= obs_op_adj_pdaf = 

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

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`.

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.