The NOAA Earth Prediction Innovation Center (EPIC), in collaboration with NOAA’s Physical Sciences Laboratory (PSL), is pleased to announce the development and integration of an idealized aquaplanet test case into the Unified Forecast System Weather Model (UFS-WM) Hierarchical System Development (HSD) framework. Historically, aquaplanet idealized tests have been widely used to study interactions between the ocean and atmosphere by simplifying the complex interactions which exist in a fully-coupled Earth System Model (ESM) (i.e., by restricting interactions to ocean-atmosphere components, while removing land, sea ice, and other feedbacks). This reduction in complexity enables researchers to isolate key physical processes and discern more robust insights into the impact of their scientific and code innovations upon forecast skill and model performance. Two key features of most aquaplanet experiments are (1) the global replacement of all land and sea-ice with ocean and the removal of topography, thereby approximating the earth’s surface as an aquaplanet, and (2) the prescription of a fixed, analytically variant Sea Surface Temperature (SST) that is kept constant throughout the model run. These studies have demonstrated that only two years of simulation were sufficient to obtain meteorological features such as global and zonal means, eddy patterns, and precipitation distributions and have demonstrated that model changes did not impact global circulation, though they did alter cloud properties.
EPIC and PSL’s goal was to develop a comparable test for the UFS-WM in order to enable the broader Earth System modeling community to conduct meteorological research with an easy-to-deploy model configuration and workflow. To generate initial conditions for this test case, several modifications were required to align with the general structure of standard aquaplanet cases. Wind initial conditions, orography, and land grids (replaced by ocean), aerosols and trace gas (CO₂, O₃ etc.) and trace water (RH, SH etc.) were all set to zero. The model radiation code was modified to prescribe a constant solar zenith angle and the declination was set to zero. From this state the model was spun-up for 90 days, and the resultant restart files were retained as new initial conditions. In this test case configuration, we used a coarse ~ 2° × 2° grid (C48 resolution), in which C48 refers to cubed‑sphere grid with 48 x 48 cells per cube face. If there is community interest, future work may include development of higher resolution configurations, such as 1° × 1° (C96), or even 0.5° × 0.5° (C192).
The idealized aquaplanet test case is available now in a tag of the UFS-WM. Documentation to assist users with configuring and running the test case is available on the UFS-WM ReadtheDocs HSD section. Required input data, fix files, and model configuration files are pre-staged on all NOAA Tier 1 platforms. New plotting scripts are also available for users who wish to visualize their model outputs for long-term experiments to understand Earth system dynamics. Additionally, the test case is available to run via an Intel container, with the container and all necessary data to run the test case is shared via the NOAA EPIC S3 bucket.
