Why Do Some Supercells Produce Tornadoes While Others Don't?

[nickgrillo]

How come some supercells produce tornadoes, while other's don't? All supercells rotate...so how come some supercells don't produce tornadoes?

 

[Andy Wehrle]

Nick, your question is just about the last huge mystery in tornado forecasting. If we knew that, tornado warnings would be far more accurate.

Some basic ideas behind it may include:
Some mesos may be undercut by outflow before they can produce (Doppler radar doesn't scan low enough to see this so the NWS still tornado-warns these storms).

Elevated supercells (those whose updrafts are not rooted near the surface because there is not enough instability available there) can be prolific hail producers but are not likely to develop tornadoes.

And sometimes you will have a seemingly perfect supercell-surface based, ideal balance between inflow and outflow, "flying eagle" echo on radar, wall cloud spinning at dizzying speeds...and still a tornado doesn't happen. The general theory is that there is insufficent stretching of the low level vorticy...but the actual mechanisms behind this are largely a mystery.

 

[Shane Adams]

One current theory that's quite popular is the warm RFD theory, which suggests that only warm/moist RFDs will produce a tornado. I think that may have something to do with it, but I don't feel comfortable automatically dismissing an RFD's tornado potential because it isn't tropical; some degree of cooling is neccesary to make it what it is, which is a downrush of cooler (relative to the surface inflow) air that sometimes wraps into the circulation.

I've always believed the secret to tornado or not lies more at the surface than upstairs. I think it's got more to do with LL helicity; the placement of boundaries and how they interact with storm motion. One thing I think everyone would agree on is there's no set order of events for tornado genesis. I think it's been proven that TG can occur through multiple means.

 

[Jeff Snyder]

Some basic ideas behind it may include:
Some mesos may be undercut by outflow before they can produce (Doppler radar doesn't scan low enough to see this so the NWS still tornado-warns these storms).

Interestingly enough, Markowski et al 2002 found, through an examination of VORTEX RFD sampling cases, that nearly all of the tornadoes occurred AFTER the mesocyclone became completely occluded: " "

I agree that it is becoming increasingly clear that the thermodynamics of the RFD play at least a signficant role in tornadogenesis or tornado failure. In fact, in-field subjective observations of 'warm' RFDs near tornadic supercells (Garret and Rockney 1962, Williams 1963, Fujita et al. 1977, to name a few) have been around for a while. More recently, PROJECT ANSWERS 2003 (Grzych et al. 2004, Lee et al. 2004, and others) found similar results as Markowski et al. 2002, in that there was a tendency for tornadic supercell RFDs to be "warmer" than non-tornadic supercells RFDs... By "warmer", I mean less of an equivalent potential temperature or potential virtual temperature deficit (RFD - inflow).

At any rate, my chasing experiences largely agree with findings from VORTEX and ANSWERS 2003, in that the torandic supercells I've chased tended to be "warm" relative to the inflow (5-12-04 Attica, 6-12-04 Mulvane, 7-14-03 Lake Crystal MN, etc), while those that were only weakly tornadic, as in a brief tornado or two, tended to have 'colder' RFDs (3-27-04 being a nice example).

I think the "secret" lies in both RFD thermodynamics as well as near-surface (0-1 km) shear profiles. Then again, I suspect that the two are not entirely independent of each other... Unfortunately, the operational use of RFD thermodynamics is unlikely given the pourous surface observation network across the Plains, while, in regards to near-surface wind profiles, we don't really have a way of measuring high-resolution (spatial and temporal) wind profiles near the surface... I mean, it seems that the best we have is the profiler network, which only gives a vertical resolution of 1 km every 6-minutes... We need 100 meters every 1 minute, I believe, given the oft-changing low-level shear profiles observed near supercells.

 

[Kevin Scharfenberg]

It's important to point out that the warm RFD theory refers to "strongly tornadic" storms (i.e., long-lasting or strong tornadoes ). It has been found that weak/brief tornadoes can spin up from mild or cold RFDs.

Because we don't have thermometers on a 1 km grid horizontally and a 500 m grid vertically throughout the plains, we'll have to learn what large-scale conditions favor warm RFDs. I agree about the importance of strong 0-1 km shear in getting a strongly tornadic storm. I'd also throw in having strong instability in the 0-1 km layer.

 

[Joe Nield]

Originally posted by Jeff Snyder
Unfortunately, the operational use of RFD thermodynamics is unlikely

It is possible that there are dual-pol radar signatures unique to warm RFDs. According to a discussion I had with Paul Markowski via email after a teletraining session last year, there are already people working on this.

Now we just need to start deploying dual-pol in the field. :lol:

 

[Mike Smith]

Good news. Dual-pol radars ARE being deployed in the field. WHNT-TV in Huntsville has deployed a dual polarization radar -- the first operational dual-pol radar in the United States.

Based on things I have heard, there will likely be additional dual-pol deployments by TV stations and other private sector organizations in the coming year or two.