Hierarchical system development (HSD) refers to the ability to engage in development and testing at multiple levels of complex prediction software such as the Unified Forecast System (UFS). It is critical for research because it enables the research community to have multiple entry points into development that reflect their interests. Those interests may include aspects of specific system components such as atmospheric physics, ocean and ice dynamics, or data assimilation for land models and other earth system components. Research interests may also include more integrative topics such as coupled model development or strongly coupled data assimilation. Research efforts such as the Community Earth System Model already have established many features of hierarchical development for modeling

HSD is likewise critical for operations because both localized and integrative, coupled processes must be improved to develop excellence in forecasts. For example, there is evidence that improved atmospheric microphysics and atmospheric coupling to ocean, land, and ice components, respectively, are necessary to achieve the best medium-range weather forecasts.

Research-to-operations-to-research (R2O2R) refers to the interactions between these communities that are needed to support their different goals and ways of working. The UFS strategy for fostering positive interactions is through participatory governance and shared infrastructure. In this kind of community approach, priorities can be developed and communicated, the research community can access and test aspects of the operational system, and the needed scientific and technical advances from research can be transitioned into operations. HSD is at the heart of R2O2R because it ensures that every advance proposed by the research community has a clear development and test path that includes integration into end-to-end applications.