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# Re: [ferret_users] alongshore and cross-shore components

```>  How to calculate alongshore and cross-shore components of winds at a
> stretch around Indian sub-continent?

The only simple way I can think of is to use the gradients
of topography to define cross- and along-shore directions:

set data etopo5
let Hx0 = rose[x=@DDC] !! x gradient of topography
let Hy0 = rose[y=@DDC] !! y gradient of topography
let norm = (Hx0*Hx0 + Hy0*Hy0)^0.5
let Hx = Hx0/norm
let Hy = Hy0/norm
let wind_cross = taux*Hx + tauy*Hy
let wind_along = taux*(-1)*Hy + tauy*Hx

Here, the crossshore component is "tau dot n"
and the alongshore component is "tau dot (k x n)",
where n is a unit vector parallel to grad(H)
and k is the unit vector pointing upward.

You may need to smooth the topography before taking
the derivatives.  These quantities are cross-
and along-shore only right on the coast line;
elsewhere it's defined according to local isobaths.

Ryo

```

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