Re: [ferret_users] Sigma to Pressure coordinates

[Paul Young <paul.young@xxxxxxxxxxxxxxxx>]

Hi,

You'll need to use the ZAXREPLACE function. I do something similar to
what you're after in the script below. Note, to use this script you'll
need an array of the actual or sigma pressure on the model grid (in
pascals). If you use the 'actual' pressure, you'll get the true height
of the model level, but this may make zonal mean plots look a little
funny as they'll be places where the zonally average height is
significantly higher than 0km (e.g. South Pole). Therefore, I tend to
use a 4D array of sigma pressure levels (i.e. the same pressure column
for each location) to make the zonal mean plots. I'm sure there might be
a simpler method for this though....

PROBLEM (for someone else?): 
---------------------------
I can't get the BOUNDS/ qualifier to work when I define the z-axis, so
the first level is plotted slightly above the surface. This is because
my 4D pressure field gives the pressure of the middle of the model
level. I've tried this:

define axis/z=0:20/units=km/bounds kmz = \ 
{0,0.04, 0.04,0.31, 0.31,0.70, 0.70,1.26, 1.26,2.01, 2.01,3.02, \
3.02,4.20, 4.20,5.46, 5.46,6.71, 6.71,7.91, 7.91,9.09, 9.09,10.50, \
10.50,12.27, 12.27,14.63, 14.63,18.22, 18.22,22.62, 22.62,27.47, \
27.47,32.33, 32.33,53.30}

..but with no joy!

----------------
! height.jnl
!
!   Args:   1 = variable to regrid
!	    2 = pressure field
!           3 = zmin (e.g. 0km)
!           4 = zmax (e.g. 20km)
!           5 = name of new regridded variable
!
! (credit to Glenn Carver, CAS, U. Cambridge)
!
\cancel mode verify

let/quiet zmin = $3
let/quiet zmax = $4

! set increment. 
let/quiet zinc = 0.1

define axis/z=`zmin`:`zmax`:`zinc`/units=km kmz

!Formula to convert pressure (in Pascals) to height (in km)
let conv2ht = (44330.77*(1.-($2/101325.0)^0.19026))/1000.0

let $5 = zaxreplace($1,conv2ht,z[gz=kmz])

message/continue Interpolated variable is now in x_km
message/continue Regular height (km) axis is in variable kmz

! end of script

set mode verify






On Wed, 2007-04-25 at 11:52, Marshall L. Mdoka wrote:
> Hi All,
> 
> I am new to Ferret and was advised that I can try to use it to solve a 
> problem I have. I am intending to analyse some modelling results I have. 
> There are CTL files which are in sigma coordinates and I would like to 
> convert them pressure coordinates for all the atmospheric levels. May 
> you please help me come up with a script.
> 
> Thanking you in advance.
> 
> Marshall