|Fig 1 -- 0.5 degree base reflectivity from KOUN at 0103Z, May 22, 2011.|
So what are those returns? It's something called ground clutter. Here's what happens. Sometimes the vertical density profile of the air in the lower atmosphere arranges itself in such a way that radar beams emitted into the air get bent in odd directions. With the right conditions, sometimes the beams can get bent into the ground.
When an emitted radar pulse hits the ground, it bounces back and returns mostly along the same path upon which it traveled out. As such, the radar interprets this as a very strong return at the range where the radar beam hit the ground. This sort of phenomenon is more common in coastal areas and also when there are strong inversions in the low levels of the atmosphere. So all of those chaotic (but still strong looking!) returns west of the radar are actually ground clutter--areas where the lowest radar beam is hitting the ground.
If you spend a lot of time looking at radar images, ground clutter is often easy to spot. For one, it moves chaotically and not uniformly like precipitation does, and often wildly fluctuates in the strength of the returns. So seeing all these random strong returns dance around in a radar animation is usually strong evidence that you're seeing ground clutter. But there's another way to check for ground clutter--look at the accompanying radar velocity image. Here's the velocity image from the same time as the reflectivity image above.
|Fig 2 -- 0.5 degree base velocity from KOUN at 0103Z, May 22, 2011.|
That makes perfect sense, though. There is no Doppler shift when the radar beam hits the ground. After all--the ground isn't moving, so why would it have a velocity value? Any strong thunderstorms are most certainly going to have some pretty strong velocities associated with them. So strong radar returns with zero velocities associated with them are almost certainly not precipitation. They're almost always some sort of clutter.
In fact, the NEXRAD radars have built in algorithms that look for just this combination--strong reflectivity returns in areas of zero velocity--to remove clutter from the radar. So, it's not as common to see this kind of clutter on radars as it used to be. However, the radar I'm showing in the images above (KOUN) is an experimental testbed radar used by the Radar Operations Center, so sometimes they don't have all the algorithms enabled. As such, it's kind of fun to watch it once in a while to see all these random radar phenomena that we never get to see on other radars.
If the right environmental conditions persist, ground clutter can also hang around for a while. Here's the reflectivity image from half an hour later:
|Fig 3 -- 0.5 degree base reflectivity from KOUN at 0133Z, May 22, 2011.|
|Fig 4 -- 0.5 degree base velocity from KOUN at 0133Z, May 22, 2011.|