Re: how to calculate potential vorticity

Please note: tThe 1st rule of data processing is to *look at your data*.

[1]
The calculation of potential vorticity requires a vertical derivative.

You have:

  a = addfile("uwnd.2005.nc","r")
  u = short2flt(a->uwnd(152,{200},:,:))

which means you are reading in the 153rd time step and the 200hPa level
only.
In fact, you have only a two-dimensional array: u(lat,lon). The same holds
for temperature and v. So, you do not have multiple vertical levels.
Hence,
a vertical derivative in not possible!!

Use
      printVarSummary( u )
to verify.

The actual data variables are: (time, lev,lat,lon)

[2]
There were many syntax errors which you did not even try to correct.
Please do not send us code with multiple syntax errors.
We like to help but *** our time is valuable too!!!! ***

[3]

Please read the Mini-Language manual at:

http://www.ncl.ucar.edu/Document/Manuals/

[4]
The approximation you were using is good only on isentropic surfaces.
The formulation
for pressure levels is significantly different.

I have a PV code which I will send you offline.
*NOTE* the is unsupported. I will not answer any questions.

kalim ullah wrote:
> Dear Wei,
> I am feeling difficulty to calculate the potential vorticity as you
> suggested me. Now I am sending you my script file I hope you will
> check it and suggested me like before.
> thanks in advance
> kalim
>
> --- On *Mon, 10/19/09, Wei Huang /<huangwei@ucar.edu>/* wrote:
>
>
> From: Wei Huang <huangwei@ucar.edu>
> Subject: Re: how to calculate potential vorticity
> To: "kalim ullah" <kalimrid@yahoo.com>
> Date: Monday, October 19, 2009, 7:09 AM
>
> Kalim,
>
> Where "phi = latitude * PI/180".
> Do not scale your vrt before pv, but after you get pv.
>
> Assume your have KZ levels,
>
> where theat = T * (100000/p) **(R/cp).
>
> then:
> at level 1: d(theta)/dp = (theta(2) - theta(1))/(P(2) - P(1))
> at level KZ: d(theta)/dp = (theta(KZ) - theta(KZ -1))/(P(KZ) -
> P(KZ -1))
> and other levels: d(theta)/dp = 0.5 * ( (theta(K+1) -
> theta(K))/(P(K+1) - P(K))
>
> + (theta(K) - theta(K-1))/(P(K) - P(K-1)))
>
> Where P is in Pa.
>
> Hope this help.
>
> Wei Huang
> huangwei@ucar.edu
> <http://us.mc537.mail.yahoo.com/mc/compose?to=huangwei@ucar.edu>
> VETS/CISL
> National Center for Atmospheric Research
> P.O. Box 3000 (1850 Table Mesa Dr.)
> Boulder, CO 80307-3000 USA
> (303) 497-8924
>
>
>
>

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Received on Mon Nov 9 14:03:04 2009