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# PDAF_assimilate_3dvar

This page documents the routine `PDAF_assimilate_3dvar`

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 Modification of the model code for the ensemble integration and its sub-page on 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 for using the parameterized 3D-Var is:

SUBROUTINE PDAF_assimilate_3dvar(U_collect_state, U_distribute_state, & U_init_dim_obs, U_obs_op, U_init_obs, U_prodRinvA, & U_cvt, U_cvt_adj, U_obs_op_lin, U_obs_op_adj, & U_prepoststep, U_next_observation, outflag)

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_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 ETKF.
- U_cvt: The name of the user-supplied routine that applies the control-vector transformation (square-root of the B-matrix) on some control vector to obtain a state vector.
- U_cvt_adj: The name of the user-supplied routine that applies the adjoint control-vector transformation (with square-root of the B-matrix) on some state vector to obtain the control vector.
- U_obs_op_lin: The name of the user-supplied routine that acts as the linearized observation operator on some state vector
- U_obs_op_adj: The name of the user-supplied routine that acts as the adjoint observation operator on some state vector
- 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`

: The integer status flag. It is zero, if the routine is exited without errors.

The user-supplied call-back routines are described on the page on [ImplementAnalysis_3DVar_classical implementing the analysis step of 3D-Var].

It is recommended that the value of `status_pdaf`

is checked in the program after PDAF_assimilate_3dvar is executed. Only if its value is 0 the initialization was successful.