= PDAF3_assimilate_lenkf =

This page documents the routine `PDAF3_assimilate_lenkf` of PDAF. The routine is part of the advanced PDAF3 interface that was introduced with PDAF V3.0. 

|| **Note:** This routine exists for backward-compatibility for the LEnKF with PDAF V2.3, in particular for existing implementations using `localize_covar_pdafomi`. We recommend to use the new PDAF3 implementation variant that avoid the need to implement the additional call-back routine `localize_covar_pdafomi`. ||

The routine is used to execute the analysis step of the localized EnKF (LEnKF). The routine uses the functionalities provided by PDAF-OMI to yield a minimum number of arguments.

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

The general aspects of the filter specific routines `PDAF3_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 both the fully-parallel and the flexible parallelization variant of the data assimilation system. 

Here, we list the full interface of the routine. 

The interface is:
{{{
  SUBROUTINE PDAF3_assimilate_lenkf(U_collect_state, U_distribute_state, &
                                 U_init_dim_obs_pdafomi, U_obs_op_pdafomi, &
                                 U_localize_covar_pdafomi, &
                                 U_prepoststep, U_next_observation, status)
}}}
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_pdafomi`: The name of the user-supplied routine that initializes the observation information and provides the size of observation vector
 * `U_obs_op_pdafomi`: The name of the user-supplied routine that acts as the observation operator on some state vector
 * `U_localize_covar_pdafomi`: The name of the user-supplied routine that Applies covariance localization to the matrices HP and HPH^T^
 * `U_prepoststep`: The name of the user-supplied pre/poststep routine as in `PDAF_get_state` (Note that the place of `U_prepoststep` has changed compared to the previous routine `PDAFomi_assimilate_lenkf`)
 * `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`: The integer status flag. It is zero, if `PDAFomi_assimilate_lenkf` is exited without errors. 

For the EnKF, the user-supplied routines are essentially executed in the order they are listed in the interface to `PDAFomi_assimilate_lenkf`. 

The user-supplied call-back routines are described on the page on [ImplementAnalysislenkf implementing the analysis step of the local EnKF].

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