tvrh2q(tvexpr,rhexpr)
 
Given virtual temperature and relative humidty, tvrh2q
returns specific humidity, q, in g/g. Specifically: 
tvexpr     
A GrADS expression that represents virtual temperature, in Kelvin. 
rhexpr     
A GrADS expression that represents relative humidty, in percent 
(values from 0 to 100). 
This function works only on gridded data.
tvrh2q
assumes that the Z coordinate system is pressure in mb. If Z is a varying
dimension, the pressure valid at each grid point is used. When Z is a
fixed dimension, the Z value from the current dimension environment is
used. 
Note that it is possible to provide values from an incorrect pressure level by overriding the current dimension environment:
In this case, the tvrh2q function assumes a pressure of
500mb, which is the current setting for the Z dimension
environment. However, tvexpr and
rhexpr are providing data from the 850mb level, 
so the function will produce incorrect results.