Implementing the Analysis Step specific for global filters and LEnKF

Overview

This page describes the recommended implementation of the analysis step specific for global ensemble filters and LEnKF (Thus the filters: ESTKF, ETKF, SEIK, EnKF, NETF, PF, and LEnKF).

We recommend to only use this variant if one only runs global filters. As soon as one also intends to use local filters, like the LESTKF or LETKF, we recommend to use the implementation using the using the universal routines of the PDAF3 interface.

The PDAF3 routines for the analysis step only distinguish whether the online or offline mode is used.

For the analysis step we need different operations related to the observations. These operations are requested by PDAF by call-back routines supplied by the user and provided in the observation modules using PDAF-OMI. The names of the routines that are provided by the user are specified in the call to the assimilation routine PDAF3_assimilate.

For completeness we discuss here all user-supplied routines that are specified as arguments in the assimilation routines.

Assimilation routines

PDAF3_assimilate_global

This routine is used both in the fully-parallel and the flexible implementation variants of the data assimilation system. (See the page Modification of the model code for the ensemble integration for these variants)

Here, we list the full interface of the routine. Subsequently, the user-supplied routines specified in the call are explained.

The unversal interface is the following:

  SUBROUTINE PDAF3_assimilate_global(collect_state_pdaf, distribute_state_pdaf, &
                init_dim_obs_pdafomi, obs_op_pdafomi, &
                prepoststep_pdaf, next_observation_pdaf, status)

with the following arguments:

• Routines to transfer between model fields and state vector:
  • collect_state_pdaf:
    The name of the user-supplied routine that initializes a state vector from the array holding the ensemble of model states from the model fields.
  • distribute_state_pdaf:
    The name of a user supplied routine that initializes the model fields from the array holding the ensemble of model state vectors. (The same routine is also used in PDAF_init_forecast.)
• Observation routines using PDAF-OMI:
  • init_dim_obs_pdafomi:
    The name of the user-supplied routine that initializes the observation information and provides the size of observation vector
  • obs_op_pdafomi:
    The name of the user-supplied routine that acts as the observation operator on some state vector
• Prepoststep and initialization for next forecast phase
  • prepoststep_pdaf:
    The name of the pre/poststep routine as in PDAF_init_forecast. (The same routine is also used in PDAF_init_forecast.)
  • next_observation_pdaf:
    The name of a user supplied routine that initializes the variables nsteps, timenow, and doexit. (The same routine is also used in PDAF_init_forecast.)
• Status flag
  • status:
    The integer status flag. It is zero, if the routine is exited without errors.

Note:

• The order of the routine names does not show the order in which these routines are executed. See the section on the order of the execution at the bottom of this page.

PDAF3_assim_offline_global

For the offline mode of PDAF, the routine PDAF3_assim_offline_global is used to perform the analysis step. The interface of the routine is identical with that of PDAF3_assimilate_global, except that the user-supplied routines U_distribute_state, U_collect_state and U_next_observation are missing.

The interface is:

  SUBROUTINE PDAF3_assim_offline_global( &
                init_dim_obs_pdafomi, obs_op_pdafomi, &
                prepoststep_pdaf, status)

PDAF3_put_state_global

This routine exists for backward-compatibility. In implementations that were done before the release of PDAF V3.0, a 'put_state' routine was used for the flexible parallelization variant and for the offline mode. When the 'flexible' implementation variant is chosen for the assimilation system, the routine. The routine PDAF3_put_state_global allows to port such implementations to the PDAF3 interface with minimal changes. The interface of the routine is identical with that of PDAF3_assimilate_global, except that the user-supplied routines U_distribute_state and U_next_observation are missing.

The interface when using one of the local filters is the following:

  SUBROUTINE PDAF3_put_state_global(collect_state_pdaf, &
                init_dim_obs_pdafomi, obs_op_pdafomi, &
                prepoststep_pdaf, status)

User-supplied routines

Here, all user-supplied routines are described that are required in the call to PDAF3_assimilate_global, PDAF3_assim_offline_global or PDAF3_put_state_global. For some of the generic routines, we link to the page on modifying the model code for the ensemble integration.

In the subroutine interfaces some variables appear with the suffix _p (short for 'process'). This suffix indicates that the variable is particular to a model sub-domain, if a domain decomposed model is used. In addition, there will be variables with suffix _l (indicating 'local').

Call-back routines that end on _pdaf are regular call-back routines from the core part of PDAF, while call-back routines that end on _pdafomi handle observations within PDAF-OMI.

collect_state_pdaf (collect_state_pdaf.F90)

This routine is independent of the filter algorithm used.

See the page on modifying the model code for the ensemble integration for the description of this routine.

distribute_state_pdaf (distribute_state_pdaf.F90)

This routine is independent of the filter algorithm used.

See the page on modifying the model code for the ensemble integration for the description of this routine.

init_dim_obs_pdafomi (callback_obs_pdafomi.F90)

This is a call-back routine initializing the observation information. The routine just calls a routine from the observation module for each observation type.

See the documentation on callback_obs_pdafomi.F90 for more information.

obs_op_pdafomi (callback_obs_pdafomi.F90)

This is a call-back routine applying the observation operator to the state vector. The routine calls a routine from the observation module for each observation type.

See the documentation on callback_obs_pdafomi.F90 for more information.

prepoststep_pdaf (prepoststep_ens_pdaf.F90)

The routine has already been described for modifying the model for the ensemble integration and for inserting the analysis step.

See the page on modifying the model code for the ensemble integration for the description of this routine.

next_observation_pdaf (next_observation_pdaf.F90)

This routine is independent of the filter algorithm used.

See the page on modifying the model code for the ensemble integration for the description of this routine.

Execution order of user-supplied routines

The user-supplied routines are executed in the order listed below. The order can be important as some routines can perform preparatory work for routines executed later on during the analysis.

Before the analysis step is called the following is executed by PDAF3_assimilate_global and PDAF3_put_state_global:

1. collect_state_pdaf (called once for each ensemble member)
2. prepoststep_pdaf (Call to act on the forecast ensemble, called with negative value of the time step)

At the analysis time, the observations are initialized by the routines:

1. init_dim_obs_pdafomi
2. obs_op_pdafomi (Called dim_ens times; once for each ensemble member)

Now the analysis step is entered. Afterwards, it is executed:

1. prepoststep_pdaf (Call to act on the analysis ensemble, called with (positive) value of the time step)

In case of the routine PDAF3_assimilate_global, the following routines are executed after the analysis step:

1. distribute_state_pdaf
2. next_observation_pdaf