What’s that on the radar?

I appreciate viewer email and especially when it comes with examples! Check this out from William:

Hi Frank,

When I checked the national weather radar mosaic on the evening of 2 September I observed a persistent circular standing wave pattern centered around Brownsville. By persistent, I mean over several hours.

I have attached screen captures of the radar patterns...

Cheers, William

Here are the radar attachments sent, wide and close up. You can see those semi-circle patterns that show up.

What would cause this? First, understand that the Doppler radar beam goes out something like a flashlight beam reflecting off a mirror except in this case the beam reflects off raindrops (or hail) back to the radar. Then the radar will map out where the rain is based on that reflection.

Of course, the Earth is curved so the placement of that beam to see the true storm can be tricky. In a normal situation, the beam looks like this going out to the storm:

But the beam could be too high or too low missing the storm or at least how strong it might be:

And, in some cases, like in Brownsville last Friday evening, atmospheric conditions may be such that the beam actually bends toward the ground, called ducting:

This kind of bending causes what is called Anomalous Propagation (AP), also known as ground clutter. The condition for this is usually a calm, stable atmosphere with higher pressure over the area. I looked at Brownsville for that day and temperatures were perfectly normal with a bit of rain earlier. My hunch is that high pressure formed between two areas of rain where low pressure was and that higher pressure, or denser air, bent the radar beam to the ground causing the AP. Something like this:

This would cause those anomalous semi-circles that look like a radar return, but are actually the radar beam bending and returning ground clutter images! Thanks for the question, William. Here is more for you from the National Weather Service.

Shout out to Mary Susan, Colleen, Paula, Rebecca and and Becky who all emailed to say how much they appreciated my blog on weather and pain -- they know all too well how changing weather can affect their health and were glad to finally see some scientific back-up! Thanks for the notes. If you missed that blog, it’s right here.

Have a splendid weekend as the weather promises to be warm and dry!

Frank

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