Personal AccuWeather : Doppler Radar FAQs

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Ground clutter is the most common form of anomalous data appearing on your local radar. You see data displayed but it's not raining. What you are seeing is the radar beam reflecting off of non-precipitation objects, which could include pollution, insects, moisture, buildings, trees, etc. So why keep the ground clutter? NEXRAD is still a relatively new technology and the algorithms that could take the clutter out are not advanced enough to do so without removing some precipitation. Because the clutter is in the same range as regular precipitation, it is hard for computers to tell the difference between the two. The lower levels in which ground clutter appear also provide important data to meteorologists.

Here are some examples of ground clutter to compare. Unless otherwise specified, everything you see on these radar images is ground clutter; no rain or snow is falling.

Widespread ground clutter is almost always low intensity. This means that it will generally be white, brown, or light blue on the radar image. Usually it occurs in a disk shape centered around the radar site, as in this example of the State College site (the small blue areas in the Northwest of the radar are probably rain showers). It could be scattered over a wide area as in this example. It doesn't have to be circular; sometimes mountains or trees block the ground clutter, as you can see in this image of the Roanoke VA site. Here is an example of ground clutter from the State College NEXRAD while it was in Clear-Air Mode and then Precip Mode. Notice that the latter seems to have much less ground clutter. This is because the Precip mode ignores all dbZ readings below 5dbZ; this is why widespread ground clutter shows up so much better in Clear-Air Mode. Note - the blue lines you see in both images are the mountains surrounding the radar site.

Ground Clutter that is not as widespread can have high instensities, for example consider this sample of the Cleveland site which intersects buildings as the beam fans out from the site. This "Ground Clutter" causes the Max dbZ reading to be 50dbZ - inferring heavy rain where the green data is. But this data stays in the same place everyday even when there is no rain in Cleveland.

Remember that if you are still not sure if the data you are seeing is precipitation or not, consult a nearby radar which covers the same area.