[ferret_users] Replacing sigma pres with sigma ht

[Paul Young <paul.young@xxxxxxxxxxxxxxxx>]

[apologies if you get this twice, but I think I sent it to the wrong
address originally]

Hello all,

I would like to plot my atmospheric model data zonal mean using
sigma height co-ordinates instead of the sigma-pressure levels. If
possible, I would like to add a 2nd y-axis that has log pressures on it.

So far, I've produced a 4D array that contains the sigma height (km)
levels for each model point, converted from the sigma pres levels. This
allows me to re-grid my data onto height levels using ZAXREPLACE:

        !new sigma height axis
        DEFINE AXIS/Z=0:20:1/UNITS=km newz

        !get sigma heights
        LET sigma_ht = sigma_km[D=1]    
        
        !re-grid my data
        new_var = ZAXREPLACE(old_var,sigma_ht,newz)

        !plot Jan zonal mean
        FILL/L=1 new_var[x=@ave]

THE PROBLEM: Above works ok, but I would like to add data about the
level boundaries so that the plot starts at 0km.

I have tried using different definitions of the Z-AXIS:

        !Define sigma ht bounds axis
        DEFINE AXIS/UNITS=km/BOUNDS/DEPTH newz = \  
                {0.02,0.18,..etc..,37.75}, \
                {0.0,0.04, 0.04,0.31, ..etc.., 32.33,53.30}

but this plots upsidedown (doesn't work at all without \DEPTH). I can't
get the /EDGES qualifier to work either (by just providing the
coordinates of the level edges). 

Am I using the /BOUNDS (or \EDGES) AXIS definitions incorrectly?

BONUS PROBLEM: I can't get the second (right-hand) y-axis to display a
log (sigma) pressure coordinate at all! 

Hope this is clear. Thanks for any help, and apologies again if I am
just being stupid about it.

Cheers,

Paul