As part of the efforts in the CMDV project, interfaces to integrate the MOAB unstructured mesh library with the TempestRemap remapping tool have been undertaken. Detailed information on the algorithmic and implementation aspects of this effort have been written in a manuscript submitted to Geoscientific Model Development [1]. This work has led to the development of a new offline remapping tool called mbtempest, which exposes the functionality to compute the supermesh or intersection mesh between two unstructured source and target component grids, in addition to using this supermesh for computing the remapping weights to project solutions between the grids. This functionality is part of the critical worflow with E3SM, where the generated remapping weights in the offline step are consumed by MCT at runtime to seamlessly transfer solution data between components (atm↔ocn, atm↔lnd, etc).
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The first -l option specifies the source grid to load, and the second -l option specifies the target grid to load. mbtempest can load Exodus, .nc and h5m files in serial, while the parallel I/O is generally optimized only for the native HDF5 format (h5m). So we recommend converting meshes to this format and pre-partitioning using the instructions above. The option -i specifies the output file for the intersection mesh that is computed in MOAB using the advancing front algorithm in parallel. Due to the distributed nature of the mesh in parallel runs, the h5m file cannot be directly used with TempestRemap's GenerateOfflineMap tool. However, if mbtempest is run in serial (first case above) or in parallel, the intersection mesh in Exodus mesh format can be written out using TempestRemap underneath by performing a MPI_Gather on the root process. This process does not scale due to the aggregation of the entire mesh and associated metadata for this process and we recommend using the fully parallel mbtempest workflow directly if possible.
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We are currently working on a Python script to convert the MOAB h5m file to a SCRIP file to be consumed in E3SM directly. As a future task, we will also be investing effort into writing a SCRIP file in parallel through a custom I/O interface using PNetCDF.
References
1 Mahadevan, V. S., Grindeanu, I., Jacob, R., and Sarich, J.: Improving climate model coupling through a complete mesh representation: a case study with E3SM (v1) and MOAB (v5.x), Geosci. Model Dev. Discuss., https://doi.org/10.5194/gmd-2018-280, in review, 2018.