Sunday, February 12, 2006

Column 9: Tornado warnings for bow echo storms (originally posted October 17, 2005)

I have been very actively watching warning trends in the United States for 3 years now, and one thing they do baffles me somewhat:
They issue tornado warnings for bow echo severe thunderstorms.

Now, bow echoes are very serious things. They tend to cause widespread wind damage, and the winds in them can reach upwards of 200 km/h, into F2 territory.

But if there's likely no tornado involved and no report of such, why do some offices issue such warnings? I just came back from a conference where they explained one reason, thereby confirming one of my hunches, but I have four.

First off, a lot of warnings are semi-automatically issued based on RADAR algorithms. While useful, I find these algorithms are, at best, hints that the meteorologist should look into something more closely. Bow echoes have a tendency to produce what's called a tornadic vortex signature, or TVS, on many US RADARs. The reason is this: the TVS, among other things, looks for a certain amount of pixel to pixel (aka gate to gate) wind shear or difference in the storm-relative velocity output. In other words, it looks for strong storm-relative winds toward a storm, along with strong storm-relative winds away from the storm. Or, alternatively, it looks for a large difference between the winds toward and away from a storm. And this is where the trouble comes in with the algorithm.

An example of this phenomenon is at this page, which shows the storm-relative "rotation" in the RADAR loops. Now, for the record, I don't recall and can't figure out how to find out if a tornado warning was issued for this storm, but you get the idea.

My second hunch is that they issue tornado warnings for such storms based on the conceptual model of such a storm. In a bow echo, you often have a very narrow channel of strong winds. This, of course, induces strong rotation--especially if the surrounding air is relatively calm. To see this idea in action, all you have to do is swoosh your hand through calm water, and notice the eddies that form.

The third idea I have on why they issue tornado warnings for bow echoes is this: the winds can sometimes be so strong that they can cause tornado-like damage. Like I mentioned before, wind speeds can sometimes reach upwards of 200 km/h, so if your roof is ripped off, you don't care whether it was caused by a tornado or by straight-line winds--you just care about your now-unattached roof. (Although apparently, for some reason, insurance companies seem to care whether it was straight-line wind or a tornado, but I don't see why--in both cases, it was wind!) The problem with this idea, though, is that people become less convinced, then, that straight-line winds can cause such severe damage--hence the oft-heard exclamation "it must have been a tornado".

Finally, the fourth possible reason is that there's some reluctance to switch mindsets when you're in warning mode. Often a storm will start as a supercell, possibly producing tornadoes. One of the normal transitions of a supercell is for it to become a bow echo. When you've been issuing tornado warnings for a storm, it becomes difficult sometimes to recognize that it has made such a transition, despite all the RADAR signatures to the contrary. It's a psychological barrier, if you will.

I suppose when it comes right down to it, it doesn't too much matter what kind of warning gets issued--as long as a warning is issued.

After all, it's what we're there for.


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