What exactly is the Hook Echo?

[John Cameron]

When we see a hook echo on radar, are we seeing heavy precip bring drawn around the mesocyclone? Or, is it being drawn around the tornado vortex? Is it precip under the influence of RFD? Is it precip being shoved upwind by focused, accelerating inflow?

I've never heard the explination of what causes what we're seeing on radar. I thought this may be a good "beginner" topic.

 

[Joey Ketcham]

A hook echo is the result of precipitation being wrapped around the area of rotation, on radar it appears just as that.. a hook.

This is a radar image which shows a hook echo on the southwest side of the storm just west of Newcastle, OK.

 

[Skip Talbot]

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.

 

[Stephen Locke]

This grab has been detailed;

 

[John Wetter]

hook echo—A pendant, curve-shaped region of reflectivity caused when precipitation is drawn into the cyclonic spiral of a mesocyclone. <table><tbody><tr><td><dd>The hook echo is a fairly shallow feature, typically extending only up to 3–4 km in height before becoming part of a bounded weak echo region (BWER).
Fujita, T., 1958: Mesoanalysis of the Illinois tornadoes of 9 April 1953. J. Meteor., 15, 288–296. </dd></td></tr></tbody></table>

 

[Aaron Kennedy]

The picture is a bit misleading as you really need to look aloft to see the BWER. I'll add that during my M.S. thesis, only 20% of hook echoes were tornadic, so you defintely don't need a tornado.

As a matter of fact, I would even hypothesize many appendages we label as hook echoes may have less to do with near-sfc rotation. Instead, they form as precipitation is advected around the mesocyclone aloft and then fall out to the surface. I've seen plenty of appendages form more of an L like echo with the echo trailing the updraft. In other cases, you'll see the mesocyclone (incipient tornado cyclone?) strengthen aloft, precip wraps and becomes collocated with the rotation and THEN descends to the surface (like a DRC or what some call a "finger"). In other cases, this intensification in vorticity seems to occur at all levels, and there is no descent in the reflectivity field.

 

[Scott Nelson]

Not stepping on John's thread here but would a storm producing multi vortex tornadoes have a multiple hook signature/echo too? Storms that generate this power fasinate me more than anything.

 

[Karla Dorman]

So: is the hook echo after the tornado (as depicted on radar) behind the actual circulation/tornado? Appreciate any answers/help ...

 

[Skip Talbot]

The hook does not have to correlate with the tornado at all. A mesocylconic tornado can spawn north of the hook in an area with no radar reflectivity (I think this is more common actually) or even further north of there in the precip core, or back inside of the hook (like in many HP's). Also remember that non mesocyclonic tornadoes including gustnadoes and spouts can form in a wide variety of places along the storm. All the hook indicates is that the storm may have a mesocyclone, and these storms have a tendancy to be the tornado producers. I wouldn't try to directly pin a tornado on a hook, especially since your are not looking at the ground level slice of the storm.

 

[L.B. LaForce]

yes, here is a smoothed image from GR3 showing the same hook echo as posted above.

 

[Glen Romine]

The hook appendage tends to have a cyclonic/anti-cyclonic vortex pair straddling the tip of the hook - where the cyclonic side is the most likely place for tornado development (though many examples exist where the circulation was there, but no tornado). So, I guess you could think of this as a hook within a hook. There are lots of good examples of the inner complexity of hook appendages in the literature. Here is one that you might want to look at:

http://ams.confex.com/ams/pdfpapers/81978.pdf

 

[Steve Dedman]

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. Granted, the 5/3/99 OKC event was as extreme as they get, and you won't see that radar depiction most of the time. But it does help to pinpoint the exact location of the vortex in relation to the rest of the hook structure of this storm. Other storms will vary.

Now a follow-up question:

Would it be accurate to say that a vortex located in the "end" of the hook is more likely to be in the "sweet spot" of the storm for significant tornadogenesis, as opposed to other areas of the storm? IOW, would a significant tornado be more likely from this spot in the hook than in another area, such as one with very little reflectivity or up into the precip core?

My gut says yes.

 

[John Wetter]

Not stepping on John's thread here but would a storm producing multi vortex tornadoes have a multiple hook signature/echo too? Storms that generate this power fasinate me more than anything.

Short answer: No. The 88D cannot resolve the tornado vortex except in fairly extreme cases (eg. Greensburg). The hook echo is much more a function of the Mesocyclone than the tornado vortex.

 

[Brian Stertz]

Sometimes, the tornado is physically outside of the hook. This I have seen in well occluded but still very prominent hooks. The best thing I have found to find the tornado is to find the nose of the inflow notch on radar. This notch is a seemingly lowered reflectivity wedge where winds are ripping in from the southeast and east (sometimes northeast). This will point to the new or mature tornado from my experiences. Hook echoes do not always mean a supercell will drop a tornado...its the steady state ones 30 mins. plus (reference the May 23rd evening/night SC Kansas tornadic supercells) that are pretty much locks. Those were all very impressive to say the least.

 

[Dave Gallaher]

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.