Right and Left moving Supercells?

[Michael Auker]

What is the cause of splitting supercells? How can they move deviant to the deep-layer flow? And finally, why do left movers move more swiftly than right movers? Thanks

 

[Jeff Snyder]

The cause of supercell splitting lies in vorticity dynamics. The tilting and stretching of horizontal vorticity into the vertical yields a positive and negative vertical vorticity center on the south and north side of a supercell (given a wind profile characterized by easterly surface winds becoming, linearly, westerly and increasing in intensity with height). Buoyancy gradients along the edge of the updraft also play a role... The vertical pressure perturbation structure results in renewed development to the south of the cyclonic center and to the north of the anticyclonic center. Developing downdraft in the 'center' of the updraft, in concert with the outward (south/north) development leads to the 'splitting' of the single updraft into two discrete updrafts... This all depends on the wind profile (and more specifically, the wind SHEAR profile), however...

For a very good read, which explains this process in greater detail, I'll refer to a paper titled "Dynamics of Tornadic Thunderstorms" by Joseph Klemp (1987)... This publication explains the deviant propagation/movement of splitting supercells. It can be accessed at http://twister.ou.edu/MM2005/Klemp87Review.pdf ... It's a bit of science, but you can skip the equations and just read the explanations/descriptions and still get a good handle on the situation...

For those that are curious, you can find other good lectures regarding supercells and tornado dynamics (e.g. how helicity aids thunderstorm rotation, how rotation in an updraft enhances the updraft well beyond the effects possible with buoyancy alone, etc) by just going to http://twister.ou.edu/MM2005/

 

[Sarah Berling]

So here's a stupid question. What do you mean by "left moving" or "right moving"? I've heard the phrase: "The cell is moving right" but I don't know what it means. Help?

Sarah

 

[Chris Allington]

it means its moving against the surface flow of the day...or more to the right of it... for example on a day with southerly winds the storms are moving to the north...so a right turner would be moving to the NE or maybe even ENE... this greatly increases the low level shear...making the storm more favorable to produce tornadoes...

 

[Jeff Snyder]

Chris is correct, as long as one realizes that he means right of the MEAN wind, not necessarily the surface wind. Given a "typical" (pff, what's that?) southwesterly flow aloft setup over the plains, the mean wind may indeed be to the northeast. A "right-mover" denotes a storm which has turn right of the mean wind (as Chris noted), often by 20-30 degrees, though sometimes signficantly more. Cyclonic supercells also tend to move slower than the mean wind (while left-moves tend to move left AND faster than the mean wind). For many, the term "30R75" may ring a bell -- "30 degrees right and 75% of the mean wind". Different storms may not obey this rule-of-thumb, however! Low-topped or mini-supercells tend to be less developed in the vertical (thus the term low-topped LOL), and thus the "steering wind" (so to say) for those storms may be the 850-700mb layer), while more classic supercells that extend to the tropopause may be most heavily influence by the 700-400mb mean wind. Regardless, this kind of get muddied up with supercells develop strong pressure perturbation gradients, which is largely the cause of the deviant motion to begin with.

There's are a few good publications on supercell motion, some of which are on the SPC Publications list... There is also a quick powerpoint presentation by Bunkers regarding supercell motion (the Bunkers motion/technique), which can be viewed at http://www.crh.noaa.gov/unr/edusafe/scm/SCmotion.ppt .

 

[Chris Allington]

yes i forgot about the mean wind fields and all of that... thank you for correcting me... hopefully we will all see some good storms up here in NE tomorrow...

Chris