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# U_obs_op_lin

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

.

The routine `U_obs_op_lin`

is a call-back routine that has to be provided by the user. In the simplified interface the predefined name of the routine is `obs_op_lin_pdaf`

, but in the full interface, the user can choose the name of the routine.

The routine is used with the all 3D-Var methods. It has to perform the operation of the linearized observation operator acting on a state vector increment that is provided as `state_p`

. The observed state has to be returned in `m_state_p`

.

The interface is the following:

SUBROUTINE U_obs_op_lin(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) :: state_p(dim_p) ! PE-local model state REAL, INTENT(out) :: m_state_p(dim_obs_p) ! PE-local observed state

The routine is called during the analysis step. It has to perform the operation of the linearized observation operator acting on a state vector increment that is provided as `state_p`

. The observed state has to be returned in `m_state_p`

.

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.