BWER's and Rotation

[John Hudson]

I have seen several references to the presence of a BWER as being an indicator of rotation within a thunderstorm.

Unless I misunderstand it, isn't a BWER created when an exceptionally strong updraft suspends most of the precipitation within it at its summit?

If this is correct, how is a BWER indicative of anything other than a strong updraft?


John
VE4 JTH

 

[Jeff Duda]

I don't think a BWER is a direct sign of rotation. Like you said, I think it just means there is a powerful updraft suspending hydrometeors above the ground. You can associate it with tornadic potential if you see a storm with a BWER and rotation near the BWER. That gives the indication that a strong updraft could be tilting and stretching vorticity to the point where a tornado could be more likely.

 

[davecarlsen]

Essentially such a reference is a reference to the Lemon Technique.

http://en.wikipedia.org/wiki/Lemon_technique

 

[Keith Mullen]

Correct me...

Correct me if I am wrong, but the presence of a BWER isn't truly an indicator of rotation as previously noted and mentioned, but an indicator of how compact and concentrated a powerful updraft is.

A vertical rotating updraft with very strong motion (enough to remove all precip in a cylindrical core) is more likely to appear as a perfect donut-hole versus an unorganized - yet still strong - updraft.

So, in closing, it's a really good indicator to add to the checklist of what's going on - but in and to itself, is not a solo indicator.

Am I correct? Or is my theory in left field?

Thanks.

 

[Jeff Duda]

From the reference Dave linked:

* Bounded weak echo region (BWER) - Another area of markedly lower reflectivity, now bounded by an area of high reflectivity. This is observed as a "hole" in reflectivity, and is caused by an updraft powerful enough to prevent ice and liquid from reaching the ground. This powerful updraft is often an indication of, or is facilitated by, a mesocyclone. It should be noted, however, that a mesocyclone is not strictly necessary for BWER development. Storm rotation can be reliably detected by the Doppler velocities of a weather radar.[9]

The implication therein is that Meso -> BWER is false.

 

[Jeff Duda]

Am I correct? Or is my theory in left field?

Keith,
Honestly, I don't think a BWER is any indication at all of rotation. See my other posts in this thread.

 

[Keith Mullen]

That's almost...

That's almost where I was going with that. Perhaps if I clarify my statement:

A BWER is not an indicator of rotation on its own. Any storm with a very strong enough updraft can show a BWER - rotating or otherwise.

My theory is that a storm with some rotation that can keep the updraft corralled by the storms internal motion and centripital force - is going to help keep the force of the updraft more focused - thus allowing greater clearing of all precip and showing a cleaner signal.

Can you use a BWER to try to argue there is rotation? Not really. Does a BWER technically indicate rotation? Not per se. But I personally think of it as another weighing factor to "suggest" rotation, when accompanied by other data.

Does that make more sense?

 

[Jeff Duda]

Sure, it does. But I would never work that way. In my head, if I'm looking for rotation, I would look first at BV and SRV, products that would always be available on any radar that I would use that would even show a BWER. So I wouldn't rely on a BWER at all for any semblance of any sort of rotation.

As an example, I once used the radar at the rest stop on I-80 northeast of Lincoln, NE to see where I wanted to go to chase (I didn't have any data with me to look at). Although I couldn't see any velocity products with the radar at the rest stop, the resolution on it was also so poor that all but the strongest BWERs would be invisible on that radar.

 

[Dave Gallaher]

While we are discussing BWERs, I'm reposting this return from 4/10 in north Alabama. Is the differential between the surrounding precip and what I've identified as a BWER sufficient to qualify as a BWER? No one complained about it, but I've wondered what level of change is required to earn the designation. Thanks.

 

[Jeff Duda]

As far as I know, there is no technical classification for the reflectivity gradient required for a BWER. In the image you have, there is definitely a BWER.