Zhihong Tan

In recent years a robust weakening of the storm tracks has been identified in reanalysis data. This trend has been linked to coinciding changes in surface temperature extremes and precipitation over land. Here we use a surface‐based metric to quantify regional trends in the boreal summertime low‐level extratropical cyclone activity (ECA) and link this to surface impacts. We compare trends over the satellite‐era across a single model large ensemble (SPEAR‐MED Large Ensemble) and the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations, including full and single forcing simulations and prescribed sea‐surface temperature simulations. The models successfully capture the hemispheric‐wide ECA decrease and regional trends over the Pacific, North America, and Europe seen in reanalysis. However, this masks a model‐observation discrepancy over the North Atlantic, which is anti‐correlated with mean circulation trends. We then re‐examine previous links between extratropical cyclones and increasing daily maximum surface temperatures and shifts in precipitation in historical trends. ECA trends cannot account for the magnitudes of daily maximum surface temperature trends over North America or Europe but can explain a large fraction of land precipitation trends over the North Atlantic. This suggests other drivers, such as greenhouse gas radiative forcing, are more important in strengthening daily maximum surface temperature trends. In contrast, models struggle to capture the North Atlantic ECA‐related trends, primarily because they underestimate ECA trends. This can explain model‐observation discrepancies in total precipitation change over land in the West Atlantic, but not the East Atlantic.

We implemented in GFDL's global atmospheric model a parameterization for convective gravity waves, which explicitly describes the wave source based on the latent heating rates from the parameterized deep convection. The characteristics of the simulated Quasi‐Biennial Oscillation (QBO) in the equatorial stratosphere depend on the tunable parameters in the convective gravity wave scheme. In particular, a larger efficiency factor or a smaller convective area leads to a shorter QBO period. A less‐than‐one scaling factor for the convective heating depth, though unphysical, is necessary for the model to simulate a realistic QBO over the lower stratosphere. Furthermore, we relax the assumption of constant convective area and replace it with the prognostic convective area diagnosed from the convection scheme. While the model with a constant convective area simulates a similar QBO to that with variable convective area at the present‐day condition, it simulates a stronger reduction of the QBO period and the QBO amplitude in response to surface warming. Our results highlight the sensitivity of the simulated QBO in general circulation models to details of the parameterization for convective gravity waves.

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.

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.

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.

Atmospheric rivers (ARs) often cause damaging winds, rainfall, and floods. However, the physical mechanisms governing their evolution remain poorly understood. To close this gap, we perform a global Vapor Kinetic Energy (VKE) budget analysis. Using two formulations of VKE, we show that ARs are governed by similar mechanisms regardless of ocean basins. ARs intensify primarily through the conversion of potential energy to kinetic energy (PE‐to‐KE), with horizontal convergence of vapor kinetic energy providing a secondary contribution in some regions. ARs decay mainly through condensation and turbulent dissipation, while their propagation is governed by the downstream convergence and upstream divergence of vapor kinetic energy. We also find PE‐to‐KE conversion varies spatially and strengthens in regions of greater baroclinic instability or enhanced topographic lifting, for example, along North America's west coast. Collectively, these findings demonstrate that the VKE framework provides a powerful diagnostic for how physical processes shape AR evolution and regional variability.

Atmospheric boundary layer (ABL) flows over spatially heterogeneous surfaces give rise to internal boundary layers and secondary circulations that profoundly affect turbulent transport. Such processes occur at subgrid scales (SGS) in climate models, and while most land models employ parameterizations for surface heterogeneity, ABL schemes typically assume horizontal homogeneity. These schemes therefore overlook potential influences of land heterogeneity on atmospheric transport and subsequent climate impacts. This paper presents and evaluates a new parameterization that couples an eddy‐diffusivity mass‐flux (EDMF) ABL scheme to SGS land heterogeneity in the Geophysical Fluid Dynamics Laboratory (GFDL) global atmosphere‐land model AM4.0‐LM4.0. The parameterization, EDMF‐HET, combines a heterogeneity‐induced mixing length formulation derived from dimensional analysis with a statistical approach that couples convective updrafts to surface patchiness. EDMF‐HET is evaluated against the baseline EDMF and a previous AM4.0 scheme (AM4‐Lock) in global Atmospheric Model Intercomparison Project simulations and against observational data sets. Results indicate that land heterogeneity enhances continental cloud cover over most regions—an effect driven by increased entrainment and cooling near the ABL top. This increased cloudiness in EDMF‐HET slightly exacerbates AM4's global‐mean bias in shortwave cloud radiative effect at the top of the atmosphere, but the associated reduction in downwelling shortwave radiation alleviates some of AM4's regional warm bias, particularly in central North America. EDMF‐HET also reduces tropical land precipitation, mitigating AM4's excessive rainfall in these regions, and shifting the surface energy partitioning toward higher sensible heat fluxes while lowering runoff. Despite these effects, EDMF‐HET had negligible impact on the global‐mean climate, suggesting that land heterogeneity mainly redistributes mass and energy regionally.

Future precipitation projections rely heavily on climate models, underscoring the need to evaluate their ability to simulate historical precipitation changes. Using multiple atmospheric models and ensemble simulations, we estimate the forced signals driven by sea surface warming and the direct effects of greenhouse gases and aerosols, as well as the atmospheric internal variability in precipitation trends since 1980. We find that forced precipitation trends are generally consistent across models, while atmospheric internal variability significantly influences regional patterns. Additionally, a few model members can reasonably well reproduce the observed pattern of precipitation trends. We highlight some regional wetting and drying are likely driven by forcings rather than the atmospheric internal variability. Zonal‐mean trends over land reveal a “wet gets wetter, dry gets drier” paradigm in the Northern Hemisphere, while the Southern Hemisphere shows drying near 45°S associated with jet stream shifts. These results help improve our understanding of historical precipitation changes.

National Centers for Environmental Prediction turbulent kinetic energy (TKE)‐based eddy‐diffusivity mass‐flux (EDMF) scheme is implemented in Geophysical Fluid Dynamics Laboratory atmospheric model (AM4.0) for improving the physical consistency of subgrid‐scale planetary boundary layer (PBL) turbulence parameterization. The mass flux (MF) component represents vertically coherent convective structures responsible for countergradient transport in the upper PBL, which the original AM4.0's ED‐only scheme cannot represent. Consequently, AM4.0 with EDMF produces a deeper and more well‐mixed PBL, leading to better zonal‐mean vertical temperature and humidity profiles and reduced near‐surface wet bias over subtropical and midlatitude oceans. Other model performance changes are generally minor, such as similar biases in global top of atmosphere (TOA) net radiation and shortwave cloud radiative effects, small and compensating changes in low cloud amount and cloud liquid water path, improved low‐level equatorial easterlies but deteriorated extratropical westerlies, slightly increased global‐mean precipitation, and weaker TOA radiative response to uniform sea surface warming. Three adaptations of EDMF are important for its performance at AM4.0's relatively coarse vertical resolution: limiting the overshoot of MF updraft above PBL‐top, reducing the ED‐induced mixing across PBL‐top, and disabling the MF transport of TKE. Low clouds and their radiative effects are also sensitive to four EDMF parameters that control the ED in the lower and upper PBL respectively, the TKE dissipation rate, and the lateral entrainment of MF updraft and downdraft. An automatic linear tuning of these parameters slightly improves the radiative bias, especially for the coastal stratocumulus. More substantial improvements likely require formulation updates of the EDMF scheme and its coupling with other AM4.0 model components.