= PDAF_assimilate_estkf = 

This page documents the routine `PDAF_assimilate_estkf` of PDAF. 

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

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]. The routine is used in the fully-parallel implementation variant of the data assimilation system. When the 'flexible' implementation variant, the routines `PDAF_put_state_*' are used. 

The interface when using the ESTKF is the following:
{{{
  SUBROUTINE PDAF_assimilate_estkf(U_collect_state, U_distribute_state, U_init_dim_obs, &
                                 U_obs_op, U_init_obs, U_prepoststep, U_prodRinvA, &
                                 U_init_obsvar, 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_init_dim_obs`: The name of the user-supplied routine that provides the size of observation vector
 * `U_obs_op`: The name of the user-supplied routine that acts as the observation operator on some state vector
 * `U_init_obs`: The name of the user-supplied routine that initializes the vector of observations
 * `U_prepoststep`: The name of the pre/poststep routine as in `PDAF_get_state`
 * `U_prodRinvA`: The name of the user-supplied routine that computes the product of the inverse of the observation error covariance matrix with some matrix provided to the routine by PDAF. This operation occurs during the analysis step of the SEIK, ETKF, and ESTKF algorithms.
 * `U_init_obsvar`: The name of the user-supplied routine that provides a mean observation error variance to PDAF (This routine will only be executed, if an adaptive forgetting factor is used)
 * `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_assimilate_estkf` is exited without errors.

The user-supplied call-back routines are described on the page on [ImplementAnalysisestkf implementing the analysis step of the ESTKF].

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

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