[ncl-talk] How to calculate d(anyvariable)/dt from a wrf file in NCL

[Lyndz]

Dear NCL experts,

I would like to compute the potential vorticity tendency (dPV/dt) from a
wrf file.
However, there is no time dimension in the variables (although the "time"
variable can be obtained using the wrf_user_getvar)

Here's my script so far:

filename = "./wrfout_d01_2011-12-12_12_00_00"
a     = addfile(filename,"r")
times = wrf_user_getvar(a,"times",-1)                   ; get all times in
the file
it = 89                                                   ; first time step
theta = wrf_user_getvar(a,"theta",it)
pv    = wrf_user_getvar(a,"pvo",it)                     ;potential vorticity

I would like to compute dPV/dt using centered finite difference
(center_finite_diff_n).

Any ideas/solutions/examples on how can I do this in NCL?

I'll appreciate any help on this matter.

-Lyndz
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