# Skilled (#3) - [LIM3] Question about diffusion operator (#13) - Message List

Dear NEMO/LIM developers

I would like to have some explanation about diffusion operator in LIM3, and the way it is calculated

I cannot figure out what equations is actually solved in lim_hdf routine.

the advection/diffusion equation for a generic ice component is something the like ( apart from scale factors and other terms in the equation):

d X d t = - div ( X . u ) + D laplacian ( X ).

as far as I understood, first transport is solved, namely

d X / d t = - div ( X . u )

leading to a value of X at next time step, X^{(n+1)}

then the diffusion is added,

X^{(n+1) }=X^{(n+1)} + D laplacian ( X^{(n+1)} )

where X^{(n+1)} Is a known quantity. So I cannot figure out why an iterative method

is used.

The question is, what am I missing?

Thanks Ivano