| | 1 | = The flexible parallelization mode in PDAF2 = |
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| | 3 | The structure for the ''flexible'' parallelization mode. |
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| | 5 | The ''flexible'' parallelization mode allows to run the assimilation program in a way so that a model task (set of processors running one model integration) can propagate several ensemble states successively. This approach allows to use a smaller number of processes compared to the ''fully parallel'. |
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| | 7 | Implementing the ''flexible'' mode requires additional changes to the model code. These are shown on the right side of figure 3. In particular an external loop has to be added. This change does actually only affect the general part of the model code. |
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| | 9 | [[Image(//pics/da_extension2x.png)]] |
| | 10 | [[BR]]'''Figure 3:''' (left) Generic structure of a model code, (center) extension for ''fully-parallel'' data assimilation system with PDAF, (right) extension for ''flexible'' data assimilation system with PDAF. |
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| | 12 | '''Extensions for the flexible assimilation system'''[[BR]] |
| | 13 | The right hand side of Figure 1 shows the extensions required for the ''flexible'' assimilation system (marked yellow): |
| | 14 | * `init_parallel_pdaf`: This routine is inserted close to the start of the model code. If the model itself is parallelized the correct location is directly after the initialization of the parallelization in the model code. `init_parallel_pdaf` creates the parallel environment that allows to perform several time stepping loops ("model tasks") at the same time. |
| | 15 | * `init_pdaf`: This routine is added after the initialization part of the model. In `init_pdaf`, parameters for PDAF can be defined and then the core initialization routine `PDAF_init` is called. This core routine also initializes the array of ensemble states. |
| | 16 | * Ensemble loop: In order to allow for the integration of the state ensemble an unconditional loop is added around the time stepping loop of the model. This loop will allow to compute the time stepping loop multiple time during the model integration. PDAF provides an exit-flag for this loop. (This external loop can be avoided with the ''fully-parallel'' implementation variant.) |
| | 17 | * `get_state_pdaf`: Inside the ensemble loop, a call to the interface routine is added to the code. In this routine the names of user-supplied routines are declared and the PDAF-core routine `PDAF_get_state` is called. This routine initializes model fields from the array of ensemble states and initializes the number of time steps that have to be computed and ensures that the ensemble integration is performed correctly. |
| | 18 | * `put_state_pdaf`: At the end of the external loop, the call to the interface routine `put_state_pdaf` is added to the model code. The routine declares the names of user-supplied routines and calls a PDAF_core routine that is specific for each filter. E.g. for the ESTKF, the routine `PDAF_put_state_estkf` is called. This routine writes the propagated model fields back into a state vector of the ensemble array. Also it checks whether the ensemble integration is complete. If not, the next ensemble member will be integrated. If the ensemble integration is complete, the analysis step (i.e. the actual assimilation of the observations) is computed. |
| | 19 | * The ''flexible'' parallelization requires that the model can jump back in time. Jumping back in time will be required if the number of model tasks used to evolve the ensemble states is smaller than the number of ensemble members. In this case a model task has integrate more than one model state and will have to jump back in time after the integration of each ensemble member. |
