Hydrostatic Seasonal Forecast System Development within the Unified Forecast System at NOAA

NOAA is undertaking the development of a Seasonal Forecast System version 1 (SFSv1) to replace the current Climate Forecast System version 2 (CFSv2) due to its limitations and the need for enhanced forecast accuracy. The SFS's development is primarily based on the capabilities of the NOAA Global Ensemble Forecast System (GEFSv13) which utilizes a non-hydrostatic Finite-volume dynamical core and is tailored for extended-range forecasting at a 25 km resolution. SFSv1, intended to operate at approximately 100 km, with a target resolution of about 50 km, requires careful examination to ensure the effectiveness of the existing physics suite and dynamics at these resolutions. 

Within the resolution range of 50 km to 100 km, the hydrostatic assumption remains the hydrostatic assumption retains its viability, offering reduced computational costs and intimating that the non-hydrostatic alternative may be superfluous. To comprehensively evaluate these alternatives, sensitivity experiments with both AMIP-type and fully coupled experiments, have been conducted. Adjustments to the schemes and parameters governing artificial dissipation, horizontal advection, and vertical remapping have been made to align with the hydrostatic option.

The verification of atmosphere-only sensitivity experiments indicates that the hydrostatic option exhibits comparable performance compared to non-hydrostatic in terms of the prediction of large-scale environmental patterns. It also shows superior performance in predicting the quasi-biennial zonal wind oscillation at the stratosphere. However, further model refinement is deemed necessary, particularly when coupled with the ocean model to mitigate cold SST biases over the NINO3.4 region. Ongoing investigations are also addressing other issues arising from the transition to the hydrostatic option for SFS.