Thursday, May 14, 2009

Doppler Ground Clutter

There are many viewers who have asked about the junk that is around the center of Doppler radars. It looks like precipitation...but is always there....even on a clear night.

Here is a general explanation of ground clutter around the radar and anomalous propagation.

Ground clutter is a pattern of radar echoes from fixed targets around the radar. An example is the red and orange spots that are always around the Mega Doppler. There are farm buildings and grain silos near the radar site that always show up as red or orange spots that do not move in the center of our radar display. We built Mega Doppler outside of the metro so we would not the interference of buildings downtown. Ground clutter can hide or disrupt precipitation echoes near the radar antenna.

Anomalous propagation (A.P.) occurs with temperature or moisture gradients near the radar that cause part of the radar to bend abnormally. The beam of the radar can be turned into the ground or can bend upward. Both types of A.P. can make it look like there are large areas of rain or snow falling right around the radar. A.P. is most often visible at night with high amounts of moisture. Yellow and green are the most common colors that show up on the Doppler during anomalous propagation. The false returns are most common during nighttime hours when cooler temperatures and more moisture is pumped into the atmosphere adding to the bending of the radar beam.

We manipulate the power and tilt of Mega Doppler to eliminate most of the ground clutter and A.P.


A viewer from Ankeny noticed that it is really difficult to see storms on radar when they move over the northern edge of Ankeny. This is a portion of the email I sent to our Ankeny viewer... it will help explain why being close to the radar does not keep you safer in storms.

The issue with Ankeny is the growth of the area to the north of town since our radar was installed at Alleman. That area...just like so many in central Iowa... has expanded so much in the last several years. Many housing courses...etc have moved very close to the radar site. This has created an issue for those who live under the dome of ground clutter that will always be a part of any Doppler radar location.

I mentioned in the earlier email we purposely put the radar site away from the downtown area of Des Moines. Since the installation of our Doppler... Ankeny and Des Moines have nearly joined at the south...and Ankeny has really expanded to the north. Alleman has expanded right under the dome of silence around the radar site.

We use this site because Alleman is one of the highest points of the central Iowa area. It is where our TV towers are we own the property.

We try to eliminate as much ground clutter and Anomalous Propagation as possible... but if you are right under the radar site... there is really nothing we can do about those permanent returns of buildings and bending of the radar very close to the radar.

In the event of a very close storm over the our radar... you can cross-section the storm with the National Weather Service Doppler that is located over by Camp Dodge in Johnston.

You will notice they have more ground clutter than we do on a clear day... they put their radar into a "clear air" mode that really increases the junk around the center of the Doppler.... but during a storm that is right over Ankeny... this could help.

Keep that NOAA all-hazard weather radio in your home and in a place that will wake you up if the storms move through when your family is sleeping. You can even register to win a weather radio from Channel 13 by going to

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