Hi John,
I note that the discussion which is a good one has diverged somewhat from the initial intent - just so other readers can follow. And although we will attempt to discuss factors known to influence tornadogenesis, we should take Chuck Doswell's advice that we are veering further away from understanding the full dymamics associated with tornadogenesis the more we peer into it! I would like to note that this discussion is particularly related to my favourites – warm season supercells (let cold season tornado climatologies be discussed separately).
Tornadic supercells be it in Australia or Tornado Alley are atmospheric accidents. I think it is the frequency that varies significantly between the two countries (that is a point of discussion in another topic). So what factors influence the differences between supercells becoming tornadic in Tornado Alley as compared to Australia?
You mention lapse rates - I think this is one of the key common denominators that signifies an extremely unstable atmosphere. Based on purely model data, there is often significant difference in lapse rate values during supercell outbreaks sometimes painted as potential tornadic outbreaks in Australia as compared to tornadic outbreaks in the United States. Looking historically to any of the violent tornadoes in Australia, you do get the impression there is that common denominator - steep lapse rates associated with short waves and sometimes cyclogensis.
But just like the original question re CAPE in absence of windshear, trying to distinguish other factors is dangerous. But I guess comparing the occurrence or absence of such factors can perhaps try explaining the difference in climatologies between Australia and Tornado Alley.
You mention wind shear - I think this is a significant difference between the development of sculptured and organised supercells including tornadic supercells. In my chases across the various regions of Australia, rarely have I had the opportunity to experience strong low level inflow. I could count such occurrences on one hand! Given the opportunities of most seasons in Tornado Alley, the converse is true.
But low level flow is, except in extreme circumstances, is pointless in the absence of moisture. The source of strong inflow is the normally rich moisture from the warm Gulf of Mexico. In Australia the source of our strong inflow - dry desert air - potentially great for bushfires rather than supercells and tornadoes.
Lots not forget the importance of negative tilt shortwaves however subtle in association with destabilsation discussed above and its importance in initiation and explosive development of supercells. I find the absence of these encourages incorrect wind profiles and alignment of supercells.
That leads to yet another factor - the cap. Landform features in the Tornado Alley region as Chuck Doswell suggests is an atmospheric laboratory. Westward rising landmass and the elevated desert play an important role in creating a cap of warm dry air overriding and mixing with moisture along the dryline. Except in early spring outbreaks, this dryline feature virtually remains a permanent feature along the New Mexico and Texas border. Important to note, the Rocky Mountains play an important role in enhancing multiple pulses of short waves ejecting across the plains. So whilst the cap suppresses energy, eventually one of these features will help release it by weakening the cap (lowering the CIN).
Each factor above play their role collectively enhancing the development of tornadic supercells or at least the potential. But we have only touched the surface here. In the process of tornadogenesis, I doubt we know the answer yet. Much has been discussed in relation to the role of rear flank downdraft and inflow interaction, and the temperature of the RFD influencing tornadogenesis.
In summary, the problem exists: where and when will we see supercells? When and where will supercells become tornadic? In terms of the factors mentioned in this discussion, I doubt they are mutually exclusive.
Regards,
Jimmy Deguara
