# [Wrf-users] perturbation potential temperature

Agnes Mika agnes.mika at bmtargoss.com
Mon Sep 6 01:29:30 MDT 2010

```Hallo Liu,

The conversion is quite simple. The formula to convert perturbation
potential temperature to total potential temperature in K can be
found in the WRF users manual (p. 5-58 in the July 2009 edition,
under section "WRF Output Fields", sub-section "Special WRF Output

Total pot. temp. in K = T + 300 (T is the perturbation pot. temp.)

The forumla to convert total potential temperature to "normal"
temperature can be found in virtually any meteorology book. The
potential temperature is the temperature a parcel of air having a
certain pressure p and temperature ("normal" temp.) would attain if
brought dry adiabatically to a standard pressure p_0 (this is usually
taken to be 1000 mbar).  The conversion formula is:

pot. temp. = "normal" temp. * (p_0/p)^kappa

From this: "normal" temp. = pot. temp. * (p/p_0)^kappa

p_0 = 1000 mbar
p is the pressure in mbar

kappa is the Poisson constant (kappa = R/c_p), the ratio of the gas
constant R to the specific heat at constant pressure c_p. For dry air
kappa = 0.2854.

Hope this helps,
Agnes

Liu, Peng wrote:
> Dear Users,
> I ran two data set by WRF and want to compare the predicted temperature. The output temperature of WRF is perturbation potential temperature, So I have a few questions.
>
> First, how WRF sets the reference sea level temperature? I mean if reference sea level temperature keeps the same when WRF deals with different data set?
>
> Second, for two different data sets, if WRF keeps sea level temperature the same , and at the same time, top of pressure and the vertical layers are set to be the same, Does that mean the reference potential temperature for each vertical levels are the same for the two data set, and when I want to compare predicted temperature, I can directly compare the perturbation potential temperature?
>
> Third, If I can not directly compare the  perturbation potential temperature to see the difference of predicted temperature, is there any way that I can convert the perturbation potential temperature to absolute temperature?
>
> Thank you very much.
>
> Peng

--
Dr. A'gnes Mika

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