Calculating CAPE from CCL/LCL

Verhaegen Yoni

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Jun 22, 2019
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Belgium
Hi all

I have been trying to get to know how to read atmospheric soundings, but some things remain unclear. I hope someone can clarify.

So if I understand well, as long as T2m < convective temperature, you will need to use the LCL and start the moist adiabatic curve from there to calculate CAPE.

When T2m >= Tcon, you will need to use CCL. Does this mean that the CCL is actually always a LFC (level of free convection), in the meaning that the moist adiabatic curve of the parcel then starts from the intersection of the mixing ratio lapse rate/temperature curve and cannot be placed to the left of the temperature curve in a skew-t/log-p as sometimes happens with the LCL?

What happens when T2m > Tcon? Does this influence the position of the CCL on the sounding and the point from which you have to start the moist adiabatic curve to calculate CAPE compared to the situation where T2m = Tcon?

I hope I made myself clear.

Thanks anyway!
 

Jeff Duda

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Oct 7, 2008
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The CCL is not part of the formal definition of calculating CAPE. CCL is just a diagnostic value designed to estimate where cloud bases will be if the convective temperature is reached. In many cases of severe convection, storms initiate well before the convective temperature is reached (especially in June when there are major capping issues).

Officially, CAPE is defined as proportional to the vertical integral of (T'-T), where T' is the parcel temperature (defined in standard ways) and T is the ambient environmental temperature, with the integration bounds being from LFC to EL. See also the AMS glossary definition here.
 
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Verhaegen Yoni

Enthusiast
Jun 22, 2019
3
0
1
Belgium
The CCL is not part of the formal definition of calculating CAPE. CCL is just a diagnostic value designed to estimate where cloud bases will be if the convective temperature is reached. In many cases of severe convection, storms initiate well before the convective temperature is reached (especially in June when there are major capping issues).

Officially, CAPE is defined as proportional to the vertical integral of (T'-T), where T' is the parcel temperature (defined in standard ways) and T is the ambient environmental temperature, with the integration bounds being from LFC to EL. See also the AMS glossary definition here.
Ok, thanks! This made most things clear. However I'm still wondering how you can use a sounding to determine the extent to which convective clouds will develop in the vertical just purely from convection alone after the convective temperature was reached... You just use the LCL then despite the fact that CCL might be at a different level than LCL?
 

Jeff Duda

Resident meteorological expert
Staff member
Oct 7, 2008
3,025
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Westminster, CO
www.meteor.iastate.edu
The true meaning behind the process of "reaching convective temperature" is that the atmosphere eventually reaches a point where, when mixed vertically, there will be a point where RH reaches 100% (at that level we call it the CCL, but it is also an LCL) in the presence of "not too much" stability above that, such that it will be possible for clouds to form, even if they are very limited in vertical extent. Too much stability and plumes will just stop ascending pretty much immediately upon hitting the CCL and will not actually produce much cloud material. Ideally, the CCL will also serve as an LFC such that deep moist convection will be promoted as soon as the CCL is developed, but this requires conditional instability.

To determine the extent of vertical development from clouds formed above a CCL, you need to picture what the profile would look like if a CCL even developed. While it is approximately always possible to modify a sounding to make a CCL (except in cases of nearly 0 moisture content), in cases where the lower troposphere is too dry, the CCL may be at a vertical level that isn't achievable by PBL processes, and thus will not be realized. Anyway, assuming you have made the modification, examine a parcel path coming from the mixed layer. The +ve area represents the extent of possible vertical development (entrainment/precip loading not included, of course).
 
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