= prepoststep_pdaf =

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

The routine `prepoststep_pdaf` (called `U_prepoststep` in the PDAF core routines) is a call-back routine that has to be provided by the user.  
`prepoststep_pdaf` is called by `PDAF_get_state`, `PDAF_put_state_X`, and `PDAF_assimilate_X` with 'X' being the name of a filter method. The routine is called at the initial time and after a forecast (directly before computing the filter analysis step) and after the analysis step. The purpose of the routine is to give the user access to the forecast and the analysis ensembles. Typically operations that are performed in `prepoststep_pdaf` are to compute the estimated RMS errors form the ensemble and to write e.g. the state estimate (i.e. the ensemble mean state). In case of the offline mode, one will also write the model restart files in `prepoststep_pdaf`.

The interface is the following:
{{{
SUBROUTINE prepoststep_pdaf(step, dim_p, dim_ens, dim_ens_p, dim_obs_p, &
                       state_p, Uinv, ens_p, flag)
}}}
with

  INTEGER, INTENT(in) :: step        ! Current time step
                         ! (When the routine is called before the analysis -step is provided.)
  INTEGER, INTENT(in) :: dim_p       ! PE-local state dimension
  INTEGER, INTENT(in) :: dim_ens     ! Size of state ensemble
  INTEGER, INTENT(in) :: dim_ens_p   ! PE-local size of ensemble
  INTEGER, INTENT(in) :: dim_obs_p   ! PE-local dimension of observation vector
  REAL, INTENT(inout) :: state_p(dim_p) ! PE-local forecast/analysis state
                                     ! The array 'state_p' is not generally not initialized in the case of SEIK/EnKF/ETKF.
                                     ! It can be used freely in this routine.
  REAL, INTENT(inout) :: Uinv(dim_ens-1, dim_ens-1) ! Inverse of matrix U
  REAL, INTENT(inout) :: ens_p(dim_p, dim_ens)      ! PE-local state ensemble
  INTEGER, INTENT(in) :: flag        ! PDAF status flag


The routine `U_prepoststep` is called once at the beginning of the assimilation process. In addition, it is called during the assimilation cycles before the analysis step and after the ensemble transformation. The routine is called by all filter processes (that is `filterpe=1`).

The routine provides for the user the full access to the ensemble of model states. Thus, user-controlled pre- and post-step operations can be performed.  For example the forecast and the analysis states and ensemble covariance matrix can be analyzed, e.g. by computing the estimated variances. If the smoother is used, also the smoothed ensembles can be analyzed. In addition, the estimates can be written to disk. 

Hint:
 * If a user considers to perform adjustments to the estimates (e.g. for balances), this routine is the right place for it.
 * Only for the SEEK filter the state vector (`state_p`) is initialized. For all other filters, the array is allocated, but it can be used freely during the execution of `U_prepoststep`.
 * The interface through which `U_prepoststep` is called does not include the array of smoothed ensembles. In order to access the smoother ensemble array one has to set a pointer to it using a call to the routine `PDAF_get_smootherens` (see page on [AuxiliaryRoutines auxiliary routines])