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

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MaxCol22
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Joined: Thu Sep 24, 2020 1:50 am

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

Post by MaxCol22 » Thu Sep 24, 2020 1:59 am

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/Funct ... e_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.

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