Rear flank downdraft

[Joel Wright]

I'm a little bit mystified as to exactly what causes the RFD and why it's only prevelent in supercells. I know it's important to tornadogenesis. I'm more interested in how it differs from the main downdraft in the storm itself. In a supercell, is there both an RFD and an additional downdraft region in the core of the cell?

I would also like to see an overhead overview of where the RFD is relative to other storm features if possible.

Thanks for any info. :wink:

 

[Jason Boggs]

Here are a few links that might help you out...

http://www.weatherpresentations.com/RFD/in...v3_document.htm

http://www.geosciences.unl.edu/~ekuchera/p.../papers/RFD.htm

http://saladosweathercenter.com/downdrafts.html

 

[Glen Romine]

While I didn't read the middle link in its entirety, I'll advise against taking the first and third links as being very accurate. There are a few concept slides in the first link that are ok, but the study is poorly defined/described, so the results may be suspect. The ideas in the third link are simply not correct based on the current knowledge about RFDs.

Those with access to AMS journals should read the Markowski 2002 paper.

http://ams.allenpress.com/amsonline/?reque...ue=07&page=1692

Not everything there is 100% agreed upon - but is an excellent review regardless.

Here is some nice information about 'special' RFDs from Erik:

http://cimms.ou.edu/~erik/Tornadoes/Foreca...ed/Detailed.htm

What you'll have a hard time finding is agreement on what causes the RFD - or even if there is a real distinction between the RFD and the FFD and the occlusion downdraft. It seems that the closer one looks the more complicated the picture gets, take the 'blob' theory of Erik Rasmussen for instance. In general though, the FFD is embedded within rain - so the thermodynamic profile is likely saturated - leaving precipitation drag as the most likely forcing for the downdraft in the main rain core. With the RFD - you have precipitation mixing with (typically) drier environmental air particularly below the cloud base - and this leads to evaporational cooling in addition to the precipitation drag and can act to really accelerate the sinking of air. The height where this mixing and sinking begins is not agreed upon, and vertical pressure gradients associated with the storm's rotation also appear to at least in part contribute to some of the observed sinking on the back side of the storm. There is still much active research into supercell downdrafts - so stay tuned.....

Glen

 

[Jason Boggs]

Yea, I really didn't have time to read any of the links. There is so much data on the internet you never can be too sure about what to believe and what not to believe.

 

[Mike Hollingshead]

I would think every storm would have an RFD to some degree. If you move your hand upward air would have to curl and sink all around it, right? Especially if there is a lid or solid object above your hand. Say you have 40 knot winds out of the west at 500mb before and during convection. Before convection it is flowing freely at that speed. If you put a tower up in front of it that ends up moving at say 20 knots won't you get a pressure buildup on the upstream side? That wind is no longer able to flow so freely horizontally. Even if it is spreading around the updraft there would still be an added pressure in the middle of the updraft. At some point that middle point starts eroding and the first signs of an rfd cut form. Once that cup is there less will be able to flow around and the pressure would go up making the cut larger....more of a cup. If you evaporate a liguid into a gas you take up more volume right? Part of the updraft is condensing gas into a liquid(rain) and I bet on the upstream side where the dry air is encountering the updraft it is adding more volume and pressue on that side due to evaporation(dry air mixing with the moisture in the updraft). So adding more volume on that side from evaporation along with the given shear pressure building would leave only one way for this air to be going, especially with any cut forming(a lot is no longer able to flow around). Also, if you compress air it heats...warm RFD? Then that RFD only acts like a bigger scoop giving you more updraft/moisture.....giving you more to evaporate..giving you more RFD. I often think low level shear is less important than the moisture and temp differences from sfc to some mid-level height. I think they'd only be imporant in that you often will have good low level shear when you get that large moisture and temp difference as you go up(dryness being more important than temp difference). What I mean is it is sort of hard to get dry air(I'll say very dry) over very moist air without having shear in place in most cases. I think that dry air aloft is more important than the low level shear....but I have absolutely nothing to support anything. Ok, I'll stop talking out my buttocks.

 

[Glen Romine]

Vertical pressure gradient fields of supercells get very complex, particularly for non-ideal shear profiles. There are both linear and nonlinear forcings - and it is tough to make a general model so often special case scenarios are developed (e.g., straight-line, quarter turn, half circle and full circle hodographs). Mike is drifting around the linear forcing side - which as previously discussed you can get a buildup of air on the upshear side of the storm and subsequent relative high pressure aloft (so linear forcing is interaction between the updraft and the environmental shear) in the case of a straightline hodograph. More typically, the hodograph veers with height, which rotates the high and low couplet aloft clockwise (with the 'low' now on the south side, this favors rightward storm propagation). The problem with the RFD arising from the downward perturbation pressure gradient is that if air is forced to the surface from mid-levels - it would be hot and dry by the time it made it to the ground. What is observed, however, among the storms favorable for strong and long-lasting tornadoes is that the RFD is warm and moist, and more typically cold and moist in most nontornadic supercells. So, this makes the paradigm of air sinking from mid-levels inconsistent. What appears more likely is that the colder the air in the RFD, the higher it's origins with cooling largely from evaporation of rain, and warm RFD's are probably originating from a much shallower height.

Glen

 

[Joel Wright]

Thanks guys for your imput!