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