extracting non-uniform pressure levels

[Mike Fiorino <fiorino@xxxxxxxx>]

Dear Ferreteers,

I'm a ferret newbie using the opendap feature to pull individual 
pressure levels and times from an ncep nomads server of global forecast 
system (atmosphere) model runs, e.g.,

yes?  ! NOAA/PMEL TMAP
yes?  ! FERRET v5.60
yes?  ! Linux(g77) 2.4.18 - 04/09/04
yes?  ! 19-Apr-04 17:03
yes? use 
"http://nomad2.ncep.noaa.gov:9090/dods/gfs/archive/gfs20040228/gfs_00z";
yes? contour/lev=(240,300,5) tmp[z=1000,l=1]
yes? contour tmp[z=500,l=1]
yes? list/file="/tmp/t.nc"/form="cdf"/clobber tmp[z=1000,l=1]
LISTing to file /tmp/t.nc
yes? list/file="/tmp/t.nc"/form="cdf"/append tmp[z=500,l=1]
LISTing to file /tmp/t.nc
**TMAP ERR: Requested data range is outside of data set limits
            Expected: TMP[K= 14 : 14 ] Found: TMP[K= 26 : 26 ]
yes? list/file="/tmp/t.nc"/form="cdf"/append tmp[z=1000,l=2]
LISTing to file /tmp/t.nc

I can plot the 1000 (z=1000) and 500 (z=500) mb temperatures 
(variable='tmp') at the first time (l=1) and dump the 1000 mb to 
/tmp/t.nc, but not  500.  I can dump the next time (l=2) and ncdump 
shows that two times have been written.  I can open the file in in both 
ferret and GrADS and make identical plots so the data are OK.

I know this is related to the z axis and how time is the only 
"unlimited" dimension, the question is how to define an axis of 
arbitrary pressure levels that decrease with k.  After searching the 
mail archive and the docs, I tried defining the levels in a text file, 
reading in and setting a z axis, but ferret complained about the levels 
being non monotonic which is true, but what I want.  I also tried 
dumping as "stream" but ferret doesn't allow append for binary output...

Thanks in advance for any help. /R Mike