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Lead Story: NOAA Selects GFDL’s Dynamical Core

Autumn 2016

In August 2014, numerical weather prediction modelers attended a workshop to discuss dynamic core requirements and attri- butes for the NGGPS, and developed a battery of tests to be conducted in three phases over 18 months. Six existing dynamical cores were identified as potential candidates for NGGPS.

During Phase 1, a team of evaluators ran benchmarks to look at performance, both meteorological and computational, and the stability of the core. The performance benchmark measured the speed of each candidate model at the resolution run currently in National Centers for Environmental Prediction (NCEP) operations, and at a much higher resolution expected to be run operation- ally within 10 years. They also evaluated the ability of the models to scale across many tens of thousands of processor cores.

Assessment of the test outcomes from Phase 1 resulted in the recommendation to reduce the candidate pool to two cores, NCAR’s Model for Prediction Across Scales (MPAS) and GFDL’s Finite-Volume on a Cubed Sphere (FV3), prior to Phase 2.

In Phase 2, the team evaluated the two remaining candidates on meteorological performance using both idealized physics and the operational GFS physics package. Using initial conditions from operational analyses produced by NCEP’s Global Data Assimila- tion System (GDAS), each dynamical core ran retrospective forecasts covering the entire 2015 calendar year at the current opera- tional 13 km horizontal resolution. In addition, two cases, Hurricane Sandy in October 2012, and the May 18-20, 2013 tornado outbreak in the Great Plains were run with enhanced resolution (approximately 3 km) over North America. The team assessed the ability of the dynamical cores to predict severe convection without a deep convective parameterization, using operational initial conditions and high-resolution orography.

The results of Phase 2 tests showed that GFDL’s FV3 satisfied all the criteria, had a high level of readiness for operational imple- mentation, and was computationally highly efficient. As a result, the panel of experts recommended to NOAA leadership that FV3 become the atmospheric dynamical core of the NGGPS. NOAA announced the selection of FV3 on July 27, 2016.

Phase 3 of the project, getting underway now, will involve integrating the FV3 dynamical core with the rest of the operational global forecast system, including the data assimilation and post-processing systems. See results, modeling_nggps_implementation_atmdynamics.

Contributed by Jeff Whitaker.

Hindcast of the 2008 hurricane season, simulated by the FV3-powered GFDL model at 13 km resolution.

NGGPS Dynamical Core: Phase 1 Evaluation Criteria

  • Simulate important atmospheric dynamical phenomena, such as baroclinic and orographic waves, and simple moist convection
  • Restart execution and produce bit-reproducible results on the same hardware, with the same processor layout (using the same executable with the same model configuration)
  • High computational performance (8.5 min/day) and scalability to NWS operational CPU processor counts needed to run 13 km and higher resolutions expected by 2020
  • Extensible, well-documented software that is performance portable
  • Execution and stability at high horizontal resolution (3 km or less) with realistic physics and orography
  • Evaluate level of grid imprinting for idealized atmospheric flows

Phase 2 Evaluation Criteria

  • Plan for relaxing the shallow atmosphere approximation (deep atmosphere dynamics) to support tropospheric and space-weather requirements.
  • Accurate conservation of mass, tracers total energy, and entropy that have particular importance for weather and climate application.
  • Robust model solutions under a wide range of realistic atmospheric initial conditions, including strong hurricanes, sudden stratospheric warmings, and intense upper-level fronts with associated strong jet-stream wind speeds using a common (GFS) physics package
  • Computational performance and scalability of dynamical cores with GFS physics
  • Demonstrated variable resolution and/or nesting capabilities, including physically realistic simulations of convection in the high-resolution region
  • Stable, conservative long integrations with realistic climate statistics
  • Code adaptable to NOAA Environmental Modeling System (NEMS)/ Evaluated Earth System Modeling Framework (ESMF)
  • Detailed dycore (dynamical core) documentation, including documentation of vertical grid, numerical filters, time-integration scheme and variable resolu- tion and/or nesting capabilities.
  • Performance in cycled data assimilation tests to uncover issues that might arise when cold-started from another assimilation system
  • Implementation plan including costs