Radar Blooms

I’m seeing this question pop up time and time again from weather enthusiasts — what the heck is going on with this radar site!?!? Typically, the image in question shows a familiar radar graphic with city names and county and state borders but with one noticeable glitch— areas of large nearly symmetric blobs of greens and blues.

As a meteorologist, I see this anomaly often — so much so that I don’t think much of it. But to John or Joan Q. Public, the radar blob can be a little disconcerting. No need to worry; it’s a perfectly normal anomaly.

Anomalous Propagation

The technical term is called anomalous propagation or simply, ground clutter. Anomalous propagation or AP happens when a radar beam is emitted and is then refracted or deflected.

In some instances, the deflection holds the emitted radar beam close to the earth’s surface and the beam is capable of traveling for an extended period of time for long distances. This is called superrefraction.

Other times, the deflection is so strong that the emitted radar beam is sent right back down to the surface of the earth — this is known ducting.

In both the superrefraction and ducting scenario, the beam consequently sends back high echo returns; returns that are greater than your typical precipitation echo. These high returns occur throughout the duration of the full radar scan and graphically the end result is an erroneous blob called ground clutter.

Beam Deflection

Deflection occurs due to the radar beam encountering a stable, typically shallow layer of the atmosphere. This stable layer is called an inversion which develops when the decrease in density with height is less than normal. In other words, the air at the surface is colder than above and the temperature of the atmosphere actually increases with height.

Inversion

Inversions most often develop at night under calm, clear conditions. During the day, the sun’s energy heats the surface of the earth. At the same time, the surface emits that captured infrared radiation (IR) right back. At night, however, we lose the surface heating from the sun but the ground continues to emit the IR back out. With little clouds overhead, the infrared radiation easily escapes into space. This net loss of heat enables the air very close to the surface to cool, moreso than the air just above. As a result we have stability (cold air is more dense than warm air) and a temperature inversion has formed.

Blooming

The top image in this article shows radars blossoming with AP from the East Coast of the United States into the central and southern Plains during the evening of October 10, 2007. The animation here shows the same thing on the evening of October 7, 2005.

As the sun sets east to west across the United States, the process of surface cooling begins and inversions quickly emerge. As the inversions set in, anomalous propagation takes hold thanks to superrefraction and ground clutter lights up on the various radar sites going east to west with the setting sun. With superrefraction in progress, the full scan of the radar beam is encountering large amounts of scattering “noise” such as mosquitos, aerosols, a variety of other particulate matter, and even birds.

So there you have it. Hopefully a digestible explanation of why radar sites sometimes seem to go haywire with an explosion of ground clutter. Radars are able to detect a variety of objects in the sky both weather and non-weather related. If you’d like to know more, I recommend this article explaining some of the other very interesting features that get picked up on doppler radar.