Re: [ferret_users] calculate vertical velocity in ferret

[Ryo Furue <furue@xxxxxxxxxx>]

Hi Yangxing and Ferreters,

|     I have gridded ocean data with variables including
| u,v, temp, ...
| Unfortunately I need vertical velocity w which is not
| an output of model. One way to get w is to use
| continuity equation. Does anyone have any idea how to
| do it in ferret, also how is potential error produced
| using continuity equation to calculate w?

If you use the same finite-difference scheme as the model,
you will get exactly the same w field as the model's,
bar potential tiny numerical error due to the computer
representation of floating point numbers.  Assuming that
you are using the finite-difference model, that is.

So, to answer your question, you need to know what finite
difference scheme your model uses.  I could show you how
typical C-grid and B-grid models finite-difference the
continuity equation, but I'm afraid I don't know how to do it
in Ferret.  I don't think Ferret's centered difference will do,
because derivatives are defined between the two neighboring
gridpoints for the continuity equation.

Ryo