[ferret_users] plotting and interpolating data given on curvilinear grids

[Martin Schmidt <martin.schmidt@xxxxxxxxxxxxxxxxx>]

Hi Ferreters,

I am trying to use ASCAT data downloaded from

ftp://podaac.jpl.nasa.gov/pub/ocean_wind/ascat/preview/L2/12km/

The files are netcdf files in curvilinear form, lat, lon and data are 
defined on a common abstract (x,y) grid.
It does not have a geophysical meaning.

Ferret should be able to deal with such data. Indeed

use ascat_20100111_005400_metopa_16755_eps_o_125_1018_ovw.l2.nc
shade/d=1 WIND_SPEED, lon,lat; frame /file=shade.gif

works and gives figure shade.gif.It does not look really bad.

Now I want to put the data on a regular grid in geographic coordinates. 
The GFDL routine curv_to_rect_map
should be fine.
yes? define axis/x=0:360:1/modulo/units=degrees xax
yes? def axis/y=-85:85:1/units=degrees yax
yes? let lonlatout = y[gy=yax] + x[gx=xax]
yes? let lonin = lon[d=1]
yes? let latin = lat[d=1]
yes? let map = curv_to_rect_map ( lonin,latin,lonlatout,1)
yes? save/clobber/file=curv_map.nc  map
(1 degree resolution)

After some coffee:

yes? cancel var/all
yes? use curv_map.nc
yes? let wo=curv_to_rect(wind_SPEED[d=1], map[d=2])
yes? shade wo; frame /file=curv_to_rect.gif

Both figures are in the attachment. The results have something to do 
with each other but they are also pretty different.

So my question:
I do not really believe in the figure shade.gif. The reason is, that I 
may plot (lon,lat) pairs into
the figure (plot/vs/ov lon,lat. In shade.gif there remain broad bands of 
coloured areas, in curv_to_rect.gif most data are covered by the symbols.
So I guess, that the shade command implies some inappropriate 
interpolation.
Does anyone have experience with plotting and processing such curviliear 
data?

It is great to have the interpolation tool curv_to_rect. But is there a 
chance to optimize the code
for a speed up of the curv_to_rect_map routine? I would like to process 
years of data on a finer grid and it is very slow with 1 degree resolution.
Unfortunately the weights are changing for every file.

Is there another and faster way to do this with ferret?

Greetings,
Martin Schmidt

Attachment: curv_to_rect.gif
Description: GIF image

Attachment: shade.gif
Description: GIF image