Sunday, June 20, 2010

Supercell formation observation

I have seen a number of supercell initiations this year and it occurred  to me that there is a distinction between a pure supercell initiation and a cluster of cells conglomerate supercell initiation. What do I mean?

Earlier this year in OK and way back in Nov 2005, dryline or near boundary initiation of supercells follow what appears to be a very linear low level reflectivity storm structure. From there these storms take time to evolve classic supercell structure at least in the low levels. (mental note: is this shear-instability dependent?)

This other mode evolves differently. Multiple, close proximity storms initiate and seemingly "merge" or form a conglomerate. The pictures to the left illustrate this process. The southernmost storm in the cluster appears to dominate. The one tornado was reported around 2200 UTC.

It is odd, but it appears that "upscale" growth occurs via merging. The assumption I have been using without explicitly stating it, is that supercells appear to need to reach a critical size to become productive in terms of tornadoes. I wonder what the supercell size distribution is, but from what I have observed via radar, it can not be a wide distribution, and I wonder if the tornadic supercells fit neatly into a portion of the middle of that distribution. A certain size might be necessary to withstand shear, and may be dependent on the shear-instability relationship.

Another issue is how crazy it is to have all those storms to the north produce very little wind or hail reports while no more than 2 counties south you have a supercell. This is a weak example of that, but there are many scenarios where storms are in a similar environment, yet only a few random cells are rotating. Maybe this is size related, or more in line with Markowski and Richardson's work, in which locally the SRH is very different via different hodographs or storm motions or both.