[gradsusr] Suggestions for calculating a -20C Height Field?

[L.B.]

Anyone have any ideas for calculating a field which contains the -20C
isotherm heights in meters?

I have thought of a few ways, but they are only giving me pressure levels.
I tried using the fndlvl function:

define m20lev = fndlvl (tmpprs, const(tmpprs,253.15), z=1, z=80)

This just gives me the pressure levels of the -20C isotherm. I suppose I
could use teh hypsometric equation after using the fndlvl function.

I also thought of using the zinterp.gs script, but again, this wants me to
input the pressure level to interpolate to heights. My pressure levels
vary, so this will not work. I fell like I'm missing something really
obvious, but I can't put my finger on it.
*-------*
William (L.B.) LaForce
Meteorologist
www.tornadoinsanity.com
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