Initialization of PDAF and the ensemble by PDAF_init

Overview

After the adaption of the parallelization, the initialization of PDAF has to be implemented. PDAF_internally, the initialization is performed by the routine PDAF_init. Typically, the initializations of all variables required for the call to PDAF_init can be collected into a single subroutine. This implementation strategy is useful, as only a single addition subroutine call has to be added to the model source code. In the example in testsuite/src/dummymodel_1D the routine in the file init_pdaf.F90 shows this strategy.

Inserting init_pdaf

The right place to insert a routine init_pdaf to the model code is in between the initialization part of the model and its time stepping loop. In the routine a number of variables have to be defined that are used in the call to PDAF_init as described in 'Required variables for `PDAF_init`'. (Please note: All names of subroutines that start with PDAF_ are core routines of PDAF, while subroutines whose name end with _pdaf are generally user-supplied interface routines) There are also a few variables that are initialized in init_pdaf but not used in the call to PDAF_init. These are variables that are specific for the data assimilation system, but only shared in between the user-supplied routines. For the example case, these variables are described in 'Other variables for the assimilation'

The example implementation allows to parse all variables through a command line parser. This method provides a convenient way to define an experiment and could also be used for other models.

Required arguments of PDAF_init

The call to PDAF_init has the following structure:

CALL PDAF_init(filtertype, subtype, step_null, &
               filter_param_i, length_filter_param_i, &
               filter_param_r, length_filter_param_r, &
               COMM_model, COMM_filter, COMM_couple, &
               task_id, n_modeltasks, filterpe, &
               U_ensemble_init, screen, status_pdaf)

The required variables are the following:

• filtertype: An integer defining the type of filter algorithm. Available are
  • 0: SEEK
  • 1: SEIK
  • 2: EnKF
  • 3: LSEIK
  • 4: ETKF
  • 5: LETKF
• subtype: An integer defining the sub-type of the filter algorithm (see example code for choices)
• step_null: An integer defining the initial time step. For some cases it can use useful to set step_null larger to 0.
• filter_param_i: Integer array collecting several variables for PDAF. The first two variables are mandatory and equal for all filters. Further variables are optional (see example code). The mandatory variables are in the following order:
  • The local state dimension for the current process.
  • The ensemble size.
• length_filter_param_i: An Integer defining the length of the array filter_param_i. The entries in the array are parsed up to this index.
• filter_param_r: Array of reals collecting flowting point variables for PDAF. The first variable is mandatory and equal for all filters. Further variables are optional (see example code). The mandatory variable is:
  • The value of the forgetting factor (required to be larger than zero)
• length_filter_param_r: An Integer defining the length of the array filter_param_r. The entries in the array are parsed up to this index.
• COMM_model: The communicator variable COMM_model as initialized by init_parallel_pdaf. If the model-communicator is named differently in the actual program, the name has to be adapted
• COMM_filter: The communicator variable COMM_filter as initialized by init_parallel_pdaf.
• COMM_couple: The communicator variable COMM_couple as initialized by init_parallel_pdaf.
• task_id: The index of the model tasks as initialized by init_parallel_pdaf.
• n_modeltasks: The number of model tasks as defined before the call to init_parallel_pdaf.
• filterpe: The flag showing if a process belongs to COMM_filter as initialized by init_parallel_pdaf.
• U_ensemble_init: The name of the user-supplied routine that is called by PDAF_init to initialize the ensemble of model states. (See 'User-supplied routine `U_ensemble_init`'
• screen: An integer defining whether information output is written to the screen (i.e. standard output). The following choices are available:
  • 0: quite mode - no information is displayed.
  • 1: Display standard information (recommended)
  • 2: Display detailed information for debugging
• status_pdaf: An integer used as status flag of PDAF. If status_pdaf is zero upon exit from PDAF_init the initialization was successful. An error occurred for non-zero values. (The error codes are documented in the routine PDAF_init.)

Other variables for the assimilation

The routine init_pdaf also initializes several variables that are not used to call PDAF_init.

The are, for example the size of the ensemble and the type of filter algorithm. The example implementation explains the full set of variables. The example also initializes a few variables that are not handed over to PDAF_init, like delt_obs. This variable is the number of time steps between sucessive analysis steps. It is meaningful to define it at this central position. Of course, this definition has to be adapted to the particluar model used.

User-supplied routine U_ensemble_init

The user-supplied routine for the ensemble initialization is called through a defined interface as is visible in the ensemble implementation. The routine is called by all MPI processes that compute the filter analysis step (i.e. those for which 'filterpe' is set to true. In the standard configuration of init_parallel_pdaf these are all processes of the first model task (i.e. task_id=1). For the ensemble based filter (that is, not for the SEEK filter), the routine has to initialize the ensemble of model states. If domain decomposition is used, the full ensemble for the local sub-domain of the MPI process has to be initialized. (In case of the SEEK filter, the initial state estimate as well as decomposed covariance matrix in form of the the array of EOF modes and the inverse of matrix U have to be initialized.) The ensemble initialization routine is only called if the internal error flag of PDAF_init is zero. The flag is provided to the ensemble initialization routine. The user can set its value depending on the success of the ensemble initialization. Preferably, values above 102 should be used for failures to avoid conflict with the error codes defined within PDAF_init.