PDAF-OMI Observation Modules

The implementation of the observations with OMI is done in observation modules (obs-modules). For each observation type a separate module should be created.

Each obs-module contains four routines:

• init_dim_obs initializes all variables holding the information about one observation type. The information about the observation type is stored in a data structure (Fortran derived type).
• obs_op applies the observation operator to a state vector. One can call an observation operator routine provided by PDAF, or one can to implement a new operator.
• init_dim_obs_l calls a PDAF-OMI routine to initialize the observation information corresponding to a local analysis domain. One can set localization parameters, liek the localization radius, for each observation type.
• localize_covar calls a PDAF-OMI routine to apply covariance localization. One can set localization parameters, like the localization radius, for each observation type.

The template file obs_TYPE_pdafomi_TEMPLATE.F90 shows the different steps needed when implementing these routines. The main work is to implement init_dim_obs, while the other routines mainly call a subroutine provided by PDAF-OMI.

In the obs-module the subroutines are named according to the observation type. The template file uses generic names which can be replaced by the user. Having distinct names for each observation type is relevant to include the subroutine from the module in the call-back routine with ‘use’. In the header of each obs-module, the user can declare further variables, e.g. assim_TYPE as a flag to control whether the observation type should be assimilated.

obs_f data type

To ensure the functionality within each obs-module, we rely on a derived data type called obs_f that describes the observation. One instance of this data type is allocated in each obs-module as the variable thisobs. a few of the elements of obs_f are initialized by the user when the observation information is initialized on init_dim_obs_f. Further variables is set in a call to the routine PDAFomi_gather_obs. This information is then used by all other routines in the obs-module. The template file obs_TYPE_pdafomi_TEMPLATE.F90 shows the different steps needed to initialize thisobs.

The mandatory variables in obs_f that need to be set by the user are:

  TYPE obs_f
     ! ---- Mandatory variables to be set in INIT_DIM_OBS ----
     INTEGER :: doassim=0                 !< Whether to assimilate this observation type
     INTEGER :: disttype                  !< Type of distance computation to use for localization
                                          !   (0) Cartesian, (1) Cartesian periodic
                                          !   (2) simplified geographic, (3) geographic haversine function
     INTEGER :: ncoord                    !< Number of coordinates use for distance computation
     INTEGER, ALLOCATABLE :: id_obs_p(:,:) !< Indices of process-local observed field in state vector
     ...
  END TYPE obs_f

In addition there are optional variables that the be used:

  TYPE obs_f
     ...
     ! ---- Optional variables - they can be set in INIT_DIM_OBS ----
     REAL, ALLOCATABLE :: icoeff_p(:,:)   !< Interpolation coefficients for obs. operator (optional)
     REAL, ALLOCATABLE :: domainsize(:)   !< Size of domain for periodicity (<=0 for no periodicity) (optional)

     ! ---- Variables with predefined values - they can be changed in INIT_DIM_OBS  ----
     INTEGER :: obs_err_type=0            !< Type of observation error: (0) Gauss, (1) Laplace
     INTEGER :: use_global_obs=1          !< Whether to use (1) global full obs. 
                                          !< or (0) obs. restricted to those relevant for a process domain
     ...
  END TYPE obs_f

Apart from these variables, there is a number of variables that are set internally when the routine PDAFomi_gather_obs is called.

Next to the derived data type obs_f, there is a derived type obs_l for localization. This is only used internally. It will be filled in init_dim_obs_l when calling PDAFomi_init_dim_obs_l.

init_dim_obs_TYPE

obs_op_TYPE

This routine applies the observation operator to a state vector. It returns the observed state vector to PDAF. The routine is used by all filters.

PDAF-OMI provides several observation operators. For example the observation operator for observations that are grid point values is called as:

    CALL PDAFomi_obs_op_gridpoint(thisobs, state_p, ostate)

Here, state_p is the state vector and ostate is the observed state vector.

For more information on the available observation operator and on how to implement your own observation operator see the documentation of observation operators for OMI?.

init_dim_obs_l_TYPE

This routine initializes local observation information. The routine is only used by the domain-localized filters (LESTKF, LETKF, LSEIK, LNETF).

For the initialization the following routine is called:

    CALL PDAFomi_init_dim_obs_l(thisobs_l, thisobs, coords_l, &
         locweight, local_range, srange, dim_obs_l)

Here, thisobs and thisobs_l are the data-type variables obs_f and obs_l. dim_obs_l, the local size of the observation vector, is the direct output of the routine.

Implementation steps:

• Ensure that coords_l is filled in init_dim_l_pdaf
• Specify the localization variables (These variables are usually set in init_pdaf and included with use mod_assimilation)
  • locweight: Type of localization (see init_pdaf)
  • local_range: The localization weight radius
  • srange: The support radius of the localization

localize_covar_TYPE

This routine initializes local observation information. The routine is only used by the local EnKF (LEnKF).

For the initialization the following routine is called:

    CALL PDAFomi_localize_covar(thisobs, dim_p, locweight, local_range, srange, &
         coords_p, HP_p, HPH)

Here, thisobs is the data-type variable obs_f. HP_p and HPH are the covariance matrices projected onto the observations. The localization will be applied to these variables.

Implementation steps:

• Ensure that coords_p is filled in localize_covar_pdafomi
• Specify the localization variables (These variables are usually set in init_pdaf and included with use mod_assimilation)
  • locweight: Type of localization (see init_pdaf)
  • local_range: The localization weight radius
  • srange: The support radius of the localization