what's a persisting updraft?

[Menno van der Haven]

In most definitions of a supercell there's talk about a persisting rotating updraft.

For example in the Storm Spotters Glossary:
*Supercell - A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes, extremely large hail and damaging straight-line winds. They frequently travel to the right of the main environmental winds (i.e., they are right movers). Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm (Fig. 7). Storms exhibiting these characteristics often are called classic supercells; however HP storms (Fig. 3) and LP storms (Fig. 5) also are supercell varieties.

How long is persistent?
I guess that it must be long enough in comparison to the thunderstorm's life cycle to be persisent, to rule out short lived rotating (wannabee) wallclouds..... So, half an hour is persistent enough?
What do you understand under persistent?

 

[Jeff Miller]

Definition for persistent:

"1: existing for a long or longer than usual time or continuously: as a: retained beyond the usual period <a persistent leaf> b: continuing without change in function or structure <persistent gills> c: effective in the open for an appreciable time usually through slow volatilizing <mustard gas is persistent> d: degraded only slowly by the environment <persistent pesticides> e: remaining infective for a relatively long time in a vector after an initial period of incubation <persistent viruses>"

In this case, I would term a persistent upgraft in this way: It must be steady state, unchanged or strengthened by it's environment, and not showing signs of collapse or degredation for a period of time greater than one hour. This is purely my own definition.

A Persistent updraft is almost always a supercellular, singular, rotating updraft because the same conditions that allow for the supercellular structure are the same conditions that allow it to be persistent. The reason being, of course, that the tilted (due to shear) updraft is not allowed to collapse on itself, the inflow region is usually unimpeded, and the bouyant rear flank downdraft actually strengthens and/or tightens it and causes it to become more persistent under the right conditions. Multicellular storms are not "persistent" in a singular sense, but may have many short lived updrafts that persistently generate in a broad area. Single cell "pulse" storms are not by any means persistant as their vertical orientation usually causes it to be choked within a half hour to an hour due to the cooling downdraft in the single cell's dissipating stage.

Some persistent updrafts have lasted as little as an hour, some have lasted over 8 hours in the right conditions. I don't feel there's any hard core math to it, I feel radar and visual observations allow an intelligent observer to make a call as to wether or not an updraft is "persistent". Of course, one of the best signs of a persistent updraft is a persistent lowering and/or wall cloud. A rotating, persistent lowering/wall cloud is a great indicator of an impending tornado - but not always.

 

[Dave Kaplow]

Doswell's definition of a supercell, which seems as good as any, contains the following:

a supercell is a convective storm that possesses a deep, persistent mesocyclone. By "deep" I mean that the circulation meeting mesocyclone criteria is present and vertically connected through a significant (say, 1/3) fraction of the depth of the convective storm. By "persistent" I mean in comparison to a convective time scale defined by the time it takes a parcel to rise from the base of the updraft to its top (on the order of 10-20 min).