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Re: [ferret_users] how to calculate the streamfunction and velocity potential
Hi Billy and Hanh,
| ! compute the horizontal streamfunction from vertically integrated u
| ! and v fields
| ! note that u and v must occur on the same spatial points (B-grid)
| ! arg1=variable name for u
| ! arg2=variable name for v
| ! arg1=x0 (must be on a gridpoint or set mode interp)
| ! arg2=y0 (likewise)
| ! psi=int(y0,y)u(x,y)dy - int(x0,x)v(x,y0)dx
| ! u=d(psi)/dy, v=-d(psi)/dx
I guess this gives you a correct streamfunction only
when du/dx + dv/dy = 0, right? (There's no function satisfying
both u=d(psi)/dy and v=-d(psi)/dx when du/dx + dv/dy is nonzero.)
Hanh mentioned velocity potential, which suggests that a Helmholz
decomposition is required:
(u,v) = grad(phi) + k x grad(psi)
To obtain phi(x,y) and psi(x,y), you need to solve these Poisson equations
div grad (phi) = div(u,v)
div grad (psi) = curl(u,v)
with some (probably arbitrary) boundary conditions.
I myself haven't done this, but a colleague of mine uses the method
described by:
Watterson, I., 2001. Decomposition of global ocean currents using a
simple iterative method. J. Atmos. Oceanic Tech., 18, 691–703.
Hope this helps,
Ryo
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