= PDAF_assimilate_prepost =

This page documents the routine `PDAF_assimilate_prepost` of PDAF. It is basically an alias for the routine `PDAF_prepost` and we generally recommend to rather use [wiki:PDAF_prepost] avoid the naming confusion that `PDAF_assimilate_prepost` holds 'assimilate' in its name, but is not performing assimilation.

The routine is typically called in `assimilate_pdaf` or directly in the model code.

The routine `PDAF_assimilate_prepost` is an alternative to the routines `PDAF_assimilate_X` where 'X' is the name of a filter method. 
The routine is used with the 'fully-parallel' implementation variant of PDAF. The purpose of `PDAF_assimilate_prepost` is to collect the ensemble information on the processes usually doing the filter analysis. However, in the case of `PDAF_assimilate_prepost`, no filter analysis step is performed when the ensemble integration is complete, but only the user-supplied pre/poststep routine is called to allow the user to analyze the forecast ensemble.

The general aspects of the filter specific routines `PDAF_assimilate_*` are described on the page [ModifyModelforEnsembleIntegration Modification of the model code for the ensemble integration] and its sub-page on [InsertAnalysisStep inserting the analysis step]. For the 'flexible' implementation variant the alternative routine `PDAF_put_state_prepost` is used.

The interface  is the following:
{{{
  SUBROUTINE PDAF_assimilate_prepost(U_collect_state, U_distribute_state, U_prepoststep, &
                                 U_next_observation, status_pdaf)
}}}
with the following arguments:
 * `U_collect_state`: The name of the user-supplied routine that initializes a state vector from the array holding the ensemble of model states from the model fields. This is basically the inverse operation to `U_distribute_state` used in `PDAF_get_state` as well as here. 
 * `U_distribute_state`:  The name of a user supplied routine that initializes the model fields from the array holding the ensemble of model state vectors.
 * `U_prepoststep`: The name of the pre/poststep routine as in `PDAF_get_state`
 * `U_next_observation`: The name of a user supplied routine that initializes the variables `nsteps`, `timenow`, and `doexit`. The same routine is also used in `PDAF_get_state`.
 * `status_pdaf`: The integer status flag. It is zero, if `PDAF_prepoststep` is exited without errors.


The user-supplied call-back routines are, for example, described on the page on [ImplementAnalysisestkf implementing the analysis step of the ESKTF algorithm].

It is recommended that the value of `status_pdaf` is checked in the program after `PDAF_prepoststep` is executed. Only if its value is 0 the initialization was successful.

PDAF also has a [PdafSimplifiedInterface Simplified Interface] providing the routine `PDAF_prepoststep_si`. In the simplified interface, the name of the user-supplied routines have predefined names and do not appear in the call to `PDAF_prepoststep_si`.  More information on the pre-defined names is provided, e.g., in the [ImplementAnalysisestkf page on implementing the analysis step of the ESTKF algorithm].