Downdraft Reflectivity "Hole"?

[Todd Renshaw]

I'm watching an area of low reflectivity embedded within the squall line passing just north of KMOB right now. Can anyone explain to me if this is an area of downdraft? If no, what causes this within an otherwise strong line of reflectivity? A factor of the storm moving above the tilt the closer it gets to the radar station?

KMOB, 0.5 degree tilt, watch for the area directly north of the radar, south and a little east of Citronelle. I'm looking at the sweeps from roughly 9:48 - 10:11 AM CST. (So 15:48 - 16:11 UTC.)

 

[Skip Talbot]

A downdraft usually shows up brightly on the radar because there is often a lot of rain and hail in it, which reflects the beam. An area of low reflectivity can indicate a string updraft, however. This is what happens with a bounded weak echo region. If the updraft in a thunderstorm is strong enough, it keeps the rain and hail from falling through it. Without the rain the beam doesn't reflect, and you get a nice little hole in the storm.

In this case, however, it looks an artifact. The hole doesn't blend well into the squall, and it doesn't keep up with the storm's movements, as it looks like it gets left behind as the storm passes. A doppler expert on here might know what's causing or if its got a special name, but I'm guessing it might be terrain interfering with the beam.

 

[Todd Renshaw]

Cool. Thanks for the info, Skip. You're thinking an artifact of the terrain? Seems like a good guess as it looked like it stayed in place at the same ground location, at least initially.

I've seen screen-grabs of supercell BWERs, but (as you already said) this didn't seem to fit. Just thinking aloud: I suppose, really, the entire line of high-reflectivity on the squall line is downdraft, right? So it wouldn't make sense that there's some sort of low-reflectivity downdraft within. (Sinking cold air with little precipitation?) But neither does it make sense to my brain that there would be an updraft forming right within the middle.

I often see radar weirdness and wonder what causes various features. I should probably keep posting as I encounter them. Might be interesting to hear the radar experts interpret? (Yes, I've seen the threads/posts on radar anomalies. So I'm fairly familiar with the common ones.)

 

[Jeff Duda]

I will agree with Skip that it sounds like an artifact of the radar system. Given the proximity of the hole to the site, I would say it's probably related to the clutter filter activating in that area. The most basic clutter filters are designed to be applied to bins with a narrow Doppler velocity spectrum that has a near-zero mean velocity. Given that the region in question was right along the zero isodop, I'd say there's a good chance it is indeed the clutter filter.

 

[Todd Renshaw]

... The most basic clutter filters are designed to be applied to bins with a narrow Doppler velocity spectrum that has a near-zero mean velocity. Given that the region in question was right along the zero isodop, I'd say there's a good chance it is indeed the clutter filter.

So if part of the storm backtracks slightly, and it just so happens to be within the rotation's zero zone, the filters are likely to reduce the dbZ value because it thinks it's ground clutter? Do I understand that right?

 

[Jeff Duda]

I'm not totally sure what you mean by this part of your statement: "So if part of the storm backtracks slightly, and it just so happens to be within the rotation's zero zone", but you are right on in terms of the second part of your statement.

 

[Todd Renshaw]

I'm not totally sure what you mean by this part of your statement ...

Don't worry about it. Just trying to understand your "near zero mean velocity, zero isodop" explanation in my layman's brain. I'm probably over-thinking it. Thank you for your help, Jeff.

 

[Jeff Duda]

The zero isodop is a line/curve along which Doppler velocities are zero not because the actual velocities of the particles are zero there, but because the motion vector there is exactly perpendicular to the radial. The zero isodop must pass through the radar site, so to look for it, start your search very near the site.

 

[Patrick Marsh]

The zero isodop must pass through the radar site, so to look for it, start your search very near the site.

While this is true for a uniform background wind, it is not a universal statement. It is easy to construct scenarios for which a zero isodop exists and does not pass through the radar site. Frontal passages are one example.

 

[Jeff Duda]

While this is true for a uniform background wind, it is not a universal statement. It is easy to construct scenarios for which a zero isodop exists and does not pass through the radar site. Frontal passages are one example.

That's true. Another example would be if there just happens to be a downburst centered on the radar site. However I think these are exceptions to the general rule.

 

[Todd Renshaw]

Very interesting. Thanks for all the help, guys!

 

[Todd Renshaw]

Just saw an example of that showing up in clear air mode. Looks like the clutter filters at it again? Really easy to spot in this example. Yes, I have screen grabs this time!

KDTX, 0.5 deg tilt, 02/14/12, 18:01 - 18:57 UTC.


Watch video >

 

[Jeff Duda]

Precisely, Todd. That's the basic clutter filter activating.