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Re: derivatives on an uneven grid
Hello Ferreters
How do you control the contour interval (begin:end) of your plot?
For example, in plotting a bathymetric data using etopo,
if once is only concern on the ocean part you begin from
0m to an arbitrary -1000m depth.
Masking the land portion
maybe good but still on the color scale, it goes from negative
value(ocean) to 0(coast) to positive value (mountains). Obviously
positive values are not needed.
Thanks in advance
truly a ferret-fan
rolando s balotro
John Donners wrote:
Hi Brent,
some weeks ago I ran into the same problem. I wrote a ferret script to
calculate first and second derivatives on an unequal grid:
let zet=z[gz=rho]
let h1=zet-zet[k=@shf:-1]
let h2=zet[k=@shf:1]-zet
let a=(-1)/(h1+h1*h1/h2)
let b=1/(h2+h2*h2/h1)
let c=(-1)*(a+b)
let rhoz=a*rho[k=@shf:-1]+b*rho[k=@shf:1]+c*rho
let aa=2/(h1*h1+h1*h2)
let bb=2/(h2*h2+h1*h2)
let cc=(-1)*(aa+bb)
let rhozz=aa*rho[k=@shf:-1]+bb*rho[k=@shf:1]+cc*rho
The variables rho and rhozz are now the first and second derivatives (in
the vertical direction) of the variable rho. The variable h1 is
the distance between the uppermost and the central
points. The variable h2 is the distance between the central and the
lowermost points. It can easily be seen that the centered difference
formulations will show up if h1=h2.
Hopefully this answers your questions. If there are any more questions
I'll be happy to answer them.
Regards,
John
------------------------------------------
John Donners
Royal Netherlands Meteorological Institute
Dept. Oceanographic Research
P.O. Box 201
Wilheminalaan 10
3730 AE De Bilt
tel. nr. : ++31 30 2206701
fax. nr. : ++31 30 2202570
e-mail : donners@knmi.nl
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