The NOAA Environmental Modeling System (NEMS) is the infrastructure underlying NOAA's Unified Forecast System (UFS), a fully coupled modeling framework that supports predictions of Earth's environment at a range of time scales. Examples of other coupled modeling systems are the Community Earth System Model (CESM) and the Met Office Unified Model.

NEMS includes infrastructure for coupling model components representing major Earth system domains and processes. A model component is a software representation of a physical domain or process, for example sea ice. It is often developed by a team of specialists in that domain. Model coupling is a software representation of feedbacks between physical processes. It involves modifying the exported fields of a component through grid, unit, temporal, and other transformations so that they can be used as the inputs for another component. These components are managed through repositories, primarily on GitHub.

In general, model components are coupled through the NEMS mediator (in other coupled modeling systems this is often called the "coupler"). NEMS also includes some specialized mediators; for example, for space weather. In some cases in NEMS, the model components are coupled "in-line", meaning that they are called directly from another model component instead of having fields sent through the mediator.

NEMS can be assembled into a number of different modeling applications (often shortened to just applications). Modeling applications are associated with a purpose, like medium-range forecasting; a set of model components; and a set of parameters that represent a range of supported options, including grids and resolutions. Different NEMS modeling applications can have different types and numbers of model components. Also, the same physical domain may be represented by different model components in different modeling applications. For example, in some NEMS modeling applications the ocean component may be the HYbrid Coordinate Ocean Model (HYCOM) and in others it may be the Modular Ocean Model (MOM).

Infrastructure

NEMS is built using the Earth System Modeling Framework (ESMF) infrastructure software. ESMF provides utilities like generation of interpolation weights and utilities for calendar and time management, and also wrappers that create a standard component calling interface. This enables model components developed at different sites to be coupled more easily.

The National Unified Operational Prediction Capability (NUOPC) Layer adds additional rules about how ESMF models interact and increases their interoperability. The NUOPC Layer covers aspects from the level of build dependencies, to standardization of initialization phases, all the way to standard names of the exchanged fields. NEMS is an example of a modeling system built using the NUOPC Layer architecture.

Architecture

The NEMS architecture is based on an ESMF component hierarchy with the application driver MAIN_NEMS at the top, calling into the NEMS_COMP component, which in turn drives the EARTH_COMP component. The EARTH_COMP drives the ATM and other components. The architecture allows for multiple EARTH_COMP instances, supporting ensemble applications such as the Global Ensemble Forecast System (GEFS).

Coupled NEMS includes atmosphere, ocean, ice, wave, land, aerosol/chemistry, and hydrologic models, with coupling interface and utilities based on the Earth System Modeling Framework (ESMF). The NEMS applications also utilize intreopereability conventions introduced by the National Unified Operational Prediction Capability (NUOPC).