"Donut" Signature on Radar

[Steve Cosby]

This morning, I pulled up radar site KSRX on Accuweather and looped the radar to see how a line of storms came through our area last night. Early in the loop, I spotted an extremely unusual signature on the radar. It looks like an explosion to me. Here is the first image, from around 11:06Z this morning. The donut appears just above the label "Glenwood".

(I can't seem to get images to display so please forgive having to click to these links)

http://stephencosby.tripod.com/weather/index.album?i=0&s=1

In the next image, the donut is clearly visible.

http://stephencosby.tripod.com/weather/index.album?i=1&s=1

Now, the donut is expanding and beginning to dissipate.

http://stephencosby.tripod.com/weather/index.album?i=2&s=1

And, at around 11:49Z, the donut is dissipating now.

http://stephencosby.tripod.com/weather/index.album?i=3&s=1

I sent an email to the NWS office in Little Rock asking, in essence, "WTF?" One of the meteorologists responded with the following:

Actually, the "donuts" show up occasionally...usually around area lakes and mostly during the early morning (toward sunrise). The culprit, or so I've been told, is bird activity (yes, the radar actually detects birds).

It was around sunrise and the donut did appear over Lake Ouachita near Hot Springs.

I did look at surrounding radars to see if they saw the donut and, lo and behold, they did. In fact, the radar around Memphis had three of them itself. Surprisingly, these all occured on a line roughly parallel to the cold front which went through last night.

Although the explanation of the NWS meterologist tracks with lakes and sunrise, I'm having a hard time believing it.

So, I turn to the intrepid Storm2K community for comment and explanation. Any thoughts would be most appreciated.

--
Steve Cosby

[ohiostorm]

Ive heard them to pick up large flocks of birds. Never actually seen one myself but its pretty wild.

[NWIASpotter]

I have heard of this before and it has happened on several occasions. Not only birds, but even swarms of insects have been detected on radar. Sunrise/Sunsets, dust, and many other things can be detected on radar. So, if the NWS employee gave you that explanation that would probably be the right answer in my opinion. He should be used to the different things that occur on their radar screen...

[Hurricaneman]

Just ground clutter

[Steve Cosby]

Just ground clutter

Ground clutter - right.

[Steve Cosby]

I have heard of this before and it has happened on several occasions. Not only birds, but even swarms of insects have been detected on radar. Sunrise/Sunsets, dust, and many other things can be detected on radar. So, if the NWS employee gave you that explanation that would probably be the right answer in my opinion. He should be used to the different things that occur on their radar screen...

Yeah, I know - it's just weird how the signature was this explosion looking thing.

But, your're right.

[senorpepr]

Actually, you'd be amazed what you can see on radar. Aircraft can be picked up on radar as well as the aforementioned birds and insects. Even on the 4th of July and New Year's Day, if the atmosphere is moist enough, you can pick up fireworks. Also, with the space shuttle coming back in the news lately, debris from the Columbia disaster in 2003 was also picked up on radar.

[ohiostorm]

Yeah I remember seeing that on radar. I believe I have it saved somewhere.

[Steve Cosby]

Ive heard them to pick up large flocks of birds. Never actually seen one myself but its pretty wild.

Well, after googling till my mouse finger hurt, I found it. URL is http://virtual.clemson.edu/groups/birdrad/comment.htm

When colonial nesters such as Purple Martins (Progne subis) finish breeding, they stage for their southbound migration in dramatic roost sites sometimes numbering in the tens of thousands of birds. During this staging period, beginning as early as late June or early July, birds disperse in the early morning to feed. As they disperse from the roost site, a striking 'roost ring' can be seen as the birds leave the roost and climb into the radar beam. This roost ring expands until the birds begin foraging after dispersing to much lower densities some distance from the roost.

Absolutely amazing. First, that the NEXRAD picks it up. But, second, who would have thought that the birds would pull this behavior.