Bare minimum thermodynamics

[Ethan Lang]

Hey all, one thing that I have been sort of researching lately is thermodynamics and what the sort of "bare minimum" is to support good supercells. I know that back a few years ago we got three tornadoes in Iowa off of about 300-700 j/kg of CAPE. I just curious as to what the bare minimum would be to support tornadic thunderstorms. What are your guys' thoughts?

 

[Jeff Duda]

There are no useful hard thresholds that distinguish between supercell environments and non-supercell environments with 100% accuracy. The definition of a supercell itself is elastic and it is difficult to measure the components of it. I recommend reading up on Chuck Doswell's essays and papers on the definition of a supercell for a deeper discussion. You can start here: http://www.cimms.ou.edu/~doswell/Conference_papers/SELS96/Supercell.html

Ultimately, you need CAPE > 0 J/kg, CIN not too high, and sufficient vertical wind shear and thermodynamic profile so that tornado-scale-and-magnitude horizontal vorticity can be generated and allowed to flourish. Again, there probably are no hard numbers that will fully answer your questions. You'd be best to think about it probabilistically. For example, if I see an environment with 500 J/kg CAPE, 30 kts of deep shear, and 7 kts of 0-1 km shear, then I'm going to base my chase decisions on an assumed very low conditional probability of seeing a tornado (say, <5%; the condition being whether storms actually form). On the other hand, if I see an environment with 3000 CAPE, 45 kts of deep shear, and 20 kts of 0-1 km shear, I'm going to assume the conditional probability of seeing a tornado is much higher (although still not 100%).

Scientifically that's about the best we can do right now. Beyond that, it's a matter of personal decision making and risk/reward analysis. "Am I willing to put in the effort to chase a setup where the ingredients are hardly there, but not totally missing?" "How many of these would I have to chase, on average, before actually seeing a tornado?" "Is it worth the effort?"

 

[Bob Schafer]

There is no simple answer. Instead of trying to start from that place, be aware that there must be a balance of many different parameters. To cite just a tiny amount of possibilities, you can have updrafts blown apart by too much shear when CAPE is 4000, or tornadoes occur with almost no instability at all but when a bunch of other things are perfect.

 

[Jonathan Beeson]

As others alluded to, there's no true "bare" minimum. Tornado formation isn't really a yes/no science, and we consistently see tornadoes form in situations that just defy common knowledge.

My personal favorite example is the activity over Northern Alabama during the early morning of February 6, 2008. We saw not one, but two long tracked supercells that produced 2 EF-4 tornadoes just before sunrise in an area of 250-300 J/KG of CAPE.

SBCAPE, while a nice tool, is somewhat overrated IMHO in the larger scheme of things. In low to moderate CAPE environments, I view lapse rates as the most important tool for judging a thermodynamic environment. When I'm chasing a dryline setup across Kansas in late May when there's 4,000 J/KG of SBCAPE, it's a little different. But in Dixie in February and March, I tend to look at lapse rates before looking at SBCAPE.

 

[Randy Jennings]

Funny you ask this... At this Saturday's TESSA National Storms Conference, one of the major points of Gary Woodall's presentation was that CAPE probably isn't a good predictor of which storms will become tornadic. Gary pointed out that things like 0-1 km shear are better indicators. That section of Gary's presentation was based on a number of papers including the Thompson, et. al. paper here: http://www.spc.noaa.gov/publications/thompson/ruc_waf.pdf .

Having said that, there is no magic values for any parameter. For starters, most of these parameters are not direct observations (unless you happen to be in an area where a weather balloon was just launched). I really like Jeff's way of looking at it as probabilities. When I chase, I often hand draw maps showing each of the ingredients and look for the union of the areas to find the location with the highest probability.

 

[Brandon Centeno]

Low CAPE, high shear events.. Tornadoes that have occurred with MLCAPE less than 500 j/kg.. You must have a balance. With such low shear, you will need strong forcing to help sustain the updrafts.

Definitely no bare minimum.. All in all there's a lot to consider. CAPE of 2k with a skinny profile (poor lapse rates) =/= CAPE of 2k with steep lapse rates (fat CAPE profile). One will have stronger accelerations. Stronger acceleration leads to stretching of pre-existing horizontal vortex tubes tilted into the vertical. Sufficient stretching will lead to the development of a supercell, which will then amplify that stretching.. You have a feedback system going on and so forth.

 

[Quincy Vagell]

Research has been done on this topic and it's fairly common for tornadoes to form via QLCS in high shear/low CAPE environments. Sometimes with very little cape at all. Research from Thompson et al. (2012) showed that 25% of EF0 tornadoes formed in QLCS segments with less than 200 J/kg MLCAPE. The minimum value was 17 J/kg. Even numerous strong tornadoes formed in similar environments. See the graph from the paper below:

Note that the median value for QLCS tornadoes is roughly 600 J/kg. The values for discrete cells producing tornadoes is much higher, but still indicates the occurrence of tornadoes with seemingly meager instability (~400 to 800 J/kg) is not all that uncommon.

http://www.spc.noaa.gov/publications/thompson/waf-env.pdf

 

[Brandon Centeno]

Highly technical, thorough yet probably more pertinent to the discussion:

http://www.spc.noaa.gov/publications/cohen/pbl-seus.pdf

Doesn't exactly answer your question, but sheds light on the topic via PhD work on southeast, cold-season environments which can yield tornadic supercells in pretty weakly unstable environments.

 

[Ethan Lang]

I too have had experience with weak CAPE days. I chased a day that the CAPE peaked somewhere around 700 j/kg and ended up seeing an ef-2 elephant trunk on a day that the SPC had a Marginal risk for. This day was what peaked my interest in this topic

 

[Alex Elmore]

At this Saturday's TESSA National Storms Conference, one of the major points of Gary Woodall's presentation was that CAPE probably isn't a good predictor of which storms will become tornadic.

There are many research articles that come to this conclusion as well. The general thought is that thermodynamic parameters do not perform well as kinematic parameters in distinguishing between tornadic and nontornadic environments. Although, CAPE can be useful for distinguishing between severe and nonsevere environments.

One thing to keep in mind with looking at parameters and their amount/distribution in environments in which a tornado did occur is how often similar environments are present but a tornado does not occur. This paper does a great job of displaying this problem:
https://journals.ametsoc.org/doi/abs/10.1175/WAF-D-16-0005.1

 

[TrumanBoyer]

Not a lot of math details here, but a minimal case for supercell tornadoes by Jon Davies.

google this: Midget Supercell Spawns Tornadoes

excellent photos here... Jon copyrights everything, or I'd just post them...

https://www.google.com/search?q=mid...TLt9rZAhVRjlkKHTRmBIEQsAQIOg&biw=1920&bih=966

He cites 200 J/kg in a 1993 article [ IF I REMEMBER RIGHT ]. I can't find it right now.

Happy Hunting.

 

[Marshall Stoner]

I think one advantage of low CAPE events is that they tend to be accompanied by very low LCLs. High boundary layer relative humidity means showers can't do much to generate a negatively buoyant cold pool. Since the limited instability that exists is being driven dynamically (cold advection aloft steeping the mid-level lapse rate) and any precip-generated cold pool is weak, supercells can remain inflow-dominant even when they're surrounded on several sides by extraneous clouds and precipitation. Low-topped supercells manage to maintain their surface-based inflow even when they're embedded in a large MCS with lots of rain around.

I'm thinking that the exact same type of kinematic environment with a hotter boundary layer and higher cape will cause the convection to quickly evolve into a squall line. This is because the strong forcing and lack of a cap makes convection too widespread. Combine widespread convection with a hot boundary layer you get a large strong cold pool. This leads to either a squall line or a cluster of non-tornadic elevated supercells. So.... paradoxically I can theoretically imagine a situation where higher CAPE is actually detrimental to tornado formation.

The high CAPE events everyone chases on the plains (classic loaded-gun events) tend to feature a cap that breaks along a specific boundary (dryline, outflow boundary, or stationary front). Cells manage to stay isolated because there's some inhibition present even after convection has initiated locally. I'm thinking the strong cap (and EML above it) is just as important as the magnitude of CAPE. I think tons of CAPE and a great hodograph will almost always lead to supercells and severe weather, but long-track tornado events need a bit more than just that. The cells have to stay isolated enough and not produce so much cold air that they become elevated or merge into a continuous MCS. As soon as supercells merge into a bowing line or become elevated the threat of a long-track tornado is over.

A recent simulation by Leigh Orf seems to indicate that all the air entering the tornado originated from the cold side of the storm. This air was none-the-less ingested into the mesocyclone and lofted to a high level. This was an extreme CAPE event, but interestingly, even the air originating in the cold pool had some remaining CAPE. It's only a simulation of a single event, but it kind of explains why a low-LCL / high-humidity boundary layer environment is helpful - given that the tornado is being driven by the ingestion of rain-cooled inflow. Further research in this area should be exciting - especially more case studies with more diverse and less conventionally ideal parameters.