Cap temperature...

[Andrea Griffa]

Hi all I wanna make with you some considerations about the measure of the cap.
First of all sometimes I think of the way to calculate the cap temperature(I'm NOT speaking about the cap INDEX): it looks like something strange. Why do I say that? I know that a way to calculate Cap temperature is with this formula:

CAP = Ambient Air Temp at LCL minus ambient Temp at LCL

But often it can be non representative of a strong inversion that could be above the LCL level and that could play the most important role in suppressing convection: in that situation you could have a little cap but also a huge amount of cin, with no convection. Do you understand what I say?

Actually if you see the value of CIN you can have a correct idea of the amount of the negative low level energy that could suppress convection.
Anyway wich is the better way to estimate the cap using the temperature?

 

[Jeff Snyder]

Andrea,

I'm not sure I've seen that formula... The only problem with that formula is that, if the parcel reaches the LCL freely (as if the cap is a little higher), then the Temp of the parcel = the temp of the environment (ambient temp), meaning that the formula would yield 0 cap strength. I usually like to think about capping in terms of CINH / CIN, but I've also thought of it in terms of the following:

Cap (degrees Celsius) = MAX (temperature of the environment - temperature of the parcel).

For example, if there's a sharp inversion, the level at which the parcel would most be negatively buoyant is the level at which Tenvironment - Tparcel is the largest (where the environment is the warmest relative to the parcel). This level can quite often occur above the LCL, though it may occur near the LCL as well. That said, I'm not sure that using the LCL is the best approach, though the cap will be weaker if its above the LCL (in which case the parcel is saturated and cooling more slowly owing to the release of latent heat) than if it's below the LCL.

Of course, like CAPE, I think it's important to look at the distribution of CINH over the depth. For example, if you have moderate CINH over a pretty deep depth, strong, persistent low-level / deep-layer moisture and mass convergence may still allow surface parcels to reach their LFC. On the other hand, if the CINH is distributed over a small depth (signifying that the parcel is much cooler than the environment at some level, which you sometimes get with sharp inversions), then it may be more difficult to push the parcel to its LFC since a surface parcel may need to be lifted through an area where it's significantly cooler than it's environment. I'd think that you'd likely need a stronger low-level convergence source to push a sfc parcel to its LFC in this latter case.

 

[Pat Lawrence]

Hi all I wanna make with you some considerations about the measure of the cap.
First of all sometimes I think of the way to calculate the cap temperature(I'm NOT speaking about the cap INDEX): it looks like something strange. Why do I say that? I know that a way to calculate Cap temperature is with this formula:

CAP = Ambient Air Temp at LCL minus ambient Temp at LCL

But often it can be non representative of a strong inversion that could be above the LCL level and that could play the most important role in suppressing convection: in that situation you could have a little cap but also a huge amount of cin, with no convection. Do you understand what I say?

Actually if you see the value of CIN you can have a correct idea of the amount of the negative low level energy that could suppress convection.
Anyway wich is the better way to estimate the cap using the temperature?

You guys have a great looking website! I'll let an expert like Jeff Snyder deal with your question, however!

Pat

 

[Andrea Griffa]

For example, if there's a sharp inversion, the level at which the parcel would most be negatively buoyant is the level at which Tenvironment - Tparcel is the largest (where the environment is the warmest relative to the parcel). This level can quite often occur above the LCL, though it may occur near the LCL as well. That said, I'm not sure that using the LCL is the best approach, though the cap will be weaker if its above the LCL (in which case the parcel is saturated and cooling more slowly owing to the release of latent heat) than if it's below the LCL.

Thanks Jeff, that was just what I was thinking about and it seemed to me a bit strange.
However this is the site where I found the formula(and it wasn't the only one)

https://ows.barksdale.af.mil/tech_ref/tfrn/...ndx_Cap_LSI.htm

So, to better understand, the correct formula should be:

Cap (degrees Celsius) = MAX temperature of the environment - temperature of the parcel at that level. It's ok?

Jeff, in this case?How could you calculate the cap?
This case is an exemple where the major inversion is above LCL level. So It's correct if I do: Temp of 750mb(inversion more pronunced) minus Temp of the same level; 11-9=2

 

[Andrea Griffa]

?

You guys have a great looking website! I'll let an expert like Jeff Snyder deal with your question, however!

Pat

Thanks a lot, Pat

 

[Jeff Snyder]

Andrea,

That seems right. For the sounding you posted, the MAX(Tenviron-Tparcel) would seem to occur at about 750mb, and that's for a mixed parcel (50mb-100mb, don't know exactly what UWy uses). I think CINH is usually a better indicator, and CINV (CIN calculated using virtual temperature) is probably better yet.

 

[Andrea Griffa]

Andrea,

That seems right. For the sounding you posted, the MAX(Tenviron-Tparcel) would seem to occur at about 750mb, and that's for a mixed parcel (50mb-100mb, don't know exactly what UWy uses). I think CINH is usually a better indicator, and CINV (CIN calculated using virtual temperature) is probably better yet.

Very nice Jeff, thanks. And what do you think about those values?They seems to me right.

0 No Cap
0.1 - 1.9 Weak Cap
2.0 - 4.0 Moderate Cap
4.1+ Strong Cap