RFD Inititation and Cap formation

[Abe Holland]

Hello All,

I have a couple of questions pertinent to supercell growth and dynamics. First i would like to ask what the mechanism drives RFD initiation. I understand that the downdraft is initiated as parcels within the storm become negatively buoyant. My question is, what causes this downdraft to descend about the rear of the storm in the form of RFD? Does the mesocyclone draw a portion of the FFD into the rear of the storm? If so, do the FFD and RFD initiate at simmilar times (it was always my belief that the FFD is well established before RFD initiation)? Lastly, what mechanism induces the formation of the cap? Does warm air from the previous day's surface heating rise to a certain level to establish it until it is eroded?

Thank You For Any Feedback,

Abe H.

 

[Chip Redmond]

There are two primary theories or methods of the RFD development. The first is the buoyancy method you touched one. Dry air is advected towards the storm from the upshear direction in the mid-levels. This runs into the updraft and interacts with the rear facing part of the storm. This develops a region of evaporation that cools the parcels on that side of the updraft making them negatively buoyant and thus sinking. When it sinks it is subject to the shear in the levels below it and therefore wraps around the updraft.

The second method is that of pressure perturbation on the upshear side of the updraft. The updraft is treated as a wall almost that the mid-level winds run into. Therefore, as the wind impacts this, it stops its motion in the downshear direction and is directed outward around the updraft. This is often called "splat" and will create a region of high pressure on the rearward side of the updraft. This coupled with surface low pressure will create a pressure gradient and flow from the upper levels to the surface (aka, downdraft). It gets wrapped around the updraft with the shear in lower levels also. This theory seems to be more influential because it doesn't cool the air and the strongest tornadoes are often associated with warmer RFD's. It will pick up precipitation on its descent and therefore can be wet. Although, it is also thought it could be a mixture of the two theories as well.

Cap is usually formed by warm air advection in the mid-levels that will prevent a parcels ascent to the LFC.

Chip

 

[Jeff Duda]

Chip did a really good job explaining two theories of RFD development. Just know that the causes of its birth are not known for sure at this time. This remains a pressing question regarding tornadogenesis and supercell dynamics research (at least, as of the time I took a course that covered this in 2008...maybe a paper came out since then clearing things up).

 

[Dave Kaplow]

In my opinion the origins of the RFD is probably the most interesting unsolved problem involving supercells. I suspect that we will need to fully understand RFDs before we can finally solve the puzzle of tornadogenesis. It's kind of mind boggling that even today we can't say for sure how and why an RFD forms, despite all the data collected in the field (VORTEX I & II, etc.) and all the detailed supercomputer simulations of supercells. I'm no scientist, but this is a subject that has always fascinated me. I know there were a couple of old ST threads on this topic that you might want to look at, but you won't find any definitive answers there or anywhere else that I'm aware of. RFDs remain something of a mystery.

 

[Jeff Duda]

In my opinion the origins of the RFD is probably the most interesting unsolved problem involving supercells. I suspect that we will need to fully understand RFDs before we can finally solve the puzzle of tornadogenesis. It's kind of mind boggling that even today we can't say for sure how and why an RFD forms, despite all the data collected in the field (VORTEX I & II, etc.) and all the detailed supercomputer simulations of supercells.

I can't say I'm surprised that RFDgenesis is not 100% known. Think about it, how do you get data from in and around an RFD? It's generally not safe for planes to fly in there, and it's difficult for people to drive into the area and launch a balloon (and that balloon probably wouldn't make it into the RFD given the intense downward motion). The only observations are from the surface, and scientists already know that RFDs aren't generated at 2 meters. The computer simulations are probably helpful in supporting some theories, but observations are needed to confirm or deny those theories.

 

[Michael Towers]

Lastly, what mechanism induces the formation of the cap?

Multiple processes that can create a cap are described here:

http://www.theweatherprediction.com/habyhints2/405/

If you’re not familiar with the site I’d recommend checking it out as you may find it a useful educational resource.