= PDAF_generate_obs = This page documents the routine `PDAF_generate_obs` of PDAF. The routine is typically called in `assimilate_pdaf` or directly in the model code. This routine is used to generate synthetic observations. It bases on the usual implementation strategy for PDAF which is described on the page [ModifyModelforEnsembleIntegration Modification of the model code for the ensemble integration] and its sub-page on [InsertAnalysisStep inserting the analysis step]. The routine is used in the fully-parallel implementation variant of the data assimilation system. When the 'flexible' implementation variant, the routine `PDAF_put_state_generate_obs' is used. The interface is the following: {{{ SUBROUTINE PDAF_generate_obs(U_collect_state, U_distribute_state, & U_init_dim_obs_f, U_obs_op_f, U_get_obs_f, U_init_obserr_f, & U_prepoststep, U_next_observation, status_pdaf) }}} with the following arguments: * `U_collect_state`: 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. This is basically the inverse operation to `U_distribute_state` used in [ModifyModelforEnsembleIntegration#PDAF_get_state PDAF_get_state] * `U_distribute_state`: The name of a user supplied routine that initializes the model fields from the array holding the ensemble of model state vectors. * `U_init_dim_obs_f`: The name of the user-supplied routine that provides the size of the full observation vector * `U_obs_op_f`: The name of the user-supplied routine that acts as the full observation operator on some state vector * `U_get_obs_f`: The name of the user-supplied routine that receives the full vector of generated synthetic observations from PDAF * `U_init_obserr_f`: The name of the user-supplied routine that initializes the vector of observations error standard deviations for for full observation vector * `U_prepoststep`: The name of the pre/poststep routine as in `PDAF_get_state` * `U_next_observation`: The name of a user supplied routine that initializes the variables `nsteps`, `timenow`, and `doexit`. The same routine is also used in `PDAF_get_state`. * `status_pdaf`: The integer status flag. It is zero, if `PDAF_generate_obs` is exited without errors. The user-supplied call-back routines are described on the page on [ImplementGenerateObs implementing the generation of synthetic observations]. It is recommended that the value of `status_pdaf` is checked in the program after PDAF_generate_obs is executed. Only if its value is 0 the initialization was successful. PDAF also has a [PdafSimplifiedInterface Simplified Interface] providing the routine `PDAF_generate_obs_si`. In the simplified interface, the name of the user-supplied routines have predefined names and do not appear in the call to `PDAF_generate_obs_si`.