U_cvt

The page document the user-supplied call-back routine U_cvt.

The routine U_cvt is a call-back routine that has to be provided by the user. In the simplified interface the predefined name of the routine is cvt_pdaf, but in the full interface, the user can choose the name of the routine.

The routine is used with all 3D-Var methods. The routine is called during the iterative optimization. It has to perform the control vector transform (preconditioning) which is usually a multiplication with the square-root of the background covariance matrix B. The routine is provided with a control vector and has to return a state vector. The routine allows a user to implement the control vector transform in form of covariance operators.

The interface is the following:

SUBROUTINE U_cvt(iter, dim_p, dim_cvec, cv_p, Vv_p)

  INTEGER, INTENT(in) :: iter           ! Iteration of optimization
  INTEGER, INTENT(in) :: dim_p          ! PE-local observation dimension
  INTEGER, INTENT(in) :: dim_cvec       ! Dimension of control vector
  REAL, INTENT(in)    :: cv_p(dim_cvec) ! PE-local control vector
  REAL, INTENT(inout) :: Vv_p(dim_p)    ! PE-local result vector (state vector increment)

Hints

• If the state vector is decomposed in case of parallelization one needs to take care that the application of the trasformation is complete. This usually requries a comminucation with MPI_Allreduce to obtain a global sun.