Bridges to Operations: Advanced Physics Testing

With funding from the Next Generation Global Prediction System (NGGPS) initiative and broad support from the community, the National Centers for Environmental Prediction (NCEP)/Environmental Modeling Center (EMC) recently replaced the dynamic core in its flagship operational model, the Global Forecast System (GFS). Version 15 of the GFS (GFSv15), implemented in operations on June 12, 2019, includes the Finite-Volume Cubed-Sphere (FV3) non-hydrostatic dynamical core in place of the long-running spectral hydrostatic core. This modeling system provides a fundamental early building block for the emerging Unified Forecast System (UFS) that is envisioned to be a full community-based Earth-System model.

The next major upgrade of the GFS, scheduled for 2021, is expected to include significant changes in model physics, posing the UFS community with a variety of challenges. Individual physical parameterizations need to be upgraded or replaced to produce superior forecast performance. Additionally, the physics suite needs to be well-integrated so that information is correctly transferred among parameterizations. Finally, the suite needs to run within the time available in the operational computing platform.

To address these challenges, three suites were identified as possible replacements for the GFS v15 suite (Suite 1). Suite 2 is the most similar to the operational suite, containing a single parameterization replacement, the planetary boundary layer (PBL) scheme. Suite 3 contains two parameterization replacements (the convective and microphysics schemes), harnessing development conducted at multiple research centers and universities, including Colorado State, Utah, NASA, NCAR, and EMC. Suite 4 contains five parameterization replacements, as it is derived from the operational RAP/HRRR modeling system, which was developed by NOAA Global Systems Division from years of community contributions through the WRF community modeling system for mesoscale applications.

In addition to the differences in physics listed in the Table above, it should be noted that the forecasts by the various configurations differ in a few other aspects, including dynamics settings and computational platforms. Additionally, Suite 4 uses the Common Community Physics Package (CCPP) as a demonstration of the UFS’s new paradigm for integrating physics and dynamics.

Runs were conducted between December 2018 and February 2019. Each suite was applied in a total of 163 model initializations, and performance for 10-day forecasts were compared objectively and subjectively. The initializations included 16 high-impact events selected by EMC’s Model Evaluation Group (MEG) along with an additional 147 dates from all seasons in 2016 and 2017. The DTC’s Global Model Test Bed (GMTB) and EMC collaboratively conducted the model runs and the output was analyzed using EMC’s verification statistics database (VSDB - the basis for all model upgrade decisions), the Model Evaluation Tools (MET) package, and a comprehensive MEG evaluation. Additionally, GMTB produced diagnostic analyses focusing on tropical cyclones, precipitation characteristics, spectral decomposition, and boundary layer properties. These diagnostic and statistical summaries were examined by an impartial panel of experts to inform their formal recommendation for next steps to EMC. Consistent with the panel’s recommendation, EMC’s final decision was to use suite 2 as the basis for developing a prototype configuration for the next GFS implementation (GFSv16). Specifically, EMC has configured this prototype with the PBL parameterization that distinguishes suite 2, along with already planned upgrades to parameterizations for gravity wave drag, land, and atmospheric radiation - and a doubling of vertical levels with extension of the model upper boundary to the top of the mesosphere. Optimization and development of this prototype in a fully cycled system will proceed in coming months, in anticipation of an early 2021 operational implementation.

For more information about this test, visit the DTC website at https://dtcenter.org/eval/gmtb/phytest2019/.