I'm missing something - CIN

[Steve Dedman]

In looking at the NAM 48 (00z, 3/7/09) here, I am seeing CIN as a negative number. On the skew-t's it's a positive number. Is this just a convention? Or am I missing an important component of the CAPE - CIN relationship? The CAPE is a positive number, so since CIN is the anti-CAPE, is it listed as a negative? And on the skew-t's the "-" is left off because everyone knows it's a negative number?

Is absolute stability at/near zero, and the further both CAPE and CIN get from zero equals more instability? IOW, the farther apart those two are = more instability?

 

[Patrick Marsh]

In looking at the NAM 48 (00z, 3/7/09) here, I am seeing CIN as a negative number. On the skew-t's it's a positive number. Is this just a convention? Or am I missing an important component of the CAPE - CIN relationship? The CAPE is a positive number, so since CIN is the anti-CAPE, is it listed as a negative? And on the skew-t's the "-" is left off because everyone knows it's a negative number?

Is absolute stability at/near zero, and the further both CAPE and CIN get from zero equals more instability? IOW, the farther apart those two are = more instability?

You need to think in terms of buoyancy (or accelerations - buoyancy is a form of acceleration). A positive buoyancy (acceleration) is what we call CAPE. This is when a parcel will continue to accelerate up toward higher altitudes. This is unstable.

CIN is what we call negative buoyancy. If a parcel has CIN (negative buoyancy) it will accelerate down toward the surface. This is stable.

If CIN = 0 and CAPE = 0 then we have a neutral atmosphere. There is no acceleration. This means a parcel will continue to do what it was previously doing. It it was sinking, it will continue to sink. If it was rising, it will continue to rise. It is was just "hanging out going nowhere", it will continue to do just that.

 

[Steve Dedman]

Thank you for the reply, fellow Sooner!

However, I don't think I phrased my question well. I get what CAPE & CIN are generally, although your post added to my understanding as well. I'm trying to get to the measurement part of it, and how to use that in predicting supercell genesis.

In reference to the scale used on the NAM and GFS CIN charts, they are measured in negative numbers. Conversely, the CAPE is shown as a positive number.

Would it look like this:

High CAPE 3500.........1000..........0.........-50.......-500 High CIN

So would a high CAPE of 3500 and a CIN of -25 (on the NAM/GFS scales) indicate extreme buoyancy with a weak *or* strong cap? What about a 3500 CAPE with a -500 CIN?

Or is it better just to throw away those scales and go by what I see on the skew-t charts, and look for a number under 50 to indicate a weak cap?

I guess what I'm looking for is how to reconcile the difference between the way the NAM/GFS models indicate the forecast level of CIN, and how it is indicated in reality on the soundings. How do those two numbers relate to one another?

 

[Patrick Marsh]

***Warning: This is a semantics post!***

[semantics] By convention, we define CAPE to be positive buoyancy and CIN(H) to be negative buoyancy. Therefore, a negative number for a value of CIN(H) is, in my opinion, incorrect. A negative CINH would be the negative of negative buoyancy which is positive buoyancy and therefore CAPE. What I think happens most of the time is that the buoyancy of a parcel is computed. Thus, we tend to have negative buoyancy at the surface and positive buoyancy aloft. We then assign the label CAPE to the positive buoyancy and CIN(H) to the negative buoyancy...and just forget to flip the sign. [/semantics]

Essentially, anytime you have CIN(H) you have a downward directed force. To get a parcel to rise, something has to exert a force upward that is greater than the downward force resulting from CIN(H). The greater the CIN(H) (CAPE) the greater the downward (upward) force. As for values, a CIN(H) of 500 is oppressively strong. You would have to overcome a lot of downward directed force before a parcel could realize the positive CAPE above and rise on it's own. Typically, a CIN(H) of 100 or greater is too much for a parcel to overcome, unless there is extreme forcing to lift the parcel through the CIN(H) area of a sounding.

If you are still at OU, and have further questions, PM me and we'll meet up. Sometimes actually examining the sounding will help make this more clear. It's just really hard to annotate a sounding online.

 

[Patrick Marsh]

I guess what I'm looking for is how to reconcile the difference between the way the NAM/GFS models indicate the forecast level of CIN, and how it is indicated in reality on the soundings. How do those two numbers relate to one another?

They *should* be the same number. CIN(H) is CIN(H) regardless if it is model computed or observed.

 

[Steve Dedman]

Cool! That's pretty much what I was looking for, and kind of what I suspected.

In practice: how will an area of high CIN(H) effect an advancing dryline that is firing supercells? Assume that the dryline is a strong one, and the CIN(H) ahead of the dryline is likewise as strong. Will the dryline go ahead and push in and weaken the cap? Or is that a maybe/maybe not/"hope today isn't a bust" question that can only be answered as the situation unfolds?

Sadly, my days in Norman are passed. I'd love to meet up and knock back a couple at O'Connell's and talk supercells!

 

[Patrick Marsh]

In practice: how will an area of high CIN(H) effect an advancing dryline that is firing supercells? Assume that the dryline is a strong one, and the CIN(H) ahead of the dryline is likewise as strong. Will the dryline go ahead and push in and weaken the cap? Or is that a maybe/maybe not/"hope today isn't a bust" question that can only be answered as the situation unfolds?

The dryline doesn't weaken the cap directly. What a dryline does is help provide focused areas of convergence, and in turn rising motion. It is the rising motion (remember air cools as it rises) that acts to weaken the cap. The sharper the dryline, the tendency for stronger forcing and greater convergence. If the vertical motions associated with the convergence is enough to sufficently cool the warm layer aloft and decrease CIN(H) you can develop thunderstorms. So ultimately, if you have a relatively strong cap in place (and in turn strong CIN(H)), you need strong convergence and strong lift to overcome it. This is the ultimate forecast problem when it comes to thunderstorm initiation on the dryline.