ROMS output on regular lat/lon

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ROMS output on regular lat/lon

#1 Post by jvratnam » Mon May 21, 2007 10:24 am


I want to convert the ROMS output on Spherical grid to a regular lat-lon grid. Is there any utility available for this.


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#2 Post by marcelcure » Tue May 22, 2007 9:34 am

Hi Ratnam,

We use PMEL's Ferret ( to do this. It is very fast, free and can be installed on desktops or 64 bits servers. The regridding to a regular lat lon grid is done in two stages :- firstly you need to make a mapping function, which can take a lot of processing power (use a server?). This is a one off operation :-

set memory/size=150 ! give ferret lots of memory to work with
use ! pull in the NetCDF history data
define axis/x=-40:10:0.1/modulo/units=degrees xax ! specify new grid
define axis/y=40:70:0.05/modulo/units=degrees yax
let lonlatout = y[gy=yax] + x[gx=xax] ! define your new lat lon grid
let lonin = lon_rho[d=1]
let latin = lat_rho[d=1]
let map = curv_to_rect_map ( lonin,latin,lonlatout,0.5) ! regrid
save/clobber/ map ! save to an nc file

Now you can use the mapping function on your data :-

use ! pull in the NetCDF history data
let out_salt = curv_to_rect(salt[d=1,K=1,L=1], map[d=2])
shade out_salt ! plot it to look at it
save/clobber/ out_salt

You can add extra 2D or 3D variables using the 'append' option to save (removing 'clobber' of course)

By the way if you want to regrid from sigma to z levels, you do this before the lat lon regridding :-

In Matlab use the zlevs.m function to generate a 3D grid of depths of each s level and then save this to a netCDF file called Then in Ferret, do something like this :-

set memory/size=150
use ! d=1
use ! d=2
def axis/z=-4500:0:100/depth/units=m z100 ! new z axis
let znew = z[gz=z100]
let zz=z_rho[d=2]*-1000 ! not sure why I do this!
let salt_depth=zaxreplace(salt[d=1],zz,znew) ! regrid the data
save/clobber/ salt_depth[k=10:40] ! save to new nc file

Ferret is not particularly intuitive, but it does the job and this kind of regridding is useful to generate model output which is compliant with OGC standards WMS/WCS/WFS.

Good luck,



Re: ROMS output on regular lat/lon

#3 Post by asz178467 » Wed Sep 25, 2019 10:27 am

may i know what is 0.5 in (let map = curv_to_rect_map ( lonin,latin,lonlatout,0.5) )

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