passing one directional "diffusion" to UDS solver
 

Hi experts
In a 3D (x,y,z) domain I need to solve a scalar equation using UDS. The scalar, f, varies only in the (z) direction, namely, df/dz=C(z)*f, where C(z) is a function which I compute in the UDF.
I thought of solving this using the Fluent UDS solver with convective flux=0, diffusion coefficient=1 and a suitable source term S.

the UDS equation will look like this: -d/dxi(df/dxi)=S

Given the definition of my scalar f, it follows that: S=-grad( C(z)*f )[z].
Here is the problem: the "diffusion" is valid only for (z), thus, both d/dx and d/dy should=0, i.e. no source for this directions.

How to pass this information to the UDS solver?

Thank you