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This paper presents a new approach for discretizing the nonhydrostatic Euler equations in Cartesian geometry using an operator-split time-stepping strategy and unstaggered upwind finite-volume model formulation. Following the method of lines, a spatial discretization of the governing equations leads to a set of coupled nonlinear ordinary differential equations. In general, explicit time-stepping methods cannot be applied directly to these equations because the large aspect ratio between the horizontal and vertical grid spacing leads to a stringent restriction on the time step to maintain numerical stability. Instead, an A-stable linearly implicit Rosenbrock method for evolving the vertical components of the equations coupled to a traditional explicit Runge–Kutta formula in the horizontal is proposed. Up to third-order temporal accuracy is achieved by carefully interleaving the explicit and linearly implicit steps. The time step for the resulting Runge–Kutta–Rosenbrock–type semi-implicit method is then restricted only by the grid spacing and wave speed in the horizontal. The high-order finite-volume model is tested against a series of atmospheric flow problems to verify accuracy and consistency. The results of these tests reveal that this method is accurate, stable, and applicable to a wide range of atmospheric flows and scales.

The paper demonstrates that quasi-biennial oscillation (QBO)-like oscillations can be simulated in an ensemble of dry GCM dynamical cores that are driven by a simple Held–Suarez temperature relaxation and low-level Rayleigh friction. The tropical stratospheric circulations of four dynamical cores, which are options in NCAR’s Community Atmosphere Model, version 5 (CAM5), are intercompared. These are the semi-Lagrangian (SLD) and Eulerian (EUL) spectral transform, finite-volume (FV), and spectral element (SE) dynamical cores. The paper investigates how the model design choices impact the wave generation, propagation, and dissipation mechanisms in the equatorial region. SLD, EUL, and SE develop spontaneous QBO-like oscillations in the upper equatorial stratosphere, whereas FV does not sustain the oscillation. Transformed Eulerian-mean (TEM) analyses reveal that resolved waves are the dominant drivers of the QBOs. However, the Eliassen–Palm flux divergence is strongly counteracted by the TEM momentum budget residual, which represents the forcing by diffusion and thermal damping. Interestingly, a reversed Brewer–Dobson circulation accelerates the downward propagation of the SLD’s QBO, whereas the EUL’s and SE’s QBOs are slowed by a mean ascent. Waves are abundant in the SLD’s, EUL’s, and SE’s tropical atmosphere despite the absence of moist convection as a typical wave trigger. Dynamic instabilities are suggested as a wave-triggering mechanism in the troposphere and wave-dissipation process in the stratosphere. In particular, there are indications that the increased occurrences of strongly negative instability indicators in SLD, EUL, and SE are related to more vigorous wave activities and higher magnitudes of the resolved wave forcing in comparison to FV.

Using a variable‐resolution option within the National Center for Atmospheric Research/Department of Energy Community Atmosphere Model (CAM) Spectral Element (SE) global model, a refined nest at 0.25° (∼28 km) horizontal resolution located over the North Atlantic is embedded within a global 1° (∼111 km) grid. The grid is designed such that fine grid cells are located where tropical cyclones (TCs) are observed to occur during the Atlantic TC season (June–November). Two simulations are compared, one with refinement and one control case with no refinement (globally uniform 1° grid). Both simulations are integrated for 23 years using Atmospheric Model Intercomparison Protocols. TCs are tracked using an objective detection algorithm. The variable‐resolution simulation produces significantly more TCs than the unrefined simulation. Storms that do form in the refined nest are much more intense, with multiple storms strengthening to Saffir‐Simpson category 3 intensity or higher. Both count and spatial distribution of TC genesis and tracks in the variable‐resolution simulation are well matched to observations and represent significant improvements over the unrefined simulation. Some degree of interannual skill is noted, with the variable‐resolution grid able to reproduce the observed connection between Atlantic TCs and the El Niño‐Southern Oscillation (ENSO). It is shown that Genesis Potential Index (GPI) is well matched between the refined and unrefined simulations, implying that the introduction of variable‐resolution does not affect the synoptic environment. Potential “upscale” effects are noted in the variable‐resolution simulation, suggesting stronger TCs in refined nests may play a role in meridional transport of momentum, heat, and moisture. Key Points Variable‐resolution models can improve the representation of tropical cyclones CAM produces realistic Atlantic TC climatology at 0.25° resolution Addition of local refinement in CAM does not impact synoptic scales

A variable-resolution option has been added within the spectral element (SE) dynamical core of the U.S. Department of Energy (DOE)–NCAR Community Atmosphere Model (CAM). CAM-SE allows for static refinement via conforming quadrilateral meshes on the cubed sphere. This paper investigates the effect of mesh refinement in a climate model by running variable-resolution (var-res) simulations on an aquaplanet. The variable-resolution grid is a 2° (∼222 km) grid with a refined patch of 0.25° (∼28 km) resolution centered at the equator. Climatology statistics from these simulations are compared to globally uniform runs of 2° and 0.25°. A significant resolution dependence exists when using the CAM version 4 (CAM4) subgrid physical parameterization package across scales. Global cloud fraction decreases and equatorial precipitation increases with finer horizontal resolution, resulting in drastically different climates between the uniform grid runs and a physics-induced grid imprinting in the var-res simulation. Using CAM version 5 (CAM5) physics significantly improves cloud scaling at different grid resolutions. Additional precipitation at the equator in the high-resolution mesh results in collocated zonally anomalous divergence in both var-res simulations, although this feature is much weaker in CAM5 than CAM4. The equilibrium solution at each grid spacing within the var-res simulations captures the majority of the resolution signal of the corresponding globally uniform grids. The var-res simulation exhibits good performance with respect to wave propagation, including equatorial regions where waves pass through grid transitions. In addition, the increased frequency of high-precipitation events in the refined 0.25° area within the var-res simulations matches that observed in the global 0.25° simulations.

Numerical weather, climate, or Earth system models involve the coupling of components. At a broad level, these components can be classified as the resolved fluid dynamics, unresolved fluid dynamical aspects (i.e., those represented by physical parameterizations such as subgrid-scale mixing), and nonfluid dynamical aspects such as radiation and microphysical processes. Typically, each component is developed, at least initially, independently. Once development is mature, the components are coupled to deliver a model of the required complexity. The implementation of the coupling can have a significant impact on the model. As the error associated with each component decreases, the errors introduced by the coupling will eventually dominate. Hence, any improvement in one of the components is unlikely to improve the performance of the overall system. The challenges associated with combining the components to create a coherent model are here termed physics–dynamics coupling. The issue goes beyond the coupling between the parameterizations and the resolved fluid dynamics. This paper highlights recent progress and some of the current challenges. It focuses on three objectives: to illustrate the phenomenology of the coupling problem with references to examples in the literature, to show how the problem can be analyzed, and to create awareness of the issue across the disciplines and specializations. The topics addressed are different ways of advancing full models in time, approaches to understanding the role of the coupling and evaluation of approaches, coupling ocean and atmosphere models, thermodynamic compatibility between model components, and emerging issues such as those that arise as model resolutions increase and/or models use variable resolutions.

We present an analysis of version 5.1 of the Community Atmospheric Model (CAM5.1) at a high horizontal resolution. Intercomparison of this global model at approximately 0.25°, 1°, and 2° is presented for extreme daily precipitation as well as for a suite of seasonal mean fields. In general, extreme precipitation amounts are larger in high resolution than in lower‐resolution configurations. In many but not all locations and/or seasons, extreme daily precipitation rates in the high‐resolution configuration are higher and more realistic. The high‐resolution configuration produces tropical cyclones up to category 5 on the Saffir‐Simpson scale and a comparison to observations reveals both realistic and unrealistic model behavior. In the absence of extensive model tuning at high resolution, simulation of many of the mean fields analyzed in this study is degraded compared to the tuned lower‐resolution public released version of the model. Key Points The effects of increasing resolution in an atmospheric model are presented Global models at 25 km can realistically simulate tropical cyclone statistics Increased resolution alone does not improve model defects

This paper describes an objective technique for detecting the extratropical transition (ET) of tropical cyclones (TCs) in high‐resolution gridded climate data. The algorithm is based on previous observational studies using phase spaces to define the symmetry and vertical thermal structure of cyclones. Storm tracking is automated, allowing for direct analysis of climate data. Tracker performance in the North Atlantic is assessed using 23 years of data from the variable‐resolution Community Atmosphere Model (CAM) at two different resolutions ( ΔX∼55 km and 28 km), the Climate Forecast System Reanalysis (CFSR, ΔX∼38 km), and the ERA‐Interim Reanalysis (ERA‐I, ΔX∼80 km). The mean spatiotemporal climatologies and seasonal cycles of objectively detected ET in the observationally constrained CFSR and ERA‐I are well matched to previous observational studies, demonstrating the capability of the scheme to adequately find events. High‐resolution CAM reproduces TC and ET statistics that are in general agreement with reanalyses. One notable model bias, however, is significantly longer time between ET onset and ET completion in CAM, particularly for TCs that lose symmetry prior to developing a cold‐core structure and becoming extratropical cyclones, demonstrating the capability of this method to expose model biases in simulated cyclones beyond the tropical phase. Key Points An objective detection technique for tracking tropical cyclone extratropical transition in gridded climate data is described Objectively calculated extratropical transition climatology in high‐resolution reanalyses closely match observational studies Tropical cyclones in CAM take too long to undergo extratropical transition highlighting model biases requiring further investigation

Simulating the Earth system is crucial for studying Earth's climate and how it changes. Modeling approaches that simplify the Earth system while retaining key characteristics are important tools to advance understanding. The simplicity and flexibility of idealized models enables imaginative science and makes them powerful educational tools. Evolving scientific community needs and increasing model complexity, however, makes it challenging to maintain and support idealized configurations in cutting‐edge Earth system modeling frameworks. We call on the scientific community to re‐emphasize model hierarchies within these frameworks to aid in understanding the Earth system, advancing model development, and developing the future workforce. Key Points Idealized models enable imaginative science that push researchers to think in new and creative ways making them useful educational tools Reduced complexity configurations remain important tools for model development in Earth system science We call on the scientific community to re‐emphasize model hierarchies to aid in the continued understanding of the Earth system

The Community Earth System Model currently contains two primary atmospheric configurations: the Community Atmosphere Model 6 (CAM6, 32 levels, ∼${\\sim} $ 40‐km top); and the Whole Atmosphere Community Climate Model 6 (WACCM6, 70 levels, ∼${\\sim} $ 140‐km top). For CAM7, a number of factors motivate a raising of the model top and enhancement of the vertical resolution and this study documents the decision making process toward this next generation vertical grid. As vertical resolution in the troposphere/lower stratosphere is increased, the role of the resolved waves in driving the Quasi‐Biennial Oscillation (QBO) is enhanced, becoming more similar in magnitude to ERA5 reanalysis. This can be traced to improved equatorial Kelvin waves and their vertical momentum fluxes. It is further shown that a model lid at ∼${\\sim} $ 80‐km does not have detrimental impacts on the representation of the QBO compared to a 140‐km top. Based on this analysis, the vertical grid for CAM7 will have an ∼${\\sim} $ 80‐km top with 93 levels, 500‐m grid spacing in the troposphere and lower stratosphere, and 10 additional levels in the boundary layer compared to CAM6. A 58‐level/∼${\\sim} $ 40‐km low‐top option will also be available. We further introduce new coupled simulations using CAM6 but with CAM7's vertical grid above the boundary layer and use these to demonstrate that basic features of the stratospheric circulation are similar to WACCM6, despite the lower model top. These simulations further show that despite the higher fidelity of the QBO, the observed connection between the QBO and the Madden‐Julian Oscillation is absent. Plain Language Summary This study explores the impacts of changing the vertical grid spacing and model lid height on the representation of the atmosphere within the Community Atmosphere Model (CAM) to inform decisions regarding the vertical grid choices for the next generation of this model (CAM7). It is shown that decreasing the grid spacing (increasing the resolution) in the troposphere and lower stratosphere can lead to a better representation of tropical waves and their role in driving the Quasi‐Biennial Oscillation (QBO)—a quasi‐periodic variation in the winds of the lower stratosphere. It is also shown that a viable representation of the stratospheric polar vortices and the QBO can be obtained with a model lid placed at approximately 80 km. Overall, this analysis motivates the decisions made with regards to the grid for CAM7 and a suite of simulations that use this new grid are described. These simulations are then assessed for their representation of the observed connection between the QBO and the Madden‐Julian Oscillation (MJO)—a mode of variability in the tropical troposphere. Despite the high fidelity of the QBO in this model, the QBO‐MJO connection remains absent. Key Points Resolved wave‐driving of the Quasi‐Biennial Oscillation (QBO) increases with vertical resolution A 93‐level mid‐top (∼${\\sim} $ 80‐km top) and a 58‐level low‐top (∼${\\sim} $ 40‐km top) grid are proposed for the next generation Community Atmosphere Model Despite an improved QBO in the mid‐top, its observed connection with the Madden‐Julian Oscillation is not reproduced

A moist idealized test case (MITC) for atmospheric model dynamical cores is presented. The MITC is based on the Held-Suarez (HS) test that was developed for dry simulations on \"a flat Earth\" and replaces the full physical parameterization package with a Newtonian temperature relaxation and Rayleigh damping of the low-level winds. This new variant of the HS test includes moisture and thereby sheds light on the nonlinear dynamics-physics moisture feedbacks without the complexity of full-physics parameterization packages. In particular, it adds simplified moist processes to the HS forcing to model large-scale condensation, boundary-layer mixing, and the exchange of latent and sensible heat between the atmospheric surface and an ocean-covered planet. Using a variety of dynamical cores of the National Center for Atmospheric Research (NCAR)'s Community Atmosphere Model (CAM), this paper demonstrates that the inclusion of the moist idealized physics package leads to climatic states that closely resemble aquaplanet simulations with complex physical parameterizations. This establishes that the MITC approach generates reasonable atmospheric circulations and can be used for a broad range of scientific investigations. This paper provides examples of two application areas. First, the test case reveals the characteristics of the physics-dynamics coupling technique and reproduces coupling issues seen in full-physics simulations. In particular, it is shown that sudden adjustments of the prognostic fields due to moist physics tendencies can trigger undesirable large-scale gravity waves, which can be remedied by a more gradual application of the physical forcing. Second, the moist idealized test case can be used to intercompare dynamical cores. These examples demonstrate the versatility of the MITC approach and suggestions are made for further application areas. The new moist variant of the HS test can be considered a test case of intermediate complexity.