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Re: [ferret_users] Calculating relative vorticity
Hi Paulo Santiago and Savin,
| Supposing you are using relative vorticity defined as
|
| zeta = du/dx - dv/dy
|
| in Ferret, you can do something like:
|
| let dudx = u[i=@ddc]
| let dvdy = v[j=@ddc]
| let zeta = dudx - dvdy
Except that zeta = dv/dx - du/dy , assuming that u and x are
eastward and v and y are northward.
I have two more caveats:
1) The formula above is correct only when u and v are defined on the
same grid;
2) The formula above is an approximation on a sphere. The real curl
is dv/dx - d(u cos(phi))/dy/cos(phi) or something like that
(please don't believe it without checking), where phi is the
latitude, dx = a cos(phi) d(lambda), and dy = a d(phi).
If you want to include the cosine factor you can use
let c = cos(pi*y[gy=u]/180.0)
let cu = c * u
let dudy = cu[j=@ddc]
let dv/dx = v[i=@ddc]
let zeta = dvdx - dudy/c
or along the lines.
Cheers,
Ryo
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