05/23/07 DISC: Plains

[rdale]

I'm wondering if it's the lack of huge (65k+) tops.

With tropopause in the 42-46000ft range over most of that area, it would be an Earth-shattering event to have storms that high up... I have a hard time believing there is a correlation between excessive storm height and wedge formation.

 

[Angie Norris]

Per Warren's question in the now thread about the lack of tornadoes yesterday,my semi-educated guess at to the lack of tornadoes in the TX panhandle yesterday was the fact that the inflow into those storms was cold. I don't remember the exact temperature spread or mechanism, but the basic concept is that to enhance tornadogenesis, the inflow into the storm must be warm. Anyone with more background here, please expand. If I can find those workshop notes, I'll try to add more later.

 

[rdale]

That's from Markowski -- the "warm RFD theory" which seems to hold quite a bit of merit. Some info in the Archive:

http://www.stormtrack.org/forum/archive/index.php/t-2459.html

 

[Angie Norris]

That's it...thanks!!!

 

[Warren Faidley]

Thanks. That makes total sense. We noticed cool inflow during the wall cloud event over Stinnett.

Warren

 

[Brian Stertz]

Yep that warm/moist RFD theory is very well grounded on my chase experiences. It seems that when that RFD is coolish...the tornadoes are brief and not the stronger types in most cases (sometimes on the High Plains you get different results though). This was cool RFD issue occurred Tuesday on that Hill City tornado. That RFD was coolish and the tornado cycle was under 10 mins. and the tornado roped out quickly. There are obviously alot more storm dynamics going on to drop a tornado, but I have really latched on to that warm/moist RFD theory....especially for the longer tracked/stronger tornadoes....and the cyclical tornado "families".

 

[Angie Norris]

Yesterday was the third time I've noticed it. First time was a storm in SW OK on 4-1-06, 2nd time was Monday afternoon/evening in the same general area as yesterday's event.

 

[Doug Lee]

The warm RFD / tornadogenesis link makes sense because warm air is more energenic than cold air, and warm moist air is more bouyant than warm dry air. If the RFD is warm and moist, it contributes to the strength of the inflow and to the strength of the updraft. A cold RFD would have the opposite effect.

Because cold air has a tendency to sink, how can an RFD become warm and moist? My guess is that all RFD's start as cold, dry air out of the top of the updraft. If the RFD encounters moist, midlevel winds on the way down, it gains moisture and experiences compressional warming. Inflow winds at the surface probably add to the warming and moisture.

I'm not a professional meteorologist, just a wannabe who trying to apply some of the things I learnd in physics courses in community college. So, my hypothesis is definitely subject to review.

 

[John Hudson]

There was an astonishingly (from my viewpoint, at least) narrow margin of temperature between the moist inflow temperature and the RFD temperature on the Greensburg supercell.

Wasn't it only 1 degree C? The small difference really surprised me.

John
VE4 JTH

 

[rdale]

The warm RFD / tornadogenesis link makes sense because warm air is more energenic than cold air, and warm moist air is more bouyant than warm dry air. If the RFD is warm and moist, it contributes to the strength of the inflow and to the strength of the updraft. A cold RFD would have the opposite effect.

Half there... The warm RFD is more bouyant, helping to stretch the vorticity aloft. And it's not that it is a "warm" RFD, but "warmer than a cool" RFD. Likely due to higher dew points being ingested into the storm. I highly recommend reading Markowski's paper on it.

 

[Bob Schafer]

Per Warren's question in the now thread about the lack of tornadoes yesterday,my semi-educated guess at to the lack of tornadoes in the TX panhandle yesterday was the fact that the inflow into those storms was cold. I don't remember the exact temperature spread or mechanism, but the basic concept is that to enhance tornadogenesis, the inflow into the storm must be warm.

A while back, I suggested that slow storm motions can hinder tornadogenesis (not always). I had reservations about yesterday's tornado potential for this very reason, as well as the weakness of the sfc L.

My theory is that a storm's updraft can essentially get choked off as the result of feeding from a "stale" PBL. Nothing to do with the PBL's thermodynamics, but from the updraft feeding from a PBL that is too "familiar". i.e., the sfc winds are too similar to the storm motion, and the updraft creates a low pressure area all around the storm base. Perhaps a couple of mb, perhaps more. Not just under the RFB's, but some distance from them as well. This L surrounding the base may extend outward for several km, IMO. The antidote is a greater difference between storm motion and sfc wind direction and/or stronger sfc wind speeds.

Consider yesterday's reports of wicked inflow. IIRC, sfc data was indicating S or SSE sfc winds at ~10-15kts (mostly), which was nothing like the inflow. Obviously, there's nothing remarkable about elevated sfc wind speeds from inflow, but I'd love to be able to see the pressure data for a grid of points spaced 0.1km apart, total size, say, 25 square km, and taken at 5 minute intervals, surrounding yesterday's storms in the NE TX PH. I'd love to know how much the pressure dropped invof the updraft regions. I'd also love to see what kind of pressure waves/sloshing occurred around the storms.

Obviously there's nothing "wrong" with S or SSE sfc winds and NE storm motion, but I theorize that when the sfc winds and storm motions are both about 10-20kts with such vectors, then the storms will struggle as those "base environment lows" ("BEL") realize some undefined critical size and depth relative to ambient parameters. What other effects could be seen from sloshing within the BEL? A storm "wants" a smooth, high pressure inflow source. We know that we rarely see tornadoes atop bizarre orographics (mountains, even low ones, etc.). Well, same thing with perturbations in the updrafts' source.

There's sooo much analysis of what's happening within storms. How much research is there about the PBL environment surrounding the storms?

Another thing was the wimpy sfc L. Same thing that ruined 2006. I was chasing SE and E of 1008mb sfc L's all 2006, and had no surprise that sfc winds were too weak to fuel producers.

Go ahead and tell me that A) I am stating the obvious to the mets, and/or I'm out of my mind.

I'm also thinking the reason for the perception of cold RFD's was that the storms were riding the WF, and drawing too much cold, dry air from their backsides. Especially given the weak warm sector winds.

 

[Jim Tang]

I just think the upper level winds were a tad too weak, leading to storms that were a little outflow dominant. The fact that storms tended to cluster together along the front/boundary didn't help things either.

 

[Jeff Snyder]

Because cold air has a tendency to sink, how can an RFD become warm and moist? My guess is that all RFD's start as cold, dry air out of the top of the updraft. If the RFD encounters moist, midlevel winds on the way down, it gains moisture and experiences compressional warming. Inflow winds at the surface probably add to the warming and moisture.

The origin of the RFD "air" or source region is, as far as I know, not known at this time. Regardless of the layer at which most of the RFD air originates, compressional warming will occur as it descends, with the air warming by the dry adiabatic lapse rate. Now, there is a complicating matter -- precipitation. Evaporational cooling within the RFD may lead to a reduction of T and increase of Td as wetbulbing occurs. As Rdale noted, it seems that the warmER (not necessarily warm) RFDs tend to be associated with tornadic supercells, while the coldER RFDs tend to be characteristic of nontornadic supercells. The equivalent potential temperature or virtual temperature of the RFD may be a couple of degrees above or below the ambient values and still be considered "warm".

It's also worth remembering that the parcels rising beneath and through cloudbase may be colder (in a virtual temperature sense) than the ambient environment! In such cases, the buoyancy is negative (e.g. the parcels would, in the absense of any other force, accelerate downward), which occurs whenever a parcel rises through a CINH or capping layer. Fortunately for chasers and storm enthusiasts, buoyancy is only a part of the story -- vertical perturbation pressure gradients exist in supercells that provide an additional, significant force for upward acceleration. From what we know about supercells, buoyancy (e.g. CAPE) may only only be the cause of ~50% of the vertical accelerations in updrafts, with VPPGs providing the other ~50%. For this reason, a supercell in an environment of 2000 j/kg CAPE may actually be more intense (in terms of updraft velocity) than a non-supercell in an environment of 3000 j/kg CAPE.

From experience in RFDs in the past 9 years of chasing, I have noted a tendency for tornadic supercells to possess relatively warm RFDs, with the nontornadic supercells usually having relatively cold RFDs.

 

[David Wolfson]

My comment from frustration several times about the setups the last few weeks was, "It would be nice sometime if the upper winds were stronger than the lower levels...." Strong RFDs/quality clear slots were also not present at least that I saw or have been reported to match the otherwise strong dynamics at Lipscomb, St. Peter, etc.

I think Jeff sums up what I as a non-expert have read about the state of knowledge on the RFD - tornado connection, which is to say, there's a lot still to be learned. So I'll presume to throw in a thought FWIW.

I'm quite sure I've read that part of the complicated hydrodynamics of the interaction between a supercell and its environment includes what I'd call a "rock-in-the-stream" component. The updraft reaches to the tropause (and even above, i.e. the often observed pre-tornadic overshoot). Faster upper winds are forced to go around the rotating updraft core and the dynamics of this interaction contributes to RFD components descending from high levels. The mass in the updraft is unable to rise further by the tropause, and so must spread horizontally and down. These downward components originating from high altitudes have relatively high potential temperature and are warm when transported to ground.

So I'd like to posit that the relatively uniform distribution of wind speed with height would not exhibit a "rock in the stream" dynamic, and that this component would be notably absent from the RFDs in these recent storms.