There are a lot of interesting anomalies that you may see on displays that show NEXRAD (or any kind of) weather radar data. Some are caused by software, some are caused by the radar misinterpreting what it sees. None are worth some of the conspiracy theories that non-scientists have come up with.
Last month, blog reader Mike asked what is responsible for the radar "bloom" (or "radar blobs") that occurs nationwide, but especially in the Southeast U.S. in Spring and Fall. What he is referring to is the gradual growth of non-precipitation objects on radar after sunset (and the data fades after sunrise). During the night, this causes a large blob around each radar site. I have uploaded some examples from that night.
I knew what Mike was referring to was a type of "Ground Clutter" - also known as false echoes - a wide-ranging problem with weather radars, I just didn't know what specifically was causing it. So, I set out to do some research on Google, but I couldn't come up with an explanation, and apparently neither could anyone else who writes blogs or web pages. In the late 1990's, I wrote several articles on radar anomalies and Ground Clutter for AccuWeather.com properties -- but I never was able to explain this one.
NOAA [JessePedia], who owns and operates the radars in the national network, has an excellent page explaining how radar beams work. It included the illustrations below about Superrefraction and Ducting (the radar beam is shown in comparison to a faded "normal" radar beam at the top of the illustrations). In both cases, the radar beam curves quicker than the curve of the Earth. I suspected this was to blame for the Radar Bloom.
In the case of "Ducting" the radar beam bends so much that it hits the earth, causing extremely dBZ returns (because the ground is much thicker than your average raindrop when the beam runs into it). dBZ, or "decibels of Z" is the way radar data (hopefully precipitation) is measured. The colors you see on radars correspond to dBZ levels, higher meaning more intense. When the radar beam hits the Earth, this phenomenon is called "high dBZ anomalous propagation" and is a real problem because, to the untrained eye, it looks just like thunderstorms.
Other websites confirmed this explaination of Ducting, but while this is great, it doesn't explain radar "bloom" which is much lower on the dBZ scale* (see below), nor does it explain why it grows and shrinks with time.
Since I couldn't get an answer online, I wrote in to the NOAA radar experts. After a couple of returned emails due to a bad form on their site, I finally got in contact with Joe Chrisman from the ROC (Radar Operations Center) Engineering Branch, who explained:
When the sun goes down and the surface begins to cool, the change in refractive index in the lowest few (to several) hundred feet of the atmosphere tend to bend the radar beam toward the surface. This bending holds the radar beam near the surface for extended distances, where it encounters scatterers that would not normally be available above the boundary layer. These scatterers include insects, bats, aerosols, particulate matter, etc., and account for the increased radar return referred to as "radar bloom."
To decode that answer a little, what he's saying is that it is, in fact, superrefraction that causes radar bloom.
In the case of superrefraction, the beam bends low to the ground but, unlike Ducting, it doesn't run into the ground (until it gets out of range anyway). With the beam so close to the ground, it keeps running into multiple insects/dust/other particulates as it moves outward from the radar. As the superrefraction becomes worse, the radar beam travels farther than it had previously, and encounters even more of these particles, causing the amount of clutter on the screen to "grow." As the superrefraction decreases in the morning, it shrinks.
Why does refraction itself (be it Super, Sub or Ducting) occur? That's a more complicated question and I'll let you read the NOAA page for a lengthy explanation. Basically, where the beam travels with respect to the Earth's curvature is determined by a complex equation of pressure, temperature and humidity that can vary greatly in small distances, and it's possible you might have more than one type of refraction occurring at the same time.
P.S. "Trophospheric Ducting" is a similar phenomenon by which radio waves propagate thousands of miles further than they normally would due to atmospheric conditions, causing, in one documented case, an FM radio in Hawaii to pick up a radio station from Mexico (if you have an FM radio in your car and have trouble picking up FM stations in your own town then you understand why that would be quite unusual).
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