= U_init_n_domains =

The page document the user-supplied call-back routine `U_init_n_domains`.

The routine `U_init_n_domains` is a call-back routine that has to be provided by the user. In the simplified interface the predefined name of the routine is `init_n_domains_pdaf`, but in the full interface, the user can choose the name of the routine.
This routine is used by all filter algorithms with domain-localization (LSEIK, LETKF, LESTKF) and is independent of the particular algorithm. 
The routine is called during the analysis step before the loop over the local analysis domains is entered. It has to provide the number of local analysis domains. In case of a domain-decomposed model the number of local analysis domain for the model sub-domain of the calling process has to be initialized. 

The interface is the following:
{{{
SUBROUTINE init_n_domains(step, n_domains_p)
}}}
with
 * `step` : `integer, intent(in)`[[BR]] Current time step
 * `n_domains_p`: `integer, intent(out)`[[BR]] Number of analysis domains for process-local model sub-domain

Hints:
 * For a full 3-dimensional localization, a local analysis domain can be a single grid point. I nhtis case `n_domains_p` would be the number of grid points in a process-local sub-domain of the model grid.
 * If the localization is only performed horizontally (a frequent choice with ocean circulation models assimilating satellite data), the local analysis domains can be single vertical columns of the model grid. In this case, `n_domains_p` is the number of vertical columns in the local model sub-domain.