Projecting variables from sigma-layer to density surfaces

["Mark Verschell" <verschell@xxxxxxxxxxxxxxxxxxxxx>]

I don't know how interesting this will be to anyone else, but the question came
up in my office about how to project sigma-coordinate model output onto density
surfaces. A couple of minutes of thought produced the answer (see the "Using
sigma and curvilinear model coordinates" web page
http://ferret.wrc.noaa.gov/Ferret/FAQ/data_management/sigma_coordinate_demo.html
for more information)

Here is an example:

use i0418b.cdf                              ! my data set X * Y * 20 sigma
                                            !   levels

let depth=h[k=@rsum]-h/2                    ! need to get depth from sigma
                                            !   levels (h is thickness for each
                                            !   sigma layer)

let density=rho_un(salt,temp,depth)         ! use built-in function and model
                                            !   salt and temperature fields

define axis/z=1022:1028:2 zdens             ! density surfaces of interest

let udens=zaxreplace(u,density,z[gz=zdens]) ! zonal velocity on density
surfaces

shade udens[z=1028]                         ! u on the 1028 density layer


Hope somebody finds this useful, Mark