ArrayDear Rob, nThanks for your response.nI don't know what's the mean of RHS and I cannot find it on the Custom Field Function Calculator plane. I need to define the U(z) in the below equation. As you can see, the U(z) is only the function of height(z) and the remaining symbols are constants. Could you please let me how to define the variable U(z)?nThanks,nDarry