Question of Terminology: Inflow Winds Vs. RFD?

[Bob Hartig]

I need to get a bit of terminology accurate. I figured a few knowledgeable heads on ST could help me resolve the following question:

Can the RFD be considered a type of inflow wind? Or is the definition of inflow limited to non-RFD winds--i.e. the backed winds that flow in from ahead of the updraft base?

Seems to me that as long as a wind is rushing in toward the updraft/tornado, it's a form of inflow wind, no matter it's origin or direction.

 

[Scott Weberpal]

RFD - Rear Flank Downdraft is a downdraft originating on the rear flank of the storm and often helps to "wrap up" a mesocyclone. Any downdraft is outflow, not inflow. The RFD "airmass" can be ingested back into the storm, but at that point it's no longer considered RFD, it's simply inflow.

 

[Dick McGowan]

Here's a quick illustration I made. This is looking W/NW

Gene Rhoden has a nice example of a tornado where you can see the RFD cutting in on a strong tornado.

 

[Bob Hartig]

Thanks, guys. The reason I asked is because my understanding is that, in tornadic storms, the rear flank downdraft tends to be relatively warm, and to flow in toward the updraft base once it hits the ground. I believe it was Project Vortex that established this action on the part of the RFD, and differentiated between colder and warmer RFDs and their behavior.

In the field, directly after a tornado had passed in close proximity, I noticed winds flowing toward the storm from the backside. I assumed those winds had the RFD as their origin. That's the kind of thing I'm talking about.

Maybe I'm splitting hairs. I guess if it's flowing in toward the storm, it's inflow, no matter where it's coming from.

 

[Chad Cowan]

Maybe I'm splitting hairs. I guess if it's flowing in toward the storm, it's inflow, no matter where it's coming from.

I think you've got the right idea there Bob, it's just a matter of semantics. The way I see it, technically, any air that gets sucked into the updraft is inflow BUT the way we generally use the term 'inflow' is to describe the warm, moist airmass that gets ingested and that hasn't been recycled. The updraft will suck in any air it can, that's why it's important for tornadogenesis to have a warm RFD, because although it won't be unstable as the 'real' inflow, it won't have the detrimental effect on buoyancy that a cold RFD will.

 

[Shane Adams]

IMO "inflow" suggests air that is actually fueling the storm, straight from the surface into the updraft, without being interferred with.

The backside RFD wrap-around (IMO) is more a case of "used up" air that has already served its main purpose (helping to bring the circulation to the ground/trigger tornadogenesis) being pulled around once it contacts the ground. It's quite common to experience strong northerlies/westerlies immediately after a tornado moves by to your position, which is simply the continuation of the RFDs path after it hits the ground and spreads. This is the reason why tornadoes start to die when the RFD winds become pulled into the storm's inflow; the colder, drier air cuts off the tornado's lifeblood (uninterrupted warm moist air flow).

 

[Matthew Grzych]

This is currently still an active area of research... but from the mid-1990's to present day quite a few measurements have been made by mobile mesonets and other in situ devices of RFD air from tornadic and nontornadic supercells. In many of the strong tornado cases, the RFD air is found to have nearly the same qualities as the inflow air (i.e. similar temperature, positive buoyancy, equivalent and virtual potential temperatures). Also, the RFD air tends to completely wrap around the mesocyclone (occlude) prior to tornadogenesis. The warm buoyant air found in tornadic RFD's is easily reingested by the updraft, angular momentum is conserved, and a tornado forms. During non-tornadic events, the air is cold and negatively buoyant. This air hits the ground and spreads out away from the updraft and tornadogenesis fails. Questions still remain as to why the RFD air has the qualities it does for the different cases. But to answer the question, I believe that inflow air is boundary layer air being ingested into the storm that has not been altered by the storm itself. RFD air has come from somewhere within the storm itself and is forced back to the surface (somehow) on the rear flank. So no, I don't believe that you can consider RFD inflow. RFD air by definition is downdraft and quite different from inflow air becase of it's source. And supercell tornadoes do not occur without RFD air Just my opinion, but I think this holds true for semantics from a storm research perspective.

Consider this scenario for a synoptic scale low pressure system: post cold frontal air is being pulled into the low pressure center from the N & NW. Warm sector air is being pulled into the low pressure center from the S & SE. There are 2 totally different airmasses converging at the low pressure center. We define these as being different because they have different sources. Same holds true for inflow air vs. RFD.

Matt

 

[Bob Hartig]

Great answer, Matt. What you've stated about warm versus cold RFDs was my understanding of the matter also, though you've explained it far better than I could.

I'm getting an overall consensus from this thread that inflow is defined by fresh, pre-ingestion air, which excludes RFD winds. Qualitatively, though, it looks like there often may not be much difference, at least in warm RFD scenarios--i.e. "In many of the strong tornado cases, the RFD air is found to have nearly the same qualities as the inflow air (i.e. similar temperature, positive buoyancy, equivalent and virtual potential temperatures)." That makes sense to me, and I suppose that a deterioration in the quality of RFD air would be one way to weaken or kill a tornado.

I can see a pretty interesting discussion developing out of this, and I'm all for it, because I can think of other questions to ask and observations to make. But as far as answering my initial question about terminology, I think I've gotten my answer. Thanks to all who have contributed!