= PDAF_diag_rsmd_nompi = 

This page documents the routine `PDAF_diag_rmsd_nompi` of PDAF, which was introduced with PDAF V3.0.

This routine computes root mean square difference (RMSD) beween two input vectors. These could be the ensemble mean state and a reference state.

The routine can be called in the pre/poststep routine of PDAF both before and after the analysis step to compute the ensemble statistics. 

This variant only computes the standard deviation for the provided state vectors `stateA` and `stateB` without considering parallelization. Thus, with a domain-decomposed parallel model, the value of the RMSD does not consider the full domain. The variant taking into account parallelization is [wiki:PDAF_diag_rmsd].

The interface is: 
{{{
SUBROUTINE PDAF_diag_rmsd_nompi(dim_p, stateA_p, stateB_p, &
     rmsd_p, status)

  INTEGER, INTENT(in) :: dim_p                 !< state dimension
  REAL, INTENT(in)    :: stateA_p(dim_p)       !< State vector A
  REAL, INTENT(in)    :: stateB_p(dim_p)       !< State vector B
  REAL, INTENT(out)   :: rmsd_p                !< RSMD
  INTEGER, INTENT(out) :: status               !< Status flag (0=success)
}}}

**Note:**
 * The routine is used to compute the RMSD with regard to some reference state. To compute the ensemble-sampled standard deviation, i.e. the estimated RMSD from the data assimilation, there are the routines [wiki:PDAF_diag_stddev_nompi] and [wiki:PDAF_diag_stddev].
 * We recommend to use this routine only in cases where the MPI parallelization is not initialized, e.g. in separate post-analysis programs. In assimilation programs, where the parallelization is initialized for PDAF, we recommend to use the routine `PDAF_diag_rmsd` for better overall compatibility.
 * The routine does not perform any MPI operations and can also be used if PDAF was not initialized by calling `PDAF_init`.