Response to tiegcm action item "Geopotential Heights and g(z)":

There are three related fields in the model:

Z:  Geopotential height as calculated in the model, using constant
    gravity (870 cm/s^2). 

ZG: Geopotential height calculated with varying gravity by sub calczg
    (addiag.F).

ZM: Geopotential height in magnetic coordinates transposed from 
    geographic Z (see sub transf in dynamo.F).

All three fields are always on secondary histories. Only Z is saved
on primary histories. All three fields are calculated at grid 
interfaces.

Z is read by the processor and used for height-interpolation, etc.
If namelist read parameter modelhts is set to 0 in the processor,
it will recalculate it using varying gravity (subs calchts and glatf
in proclat.F).