Roost Rings

The Charleston SC Doppler radar (really located south of Hampton, SC) picked up something that, if you’ve never seen them before, might send you scurrying to the news to see if something blew up, or the nearest UFO meeting to see what you missed:

Radar from KCLX (“Charleston” radar), Saturday Morning, 23 July 2022

Here is one frame zoomed in on the source, which seems to be east of Columbia, near Irmo, SC …

And here is what is there on Google Earth:

Click any graphic to embiggen.

So what is going on? The lake is a strong clue. It is a “Roost Ring”, radar returns from massive numbers of birds leaving their roosts to head out to forage for the day. Here is an article from the National Weather Service with more details (link). In this case we are seeing Purple Martins leaving Lake Murray, which holds the largest sanctuary in North America, Bomb Island, home to something like one million of these birds.

The above radar images are reflectivity (strength) on the left, and direction of motion relative to the radar station on the right. Green means it is moving towards the radar station, red away. On the zoomed in view, that “arrow” pointing towards the station is an artifact of the earth’s curvature, the height the birds are flying, and the scan angle of the radar. Another neat thing to see is that thunderstorm cluster that flared up over Myrtle Beach, visible in the top animation which covers from 5:30 to 7:30 am. At 6:15 it was nothing, by 7am it was a strong, 50dBZ return. That goes to show that this time if year it doesn’t take long for an innocent looking cloud to turn into a dangerous thunderstorm, so boaters and aviators beware! Just because the radar is clear now doesn’t mean it will be that way in 20 or 30 minutes.

FYI, there’s no threatening activity to speak of in the tropics anywhere in the world. JTWC is watching a weak circulation south of Japan, and NHC is watching activity in the Eastern Pacific, but staying offshore of Mexico and fading out before nearing Hawai’i, so no big risks there. No major earthquakes recently, and the situation in Ukraine continues to be Russia methodically working their way towards their objectives, not that you’d know that from Western media. The COVID pandemic is entering a new phase, with lots of interesting research and much better understanding of both the virus and how the vaccines interact. Unfortunately the data collection system and advice for the public, over two years after the virus appeared, is still substandard, with many states (like Georgia) not having updated their public data pages since April, and the news media jumping from issue to issue with no consistent coverage. The environment is so hostile for rational discourse that it’s hard to get the energy to write up something on either of those topics, but maybe I’ll give it a go next week.

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