Radar observation question.

[David Draun]

I'm on GRLevel3 looking at the KDVN area. The radar was in VCP 32 (clear air mode) and switched to VCP 12 (precip mode) to track the developing weak convection to the west. As the radar went into precip mode, the MS river began to produce organized ground clutter. Basically you can see the state border between Iowa and Illinois made up of radar echoes along the river. What exactly is causing this?

It shows up pretty good here too: http://radar.weather.gov/radar.php?rid=dvn&product=N0R&overlay=11101111&loop=no

 

[Nick Hartley]

KFSD's radar does a similar thing. It picks up the Buffalo Ridge to the east of Brookings and down to Worthington. Radar makes it look like storm cells forming over one area. Intriguing thing I noticed about a month ago while preparing for a chase.

It's show the Buffalo Ridge right now:
http://radar.weather.gov/radar.php?rid=fsd&product=N0R&overlay=11101111&loop=no

 

[Jeff Snyder]

David,

I think it's probably just coincidental that the VCP switch happened to occur around the time of boundary layer stabilization as diabatic cooling ramped up near sunset. While the radar was in VCP 32, you'll notice a large increase in the aerial extent of low reflectivities between 840 pm and 900 pm, right before the radar switched to VCP 12. As the sun neared setting, the surface cooled and a radiation inversion developed. In addition, I'm sure a myriad of bugs took to the sky around that time (and bugs can act as scatterers and be seen on the 88d if the concentration is high enough). I can't think of any reason intrinsic to VCP 32 vs. 12 that would lead to an increase in AP or ground clutter.

FWIW, VCP 31 is more sensitive than VCP 32 as a consequence (in a positive sense!) of VCP 31 employing a longer pulse length, so you may see more echoes if the radar switches from VCP 32 to VCP 31. Since it's a sensitivity gain, you'll see the most significant gains are longer ranges and lower reflectivities (i.e. VCP 32 will show no echoes, while very low reflectivities will show up on VCP 31)

 

[Jarrod Cook]

I'm about 100% sure what you're seeing are Mayflies hatching and taking off. ARX posted an article about this a couple years ago and had a radar loop showing them along the MS river. This is exactly what it looks like. They are also showing up along the Illinois River on the KILX radar...and I could see some on KARX too. Pretty neat!! This is the time of year for them, too.

 

[David Draun]

Ok well now I have a new question. Its about lightning this time. I have a lightning data file with my Allison House package. NLDN data near the newly formed 55,000ft bomb near Marengo, IA has CGs pounding the ground with some frequency in a two county radius well outside the storm itself. Now I know lightning can hit in an area up to 25 miles outside of a storm, especially positive bolts. But these appear to be much further. But I also question how accurate the data is.