Adding a Filter Algorithm to PDAF

This text describes the implementation strategy and internal structure of PDAF valid for version 1.7.0 and later. If you use an earlier version of PDAF, we recommend to update to the most recent version. In this text, we assume that the reader is already familiar with PDAF to the extend of experience with the implementation of a model with PDAF that is described in the implementation guide.

The internal structure of PDAF is organized into a generic part providing the infrastructure to perform ensemble forecasts and filter analysis steps. The generic part is independent of the particular filter algorithm and only distinguishes between ensemble based filters (all filters except SEEK) and mode based filters (currently only SEEK). The filter-specific routines are called through an internal interface.

Each filter algorithm consists of a minimum of 6 routines. All routines are called trough the internal interface of PDAF, except for the "put state" routine (PDAF_put_state_X where X is the name of the selected filter).

Internal Interface of PDAF

The internal interface of PDAF is stcructured as follows ('X' is the name of the filter algorithm):

The routine PDAF_init calls

• PDAF_init_filters - interface routine to PDAF_X_init
  
  • PDAF_X_init performs the filter-specific initialization of parameters and calls the user-supplied routine that initializes the initial ensemble of model states.
• PDAF_alloc_filters - interface routine to PDAF_X_alloc
  
  • PDAF_X_alloc allocates the filter-specific arrays.
• PDAF_options_filters - interface routine to PDAF_X_options
  • PDAF_X_options is an optional routine. Its purpose is to display an overview of available options for the filter algorithm.

The routine PDAF_get_state calls

• PDAF_initinfo_filters - interface routine to PDAF_X_initinfo
  • PDAF_X_initinfo displays information on the current configuration of the filter algorithm.

The routine PDAF_print_info only includes the interface to PDAF_X_memtime

• PDAF_X_memtime displays information on the execution duration of the different parts of the assimilation process as well as information on the amount of allocated memory. This functionality is optional.

The routine PDAF_put_state_X is called directly from the model code. There is a separate routine for each filter. The standard implementation calls

• PDAF_X_initinfo
  • This is the same routine that is called by PDAF_get_state. Here, it is only called, if the offline mode of PDAF is used.
• PDAF_X_update
  • This routine contains the actual assimilation or analysis step of the filter algorithm.

When PDAF_init is called, the filter algorithm is chosen by its ID number. Internally to PDAF, each filter is identified by a string that is defined in PDAF_init_filters. The interface routines have a very simple structure. In general, they select the filter-specific routine based on the string identifying the filters. When a filter algorithm is added a line for the corresponding filter-specific routine has to be inserted to each of the interface routines.