Defining the inflow layer

[Mark J. Dempsey]

Hi folks, I wonder if someone can help me to understand something. I am reading an article called Effective Storm-Relative Helicity in Supercell Thunderstorm Environments by Richard L. Thompson, Roger Edwards, and Corey M. Mead. They are defining the inflow layer for effective SRH.
"The tests were performed by beginning at the ground level in the sounding and searching upward for the first lifted parcel to satisfy the CAPE and CIN constraints, and this level was designated the “effective base”. Continuing upward from the effective base, each level in a sounding was examined until either of the CAPE or CIN constraints were violated, and this level was designated the “effective top” of the inflow layer. The vertical distance between these two levels defines the effective storm inflow layer."
I don't understand what they mean when they search upward "for the first lifted parcel to satisfy the CAPE and CIN constraints." (base level) and then what are the violated constraints? (top level).
The constraints are given by the authors: example 100 J/kg CAPE and -250 J/kg CIN. The results being a base at 900hPa and a top at 650hPA.
I think this is very important and would like to understand it better. Any help would be greatly appreciated. Thanks.
Mark J Dempsey

 

[Jeff Snyder]

Mark,

Going off the criteria given in your post (CAPE >= 100 j/kg and CINH >= -250 j/kg), the effective base is the lowest level from which a parcel satisfies the criteria. For example, if you lift the sfc parcel and find that it produces CAPE ~ 2500 j/kg and CINH ~ -10 j/kg, then you would, right away, have the effective base. However, suppose we're looking at a location north of a strong warm front, a location that is characterized by cool surface temperatures but strong low-level warm-air advection. In such a case, perhaps the surface parcel lifted would yield 0 j/kg CAPE, or only 50 j/kg CAPE with -800 j/kg CINH; it would not qualify to be a parcel of the effective inflow layer, so the next level up is examined. If the surface is 1000 mb, then look at 980 mb. Does a parcel lifted from 980 mb fit the criteria? If it does, then great -- it will be the effective base; if it doesn't, then continue on up to the next level.

Assuming an effective base is found, then the next higher level is examined to see if the criteria hold. If they do, then that level is considered to be part of the effective inflow; if they do not, then that level is set to be the effective top of the inflow layer.

I haven't looked at that paper in a while, but I assume the main goal of finding an effective inflow layer is to estimate the depth through which an updraft will ingest (and "feel the effects of") the "air". For example, this may be particularly well-suited for cases of elevated instability, where SBCAPE or MLCAPE may not be useful.

On the other hand, I don't remember that paper addressing how those criteria were established. For example, does an updraft "suck up" a layer of air that has -250 j/kg CINH? What about -160, -180, 251, or -300 j/kg? Of course, this is likely heavily affected by such things as the depth and intensity of low-level convergence, the strength of the vertical perturbation pressure gradients (associated with mesocyclones), etc, and it is likely more of a probability distribution function (PDF) than a complete binary situation (i.e. parcels with CINH lower than -250 j/kg become increasingly less likely to be ingested, as opposed to saying that all parcels with CINH > -250 j/kg will be ingested while all parcels with CINH <= -251 j/kg will not). Though I don't recall any modeling studies on this (I'm certainly not a modeling expert!), I wonder if this has been looked at in the past. I suppose it would be as easy as running a suite of simulations using slightly tweaked low-level thermo profiles and examining the theta-e of the updraft and/or running backward trajectories to find the "source layer" of much of the mass that makes its way through the updraft. Then again, we know that the shear profile can (and does) affect updraft intensity, so you'd want to examine at least a few different shear cases as well.

 

[Mark J. Dempsey]

Hi Jeff,
Thank you for your prompt and complete response. I'm understanding more completely. I'm doing my undergrad thesis paper on my storm chase this summer and I'm reading as many papers on severe storm theory and forecasting that I can find. I had never really thought about the inflow as being stratified in such a way and then to talk about it in terms of CIN and CAPE was pretty exciting. Your warm front example places the abstract back in the here and now. Thanks. I'm gonna think some more on it. There's a lot there.
I took a look at your site and liked the videos of Goshen and Aurora. I saw the Goshen tornado from about 15 miles away. It was sweet. Then we were lucky to see the Aurora Nebraska tornado up close. These were my first tornadoes in person.
Another question (if you don't mind - I promise not to be a pest!) I've been thinking about what the nature of the surface flow is like. I've been reading that there is a horizontal vorticity component. I'm not sure if, as the wind is approaching the meso region, does it roll forward like a rolling pin? Or is it spiraling forward like a corkscrew pointed away from you? I think I get the concept of horizontal shear, but the vertical shear concept gets fuzzy.
Anyway, I'm working the SLOW overnight shift so I've time to think about these things (one of the perks of the job). Thanks again for your help and have a great day!
Mark

 

[Jeff Duda]

On the other hand, I don't remember that paper addressing how those criteria were established. For example, does an updraft "suck up" a layer of air that has -250 j/kg CINH? What about -160, -180, 251, or -300 j/kg? Of course, this is likely heavily affected by such things as the depth and intensity of low-level convergence, the strength of the vertical perturbation pressure gradients (associated with mesocyclones), etc, and it is likely more of a probability distribution function (PDF) than a complete binary situation (i.e. parcels with CINH lower than -250 j/kg become increasingly less likely to be ingested, as opposed to saying that all parcels with CINH > -250 j/kg will be ingested while all parcels with CINH <= -251 j/kg will not).

Jeff,
I know the authors did look at other thresholds for defining the CAPE/CIN criteria for being in the effective inflow layer. I'll quote the paper to show why they chose the layer they did:

Four CAPE thresholds
(25, 100, 250, and 500 J kg
1) and four CIN thresholds
(
50,
100,
150, and
250 J kg
1) were tested in
16 combinations as potential bounds on the effective
inflow layer (Table 1). The least stringent parcel constraints
(e.g., 25 and 100 J kg
1 CAPE with
250 J kg
1
CIN) resulted in the highest probabilities of detecting a
nonzero effective inflow layer depth (0.96 and 0.95, respectively)
for our 835 supercell proximity soundings.
Given the importance of the probability of detection and
some concern for false alarms in operational forecasting
of supercell environments, the 100 J kg
1 CAPE and

250 J kg
1 CIN constraints were chosen to represent
the bounds of the effective inflow layer."

The paper can be found here.

 

[Mark J. Dempsey]

Thank you guys. This has been very helpful.
mark