What exactly is the Hook Echo?

[Paul Knightley]

I understood that the hook was precip descending from the top of the updraught column but immediately upwind of it, coming from the BWER aloft. The shape is to do with its interaction with the mesocyclone, especially the low-level mesocyclone, but it's not actually precip from the forward flank core being pulled around it - rather, it's a seperate area of precip to the forward flank core.

 

[Scott Taylor]

I don't know if it helps you visualize or if it just confuses the discussion, but the small area of very high reflectivity in the "ball" at the end of the hook (white on L.B.'s shot) is likely the debris cloud of the tornado.

Steve, this is not the case, as the radar isn't seeing what's going on at ground level. I've included Skip Talbot's reply below to the original question, as it clearly explains what you're seeing in the signature.

Scott

Yes, its being wrapped around the mesocyclone of the storm. The radar images you typically see hooks on are from looking at the midlevels of the storm where the mesocyclone is. To see precip wrapping around the tornado you'll need a much higher resolution radar with a beam lower to the ground (like Josh Wurman's radar). Also keep in mind that many storms with hook echoes do not produce tornadoes. RFD is also marked by the hook. As the hook approaches you will get hit by outflow which is RFD and then heavy rain and/or hail.

 

[Greg Stumpf]

Steve, this is not the case, as the radar isn't seeing what's going on at ground level. I've included Skip Talbot's reply below to the original question, as it clearly explains what you're seeing in the signature.

Actually, the 3 May 1999 storm presents a great example of a debris ball signature, but it wasn't at the time of those radar images. It occurred just after the tornado entered the densely-populated neighborhoods of southwest OKC and Moore. A very high reflectivity echo develops within the ball of the hook and ascends to mid-levels on subsequent volume scans. This can be seen on a time series of vertical cross-sections through the ball of the hook.

Use the "ball hook" = "debris signature" correlation with much caution. A precip ball on the end of the hook usually does NOT equate to a debris signature and is simply a rain-wrapped low-level meso. One should carefully analyze the 3D structure trends of the hook like with the 5/3/99 case, and correlate it with dense "ground target areas" to make a definitive conclusion. Furthermore, polarimetric radar data shows some great potential in segregating debris from other targets in the reflectivity field.

 

[Skip Talbot]

I think it should also be stressed that you should be looking for the mesocyclone, low leve rotation, and potential tornado location using the velocity scans, not the reflectivity scans. There's a lot of guesswork to deciphering the internals of a storm based on a slice through its precip core. Whereas with velocity you can actually see what kind of rotation (on a broad level) is occuring at that level.

 

[Connor McCrorey]

Wasn't the Greensburg supercell also a good example of a debris ball? I seem to remember the tip of the hook exploding on reflectivity a short time after the tornado moved over the town.

 

[Chris Lott]

Les Lemon did a presentation at TESSA on the Greensburg tornado, showing the debris ball and the "vortex hole" that appeared on radar. Here is a paper of his on the Greensburg cell, which is basically the presentation we saw.

http://ams.confex.com/ams/pdfpapers/141811.pdf

 

[John Cameron]

Les Lemon did a presentation at TESSA on the Greensburg tornado, showing the debris ball and the "vortex hole" that appeared on radar. Here is a paper of his on the Greensburg cell, which is basically the presentation we saw.

http://ams.confex.com/ams/pdfpapers/141811.pdf

That is a fantastic paper! Primarily because Les wrote it in a way that the non-met can understand it (see: me)
By the time I get to page 4 of the Lemon/Doswell paper of 1979, my head really starts to hurt.

 

[John Wetter]

Here's a classic example of a hook (even with a TVS) that dropped no tornado, that I posted from the Selma, AL storm of February 18th.

As a side Topic, Dave, you have a graphics driver problem there with that radar grab. Turn on 'force 16 bit smoothing'. See the GRS owners forum for details.

The main thing to remember with the hook echo is that it is just an indicator. Use it as a tool, not as the solution.

-John

 

[Tim Vasquez]

I'm a little late chiming in on this, but Aaron Kennedy touched briefly on the DRC (descending reflectivity core). This is an area of enhanced reflectivity that forms quite a bit higher in the storm near the mesocyclone - RFD interface and appears to have some correlation with the onset of a "hook echo" (or appendage, if we want to use proper terminology) once the DRC reaches lower levels.

If you search Stormtrack for DRC there might be some discussion about this. Also some useful references:

Rasmussen, Erik N., M. S. Gilmore, and R. Davies-Jones, 2006: A preliminary survey of rear-flank descending reflectivity cores in supercell storms. Wea. Forecasting, 21, 923-928.

Rasmussen, Erik N., Jerry M. Straka, Matthew S. Gilmore, and Robert Davies-Jones, 2006: A preliminary survey of rear-flank descending reflectivity cores in supercell storms. Wea. Forecasting, 2006, 923-938.

I think Howie Bluestein also put out a paper on this around 2001 but I'm not sure where it is. I'd be curious whether anyone has looked for DRC signatures or has had luck messing around with GRLevelX to look for this stuff.

Tim

 

[Dave Gallaher]

As a side Topic, Dave, you have a graphics driver problem there with that radar grab. Turn on 'force 16 bit smoothing'. See the GRS owners forum for details.-John

Done--thanks for pointing that out, John. I was totally unaware.

 

[John Wetter]

Tim, GR2AE I think is the only commercial product you can look for DRC's with and I've been very successful in using it for that over the last several years. The trick is finding the FAR for it or what to really use it for, if it's anything other than just another indicator. You can find it on all strong tornadoes, most moderate tornadoes and even on many weaker ones, just needing to change your isosurface to what makes sense for that strength of storm.