moved around slightly for about 25 minutes, according to the timer on the map, and then disappeared. It didn’t keep the same exact shape, but kept the same overall configuration. It seemed to be around 15-20 miles across but only a mile or less thick. I thought it might be some glitch in the radar around Joliet, but it did look like a signal for slight to moderate rain, i.e. light green and darker green. I’ve never seen anything like this, although it may be that some meteorological dopers have. What’s your best guess? xo,
C.
p.s. I did take a screenshot but I have no clue as to how to create a link to it.
Here is the WU radar map for yesterday:
https://www.wunderground.com/maps/radar/yesterday/spi
If you select IL and play it, there indeed appears to be a large circular artifact that briefly appears around Joliet at the beginning of the animation, then disappears. It looks much larger than 15-20 miles though; more like about 100 miles. Strangely, around the same time that that thing appears, a similar thing appears in southern Michigan and another partial one in southern Missouri. I don’t know what these things are.
Unfortunately I believe the link above is only good today; it literally shows “yesterday’s” radar. I’m not sure if there’s a way to get a permanent link to a particular date.
It looks to me like contamination from the radar’s sidelobes.
For those who are not familiar with radio antenna patterns and sidelobes, here is a handy-dandy wikipedia article on the subject:
The short version is that while most of a radar antenna’s energy is transmitted and received out the front of the antenna, some of the energy leaks off to the sides (hence, “side” lobe). The radar can’t tell if a reflection is coming from the main lobe or the side lobe, so a weather storm directly over the antenna could be picked up by the sidelobe, tricking the radar into thinking that it is getting a reflection from further away. As the radar antenna rotates, it keeps getting a reflection from the overhead storm, and keeps thinking that the reflection is coming from in front of the antenna and not overhead, forming the ring patterns on the radar display.
Radar transmitters and receivers do a lot to minimize sidelobe contamination, but it’s pretty much impossible to eliminate it completely.
I note that all three of the artifacts that I mentioned are roughly centered on one of the radar dishes. The Joliet circle is centered on the dish labeled LOT, the Michigan circle is centered on DTX, and the Missouri circle is centered on PAH.
I don’t doubt that engineer_comp_geek knows what s/he is talking about, but there were no storms in the area. Maybe the essence of the explanation is correct, that it’s an artifact related to the way radar works, but the returns didn’t come from an overhead storm, or any storm for that matter. I guess that other things can cause that type of glitch, too.
Side lobes also pick other reflections, such as ground clutter. For that matter, the main lobe gets ground clutter, too. It’s usually suppressed, but the suppression is not always perfect.
There is also a phenomenon known as atmospheric ducting, where the radio signals can get bent back down towards the ground if conditions are right. It’s also related to why sometimes AM radio stations can be received for much further distances. So it might be a ghost image of the ground due to ducting instead of something overhead. I wouldn’t expect the rings to be quite as uniform and circular in that case, but it’s possible, I suppose. My experience with radar was with fighter jets, not weather radars, so I am guessing a bit.
(and for the record, I’m a “he”)
ETA: It turns out that wikipedia has an article on atmospheric ducting, though it’s not one of their better articles, IMHO.
Let’s flip this around: suppose those circles were valid returns and not artifacts. That means that there was briefly weather or structures that formed a near-perfect circle ~100 miles in diameter and concentric with the radar antenna’s axis of rotation. That would be extraordinary.
It seems much more likely to me that the circles are artifacts and/or ground clutter—which is a kind of artifact.
Those are expected artifacts from the WSR-88D volumetric scan pattern and how that dataset is mapped to a 2D display. The NEXRAD system scans at multiple elevation angles, eventually compiling a composite volume “layer cake” of all those elevation scans. A volume scan takes from about 2-5 min depending on the radar mode. Typically consumer radar displays only show one elevation scan or some kind of composite.
With increasing distance from the radar site the beam will be increasing in altitude above ground due to curvature of the earth. Thus if the atmospheric conditions cause increased reflectivity at a given altitude, this may appear as a circular pattern at a fixed radius from the site. This is normal.
If you want to see better data and more specific control over elevation scans, use the app RadarScope, available for Android, iOS and Mac platforms: https://www.radarscope.app
For the Windows platform a more complex app is GRLevel3: GRLevel3 Main Page