Australian Severe Weather Forum

Going back to the occluded supercells do they also get known as cumulonimbus that develop an anvil and the anvil separates from the main storm and repeats a few times?

storm boy

The occlusion process refers to the gradual separation of an updraft no anvil. As a tornado or mesocyclone wraps rain, it gradually cuts off the warm moist air supply and gradually the updraft shrivels from the mature storm. With cyclic supercells, this process can occur several times as new updrafts dominate and other updrafts separate from the main storm. (I guess this makes supercells a family of the multicell family in some respects although the dynamics and process taking place are different).

Perhaps search the occlusion process.

Regards,

Jimmy Deguara

Ok thank you for the information Jimmy. By the way what does it mean when you do look at a developing cell and the cumulonimbus or anvil decides to disconnect from the main cell?"

Stormboy,

Storms in their natural process go through stages - the developing stage, the mature stage and the dying stage. A supercell's dynamics is fairly complicated to explain in detail with the lack of time I have because a lot of favourable conditions must come together. I would suggest do a lot of reading on supercells right around the internet and brush up before even attempting to fully comprehend what what processes lead to cut off updrafts and tornadogenesis as well as cyclic mesocyclonic behaviour.

Generally though, it is believed all storms have more than one updraft and they are in different stages of development. In supercells, the most mature stage will have the strong tornadoes and these go through the rope out stage when the updraft occludes or goes through the stage of being cut off from the rest of the storm and eventually shrivels up.

As a case study, take for instance the event on May 12 2004. David Croan and I were top the east of the supercell that exploded and began to produce tornado after tornado - we captured 5 tornadoes including spinups. Three of those tornadoes were quite strong and you can see the sequence in these set of photographs:

http://www.australiasevereweather.com/photography/photos/2004/jd20040512.html

Just for an illustration of what an occlusion looks like, check out tornado 1. You can see the base is ragged and the inflow cut is beginning to separate the tornado and its attached base from the rest of the storm above us.









Also check out the longer lasting tornado 2:















Even though the occlusion process is well under way by this last photgraph of this tornado, this is a better example where you can see the tornado is losing vertical tilt and becoming narrow. It roped out not long after this photograph. Also again you see a higher base and the cut off occurring.



Regards,

Jimmy Deguara

Those are some nice shot of a tornado above Jimmy. Are they recent? And also are you still chasing in America? Also thank you for the brief description above. I will browse the web and look further into the definitions of the super cells as well as development stages.

Storm boy.

Thanks for taking the time to explain occlusion, JD.  It's something I've neglected in my own chasing experiences - when it occurs for my purposes - and really no-one has explained it to me in other avenues as well as you have.  One always hears of storms occluding or an occluding front so it gives a better insight into how the dynamics work.  Kudos to you.

Dragging up an older part of this thread, regarding conditions resulting in thunderstorms, I now have access to the BOM forecast decision tree for thunderstorm development and threat of severe events if anyone is interested. I also have just finished deriving a program which determines a MLCAPE (Mixed layer of lowest 50hPa of the atmosphere), CIN, temperature, pressure of the LCL, Equilibrium Levels, Shears, Convective covariates from specified model/reanalysis data. If anyone would like help deriving any of these I would be happy to assist where possible.

Hi, John.  I'm def interested in the tree as per thunderstorm development....what can you offer by way of info?

John,

Is this the thunderstorm forecasting decision tree by John Colquhoun?

Regards,

Jimmy Deguara