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CAPE computation in RIP and NCL: more info on (1) entrainment, (2) adiabatic, (3) environment, (4) computation

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MaxCol22

New member
Dear WRF community,

To understand the ins and outs of the CAPE computed by NCL's wrf_user_getvar out of my WRF simulations, I would like to have a reliable source of information on this computation. Note that I use NCL v6.4.0.

In particular:

- Is this CAPE the integrated buoyancy of non-entraining and pseudo-adiabatic parcels? Or does it consider entrainment and non-adiabatic transformations? Is the water loading kept along with the rising parcel? Is ice considered?

- Would anybody know by any chance if the "environment" that is considered by the code to compute CAPE is actually the same column as the one from which the parcel was raised, or it is the neighbouring columns?

I can read some basic but useful information on the NCL website (https://www.ncl.ucar.edu/Document/Functions/Built-in/wrf_cape_2d.shtml). There is also some info on the RIP code from the UCAR website (http://www2.mmm.ucar.edu/mm5/documents/ripug_V4.html). Are these sources accurate?

There seems to be a disagreement between these 2 sources at least: the NCL website claims that the mCAPE is computed from the parcel with maximum Theta_e (equivalent potential temperature, I assume) but the RIP User guide on the UCAR webiste says mCAPE is computed from the parcel with maximum q_e (water vapour mixing ratio, I assume). This is quite different, no? Does anyone know what is actually used in NCL v.6.4.0.

I was told NCL uses the CAPE computation built by WRF developers. I was also sent a version of the rip_cape.f code but would be keen to have a discussion on this.

Other related questions:

- What does the average over 500m depth, used to define a parcel, mean for air that is very close to the surface (say first model level at 15m)?

- Is it accurate that mCAPE is calculated as the CAPE of the parcel with maximum of q_e (or theta_e) only over the lowest 3km of the column? (that should not be too restrictive I think)

Thank you for those who have already investigated this in more details and who are happy to share their knowledge.

All the best,

Maxime.
 
Hi Maxime,
I just stumbled across this post and see that it was never answered. I first would like to apologize for the very long delay - it is never our intention to overlook user inquiries. As it's been so long since you posted, before I start digging into code, I'd like to ask if you've been able to move forward since you posted this. Thanks!
 
Thank you very much for your reply.

Actually, from reading the different information, I just assumed that "CAPE is computed here by lifting the air parcel with the maximum equivalent potential temperature found in the first 3 km in a pseudoadiabatic way and without entrainment". But it is true that most of the technical/detailed questions I had remain unanswered. So if anyone can shed some light on this, I would still appreciate it (although I reckon it is a bit technical).
 
Hi,
I am going to refer you to the NCL section of the forum for your additional questions. I believe they are more-related to NCL. Post your question in that section and hopefully someone will be able to answer those for you.
 
I will comment regarding the RIP computations.

Remember that CAPE is theoretical and cannot be measured. The computations depend on a series of assumptions. It's based on parcel theory and not a layer or slab theory.

Is this CAPE the integrated buoyancy of non-entraining and pseudo-adiabatic parcels?
Yes
Or does it consider entrainment and non-adiabatic transformations?
No
Is the water loading kept along with the rising parcel?
No
Is ice considered?
No
or it is the neighbouring columns?
No, it's the single, vertical column.
Are these sources accurate?
Yes, it's all we have. However, it's always a good idea to see what the code is actually doing.
What does the average over 500m depth...
It's an attempt to account for mixing. In some circles, they use a 'mixed-layer' CAPE. A single layer may be superadiabatic or otherwise unstable. By 'mixing' it, you can get a more reasonable value for CAPE. In practice, you find the level nearest 500 m above ground, then perform a linear average of T and q for all of the levels below 500 m. The surface air would contribute to that average.
Is it accurate that mCAPE is calculated ...
3km (about 700 hPa) is reasonable for most situations. The max theta_e is usually at the surface except for overrunning or nocturnal convection. Because theta_e (or moist static energy) decreases with height into the mid-troposphere using 3km is reasonable. However, the parcel that actually enters the cloud may not have the 'max' properties. Max CAPE is an attempt to give an upper bound to the strength of the convection.

You can download the UPP to see how they compute CAPE as well. Note that a lot of these CAPE computations are copied from each other or from some ancient code. I've noticed that the mcap from RIP can be very questionable especially over the tropics.
 
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