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How PH and PHB change in time

A

Alex_Cantrel

New member
Hello,

I have a question on PH (perturbation geopotential) and PHB (base-state geopotential) variables.
I am curious how these two variables vary in time. I have a 1-week analysis in which I store these two variables to calculate the height above the ground level (Height AGL).
I would like to ask if I could use the weekly mean value of PH and PHB in each height level to get a good approximation of these two variables (and the height AGL) for the rest of the year.
Does this makes sense or the estimation will be insufficient?

Thank you very much in advance.
 
PHB is base-state geopotential, which doesn't vary with time.
PH is perturbation geopotential, which is a prognostic variable and it is time-varying. PH + PHB gives the geopotential.
Therefore, you cannot use 2-week mean PH and PHB to represent geopotential over the entire year.
 
Thank you very much for your reply.

I have one following question.
I had an error in my post-processing script and the PHB and PH variables are stored in non-consecutive levels over my domain.
So, that practically means, that I have information about the height only on one of the staggered edges of my cell and I cannot calculate the height in the cell center to get the de-staggered height level directly.

Is there any way to estimate the height in the cell center?

Thank you very much in advance.
 
You are correct that PH and PHB are on full model levels. One simple way to derive their values on half-model level is by arithematic calculation:
PH at half level = (sum of PH at full levels above and below) / 2.
WRF-NCL and WRF-Python also have functions to de-stagger PH and PHB.
 
Thank you for the reply.

Apologies, maybe, my question was not clear and correct.

Unfortunately, I don't have the original wrfout* files, but instead, I have post-processed xarrays with PH and PHB stored at specific height (non-consecutive) levels.
For example, let's assume that my original wrfout files contain 50 eta_levels, and that I am storing PH and PHB only at the 20th and 30th index (bottom_top_stag) of the original wrfout files. If I try to de-stagger these values using the integrated wrf-python or NCL tools, I will get the mean value between these indexes (please correct me if I am wrong), therefore, I will get a value close to the 25th index (e.g.), instead of the desired values that should be between the 20th and 21st staggered indexes.

So, I am trying to find a manual way to calculate, or at least approximate the PH variable in the cell centers, if it is possible.

Thank you very much in advance.
 
Last edited:
Ming Chen said:
You are correct that PH and PHB are on full model levels. One simple way to derive their values on half-model level is by arithematic calculation:
PH at half level = (sum of PH at full levels above and below) / 2.
WRF-NCL and WRF-Python also have functions to de-stagger PH and PHB.
Click to expand...
Is it required to destagger PH and PHB to obtain the geopotential height? I did PH+PHB from wrfout files to get the Geopoential Height. Is it wrong then?
 
Satiz,
PH + PHB gives you geopotential height at full model levels. There is nothing wrong here. .
if you want geopotential height on half model level, you need to unstagger it.
 
@ Alex_Cantrel,

I am sorry to tell that it is more reasonable to obtain GHT based on original wrfout data. To get data at 25th level based on data at 30th and 20th is not an appropriate approach. This is because WRF vertical levels are not homogeneously distributed and thus the 25th level is not necessarily the midpoint between 20th and 30th levels.
 
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