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# obs_op_adj_pdaf

The page document the user-supplied call-back routine `obs_op_adj_pdaf`

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The routine `obs_op_adj_pdaf`

(`U_obs_op_adj`

inside PDAF) is a call-back routine that has to be provided by the user.

The routine is used with the all 3D-Var methods. It has to perform the operation of the adjoint observation operator. It has to perform the operation of the adjoint observation operator. The operator acts on a vector in observation space that is provided as m_state_p. The resulting state vector has to be returned in `state_p`

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The interface is the following:

SUBROUTINE obs_op_adj_pdaf(step, dim_p, dim_obs_p, state_p, m_state_p) INTEGER, INTENT(in) :: step ! Current time step INTEGER, INTENT(in) :: dim_p ! PE-local dimension of state INTEGER, INTENT(in) :: dim_obs_p ! Dimension of observed state REAL, INTENT(in) :: m_state_p(dim_obs_p) ! PE-local observed state REAL, INTENT(out) :: state_p(dim_p) ! PE-local model state

Hints:

- For a model using domain decomposition, the operation is on the PE-local sub-domain of the model and has to provide the observed sub-state for the PE-local domain.
- If the observation operator involves a global operation, e.g. some global integration, while using domain-decomposition one has to gather the information from the other model domains using MPI communication.