Joseph Mouallem

This work describes the implementation strategy and technical challenges involved in integrating the Geophysical Fluid Dynamics Laboratory (GFDL)’s Land Model (LM4) with dynamic subgrid tiling capabilities within the atmospheric model, System for High-resolution modeling for Earth-to-Local Domains (SHiELD), capable of kilometer-scale global simulations. A key challenge addressed in this effort is coupling LM4, which was designed for implicit surface flux coupling, with SHiELD’s explicit physics solver. We achieve this through a refactoring of the atmospheric physics suite and code drivers, enabling implicit land-atmosphere coupling of heat and moisture within the well-established FMS coupler infrastructure. The resulting flexible architecture supports multiple model configurations from a single executable without recompilation. This extends SHiELD from an uncoupled atmospheric model, in which land processes are treated as a part of the atmospheric physics package, to a fully coupled high-resolution atmosphere-ocean-land-ice model. We demonstrate the new system through a high-resolution global simulation of Hurricane Helene's landfall where the land component realistically captures the rapid soil saturation, localized runoff generation and multi-day river flooding in Western North Carolina. These results demonstrate the technical coupling strategy, unlock new forecast capabilities, and highlight the importance of interactive land-atmosphere coupling for simulating extreme weather and hydrological events.

The U.S. operational dynamical hurricane prediction models have advanced markedly since the 1990s, evolving from vortex-resolving to convection-permitting systems as increasing computational power has enabled progressively finer grid spacing. An emerging frontier in hurricane prediction is the use of hectometer-scale horizontal grid spacing, enabling the explicit representation not only of the convection but also turbulent large eddies within hurricanes. Here we present an experimental forecast-oriented hectometer-scale model configuration based on Geophysical Fluid Dynamics Laboratory (GFDL) System for High-resolution prediction on Earth-to-Local Domains (SHiELD), incorporating innovations in both grid refinement method and subgrid-scale parameterizations tailored for the turbulent gray zone. Using Hurricane Laura (2020) as an example, we demonstrate the model reproduces the fine-scale features not resolvable in existing operational hurricane models, particularly the large eddy structures along the inner edge of the hurricane eyewall. These features are associated with vertically overturning circulations below ~3 km height and localized extreme wind gusts exceeding 90–100 m/s. Our results highlight the predictability of inner-core large eddies and their impact on most extreme wind gusts in hectometer-scale models, which are critical for wind hazard prediction and impact assessment. More broadly, this work motivates systematic evaluation of the fine-scale features resolved by the large-eddy-permitting models to advance physically grounded next-generation hurricane prediction.

This work describes the implementation strategy and technical challenges involved in integrating the Geophysical Fluid Dynamics Laboratory (GFDL)'s Land Model (LM4) with dynamic subgrid tiling capabilities within the atmospheric model, System for High-resolution modeling for Earth-to-Local Domains (SHiELD), capable of kilometer-scale global simulations. A key challenge addressed in this effort is coupling LM4, which was designed for implicit surface flux coupling, with SHiELD's explicit physics solver. We achieve this through a refactoring of the atmospheric physics suite and code drivers, enabling implicit land-atmosphere coupling of heat and moisture within the well established FMS coupler infrastructure. The resulting flexible architecture supports multiple model configurations from a single executable without recompilation. This extends SHiELD from an uncoupled atmospheric model, in which land processes are treated as a part of the atmospheric physics package, to a fully coupled high resolution atmosphere-ocean-land-ice model. We demonstrate the new system through a high-resolution global simulation of Hurricane Helene's landfall where the land component realistically captures the rapid soil saturation, localized runoff generation and multi-day river flooding concentration in Western North Carolina. These results validate the technical coupling strategy, unlock new forecast capabilities, and highlight the importance of interactive land-atmosphere coupling for simulating extreme weather and hydrological events.

This work describes the implementation strategy and technical challenges involved in integrating the Geophysical Fluid Dynamics Laboratory (GFDL)'s Land Model (LM4) with dynamic subgrid tiling capabilities within the atmospheric model, System for High-resolution modeling for Earth-to-Local Domains (SHiELD), capable of kilometer-scale global simulations. A key challenge addressed in this effort is coupling LM4, which was designed for implicit surface flux coupling, with SHiELD's explicit physics solver. We achieve this through a refactoring of the atmospheric physics suite and code drivers, enabling implicit land-atmosphere coupling of heat and moisture within the well established FMS coupler infrastructure. The resulting flexible architecture supports multiple model configurations from a single executable without recompilation. This extends SHiELD from an uncoupled atmospheric model, in which land processes are treated as a part of the atmospheric physics package, to a fully coupled high resolution atmosphere-ocean-land-ice model. We demonstrate the new system through a high-resolution global simulation of Hurricane Helene's landfall where the land component realistically captures the rapid soil saturation, localized runoff generation and multi-day river flooding concentration in Western North Carolina. These results validate the technical coupling strategy, unlock new forecast capabilities, and highlight the importance of interactive land-atmosphere coupling for simulating extreme weather and hydrological events.

We present the implementation of an inline three‐dimensional (3D) turbulent kinetic energy (TKE)‐based subgrid diffusion scheme in the Geophysical Fluid Dynamics Laboratory (GFDL) Finite‐Volume Cubed‐Sphere (FV3) dynamical core. This scheme addresses the need for a unified and physically grounded treatment of turbulence as FV3 is increasingly used across the gray zone and large‐eddy simulation scales. Building on the 3D TKE framework introduced in Zhu et al. (2025, https://doi.org/10.1038/s41612‐025‐01117‐6 ), we embed the full TKE budget calculations, including the 3D shear production, and TKE‐informed horizontal and vertical diffusion directly into the FV3 core. Key features include computation of 3D TKE shear production based on FV3 D‐grid winds, updating of TKE and diffusion coefficients within FV3 time‐stepping loop, and extension of TKE‐based horizontal diffusion to all dynamical quantities. This integration avoids reliance on external physics packages and improves consistency between resolved and subgrid processes. The inline 3D TKE scheme offers a robust alternative to numerical damping in FV3 and enables a physically based representation of 3D subgrid‐scale diffusion that is well suited for applications at grid spacings of ∼100 m and finer.

This work describes the implementation strategy and technical challenges involved in integrating the Geophysical Fluid Dynamics Laboratory (GFDL)'s Land Model (LM4) with dynamic subgrid tiling capabilities within the atmospheric model, System for High-resolution modeling for Earth-to-Local Domains (SHiELD), capable of kilometer-scale global simulations. A key challenge addressed in this effort is coupling LM4, which was designed for implicit surface flux coupling, with SHiELD's explicit physics solver. We achieve this through a refactoring of the atmospheric physics suite and code drivers, enabling implicit land-atmosphere coupling of heat and moisture within the well established FMS coupler infrastructure. The resulting flexible architecture supports multiple model configurations from a single executable without recompilation. This extends SHiELD from an uncoupled atmospheric model, in which land processes are treated as a part of the atmospheric physics package, to a fully coupled high resolution atmosphere-ocean-land-ice model. We demonstrate the new system through a high-resolution global simulation of Hurricane Helene's landfall where the land component realistically captures the rapid soil saturation, localized runoff generation and multi-day river flooding concentration in Western North Carolina. These results validate the technical coupling strategy, unlock new forecast capabilities, and highlight the importance of interactive land-atmosphere coupling for simulating extreme weather and hydrological events.

As part of the development of GFDL’s new high resolution, seamless weather to S2S to climate timescale coupled model, we present the integration of GFDL’s atmospheric model SHiELD and land model LM4, enabling a suite of Earth system interactions, including extreme hydroclimate events, ecological droughts, and fires. This work details the implementation strategy and technical challenges of integrating GFDL’s LM4 with dynamic subgrid tiling capabilities within SHiELD capable of kilometer-scale global and global-nested simulation. In addition, this effort demonstrates how GFDL terrestrial components designed for implicit flux coupling could be integrated with SHiELD physics designed for an explicit atmospheric solver. The primary objective is to extend SHiELD from an uncoupled atmospheric model, in which land processes are treated as a part of the atmospheric physics package, to a fully coupled high resolution atmosphere-ocean-land-ice-wave model leveraging GFDL’s FMS full coupler infrastructure. This enhanced coupling enables more accurate simulations of land-surface feedbacks, cryosphere and hydrological processes, and extreme weather events such as flooding and abrupt changes in aerosols emissions from fires. We demonstrate the model’s capability through validation test cases. The results underscore the importance of robust land-atmosphere coupling for high-resolution prediction and provide a framework for future development of fully coupled Earth system models of high resolution for forecast and earth system prediction applications.

Sudden Stratospheric Warming (SSW) are extreme weather events that can significantly impact weather patterns on short to subseasonal to seasonal timescales. In this study, we present a new idealized test case of a Sudden Stratospheric Warming (SSW) event implemented in GFDL's FV3 dynamical core. The initial condition features a wintertime stratospheric circulation with a westerly jet in the Northern Hemisphere and an easterly jet in the Southern Hemisphere. In the absence of tropospheric wave forcing, the model preserves the stratospheric circulation for approximately 200 days. To induce SSW, we introduce a moving mountain to generate planetary waves. Wavenumber‐1 forcing led to a vortex displacement SSW, while wavenumber‐2 forcing produced a vortex split SSW, consistent with observational data and literature. This minimal setup offers a controlled environment for studying SSW dynamics and serves as a useful testbed for evaluating the ability of dynamical cores to capture key stratospheric processes and troposphere‐stratosphere interactions.

We present a new high-resolution coupled atmosphere–ocean model, SHiELD-MOM6, which integrates the Geophysical Fluid Dynamics Laboratory (GFDL) advanced atmospheric model, the System for High-resolution modeling for Earth-to-Local Domains (SHiELD), the Modular Ocean Model version 6 (MOM6), and the Sea Ice Simulator (SIS2). The model leverages the Flexible Modeling System (FMS) coupler and its innovative exchange grid to enable a robust and scalable two-way interaction between the atmosphere and ocean. The atmospheric component is built on the non-hydrostatic Finite-Volume Cubed-Sphere Dynamical Core (FV3) with the latest version of the SHiELD physics parameterization suite, while the ocean component is the latest version of MOM, supporting kilometer-scale, high-resolution and regional applications. Validation of this new coupled model is demonstrated through a suite of experiments, including idealized hurricane simulations and a realistic North Atlantic case study featuring Hurricane Helene of 2024. By analyzing the storm intensity, its structure, and its effects on the ocean phenomena such as the upwelling and sea-level changes, the results reveal that air–sea interactions are effectively captured. Scalability tests further confirm the model's computational efficiency. This work established a unified modular cornerstone for advancing high-resolution coupled modeling with significant implications for weather forecasting and climate research.