Summary of UFS HSD

Hierarchical System Development (HSD) is a structured approach to engage in the development and testing across multiple levels of model complexity and configurations used in numerical weather prediction (NWP) modeling systems such as the Unified Forecast System (UFS). It typically facilitates multiple entry points for community collaboration and development (e.g., atmospheric physics, ocean and ice dynamics, data assimilation for land models and other earth system components), and it can include both operationally relevant and idealized configurations or individual test cases. Integration of HSD into the UFS provides a tool for users to test the impact of code innovations on forecast skill and model performance through the use of simplified configurations, isolation of processes/components, and provision of idealized initial/boundary conditions. For a more comprehensive description of HSD as it relates to UFS and NWP, interested users are referred to the Hierarchical System Development for the Unified Forecast System white paper.

Although the UFS Weather Model (WM) can be run in any of several configurations, from a single-component atmospheric model to a fully coupled model with multiple earth system components (e.g., atmosphere, ocean, sea-ice, land, mediator), initial development of HSD within the UFS WM has focused on atmosphere-only configurations (i.e., FV3-only), with the inclusion of model physics provided through the Community Common Physics Package (CCPP).

Users who wish to utilize or contribute to the UFS HSD framework can begin by obtaining the develop version of the UFS WM. It currently features three test cases (see Section III): one based on a realistic UFS Case Study (originally developed by the Developmental Testbed Center (DTC)), and two idealized configurations. Information on how to set-up and run these test cases is available here. Additionally, users desiring to add their own cases can familiarize themselves with the framework through the documentation, example cases, and tutorial video, and then check out the “General Guidelines for Adding a Test Case to the UFS HSD Framework” section of the documentation to learn how to contribute directly to the framework.

HSD Documentation and User Support

The UFS HSD documentation provides the outline of a structured framework for testing and development across a range of idealized or user specific weather model configurations. The test cases outlined in the HSD documentation demonstrate a July 2020 CAPE, baroclinic instability wave, and regional tropical cyclone cases. The documentation for these test cases is accompanied by detailed instructions for input data acquisition, machine and model configuration setups, execution, and output validation. In addition, it offers guidance for extending the HSD framework by adding new test cases, with specifications for model compile options, input data, test case specific physics parametrizations, model configuration set up, and test run execution steps. While the HSD documentation remains a work in progress, it serves as the foundational resource for researchers and developers seeking to engage with and contribute to the rapidly evolving HSD capabilities of the UFS WM as well as other UFS applications.

Apart from the documentation, users can post questions or issues in the UFS WM GitHub Discussions or Issues pages to receive additional support.

HSD Releases and Summary of Test Cases

EPIC, through collaboration with our partners or collaborators at NOAA National Weather Service (NWS) Environmental Modeling Center (EMC) and DTC, has overseen the release of three test cases as part of a continued effort to enhance the UFS HSD framework. The first HSD release (November 2024) contained the July 2020 CAPE case (originally developed as a UFS Case Study by the DTC) and baroclinic instability idealized case (developed by Xiaqiong Zhou of NOAA/NWS/EMC). The second HSD release (June 2025) was a collaboration with the NOAA/NWS/EMC/Hurricane Analysis and Forecast System (HAFS) group and focused on the integration of a regional tropical cyclone case. Furthermore, each HSD release also expanded the user documentation and tests-dev framework of the UFS WM. Additional details about each test case can be found below.

• July 2020 CAPE – This case study was used to investigate the low Convective Available Potential Energy (CAPE) bias that exists in the Global Forecast System (GFS) which is attributed to model’s differences in latent and sensible heat fluxes when compared to Global Data Assimilation System (GDAS) observations. This case is an atmosphere-only forecast run at C48 resolution with 127 vertical levels and is set to run 24 hours starting at 2020-07-23 00Z.

  

• Baroclinic Instability Case – This case was investigated and designed to evaluate the accuracy of various atmospheric models in simulating a baroclinic wave. This test aims to assess how well “dry dynamical cores”, the foundational components of weather and climate models that handle air movement and temperature changes, perform in idealized conditions. It also measures their ability to handle the development of baroclinic waves and specifically which individual models are the most accurate in doing so. This case is an atmosphere-only forecast run at C192 resolution with 127 vertical levels and is set to run a dynamics-only 24 hour forecast starting at 2019-12-03 00Z.

  

• Idealized, Regional Tropical Cyclone Case – This case study is derived from the I-HAFS configuration and is designed to support controlled studies of tropical cyclone dynamics and forecast development. The case is configured to run at 4-km resolution, with 81 vertical levels. The forecast is initialized on 24 August 2019, and initial conditions and lateral boundary conditions are provided for up to a five-day forecast duration.

  

Community Engagement 

Adding Test Cases to UFS HSD Framework

The UFS HSD framework is designed to encourage and support model enhancements through contributions not only from NOAA and NOAA-affiliated developers, but also from the broader NWP community. This includes, but is not limited to, researchers in academia and industry, graduate students, and NWP scientists and developers from other agencies and institutions. To facilitate such contributions, EPIC has developed a video tutorial covering two of the current HSD test cases in the UFS WM, and provides regularly updated technical documentation, including a dedicated section that guides the new users on how to integrate their code innovations into the UFS HSD framework. Additionally, a comprehensive training is scheduled for UIFCW 2025 and it will be recorded and posted here once available.