Explore the vast range of titles available.

The mechanics of slow slip events (SSE) in subduction zones remain unresolved. We suggest that SSE nucleate in areas of unstable friction under drained conditions, but as slip accelerates dilatancy reduces pore pressure p quenching instability. Competition between dilatant strengthening and thermal pressurization may control whether slip is slow or fast. We model SSE with 2‐D elasticity, rate‐state friction, and a dilatancy law where porosity evolves toward steady state ss over distance dc and ss = 0 + ln(v/v0); v is slip speed. We consider two diffusion models. Membrane diffusion (MD) is approximated by −(p − p∞)/tf where p and p∞ are shear zone and remote pore pressure and tf is a characteristic diffusion time. Homogeneous diffusion (HD) accurately models fault‐normal flow with diffusivity chyd. For MD, linearized analysis defines a boundary ℰ = 1 − a/b between slow and fast slip, where ℰ ≡ f0ε/βb(σ − p∞), f0, a, and b are friction parameters and β is compressibility. When ℰ < 1 − a/b slip accelerates to instability for sufficiently large faults, whereas for ℰ > 1 − a/b slip speeds remain quasi‐static. For HD, Ep ≡ εh/(β (σ − p∞)) defines dilatancy efficiency, where h is shear zone thickness and v∞ is plate velocity. SSE are favored by large εh and low effective stress. The ratio Ep to thermal pressurization efficiency scales with 1/(σ − p∞), so high p∞ favors SSE, consistent with seismic observations. For Ep ∼ 10−3 transient slip rates, repeat times, average slip, and stress drops are comparable to field observations. Model updip propagation speeds are comparable to those observed along‐strike. Many simulations exhibit slow phases driven by steady downdip slip and faster phases that relax the accumulated stress. Model SSE accommodate only a fraction of plate motion; the remaining deficit must be accommodated during coseismic or postseismic slip.

Episodic Tremor and Slip (ETS), involving transient deformations accompanied by emergent, low‐frequency tremor occurs in subduction zones around the world. ETS events increase the shear stress on locked megathrusts and may potentially trigger damaging earthquakes. Despite the clear association of tremor and slip the physical relationship between them is unresolved. Tremor appears to result from slip on small asperities on the plate interface due to either creep on the surrounding fault, or stress increases ahead of the propagating slow‐slip front. Previous studies of migrating slow slip events have not had sufficient spatial and temporal resolution to differentiate between these two models. To address this, we invert GPS data from the August 2009 ETS event in central Cascadia for the space‐time evolution of fault slip‐rate. We find a correlation in both space and time between tremor epicenters and the independently determined position of high fault slip‐rate. This supports the first hypothesis that tremor asperities are loaded directly by slow slip, rather than by stress increases ahead of the slip front, and provides new insights into the mechanics of ETS. Key Points Tremor and the actively slipping region migrate together during ETS Tremor is likely composed of individual LFE patches, loaded directly by creep Tremor can use used as a proxy for the actively slipping region in migrating ETS

Previous studies have shown that atmospheric models with a spectral element grid can benefit from putting physics calculations on a relatively coarse finite volume grid. Here we demonstrate an alternative high‐order, element‐based mapping approach used to implement a quasi‐equal‐area, finite volume physics grid in E3SM. Unlike similar methods, the new method in E3SM requires topology data purely local to each spectral element, which trivially allows for regional mesh refinement. Simulations with physics grids defined by 2 × 2, 3 × 3, and 4 × 4 divisions of each element are shown to verify that the alternative physics grid does not qualitatively alter the model solution. The model performance is substantially affected by the reduction of physics columns when using the 2 × 2 grid, which can increase the throughput of physics calculations by roughly 60%–120% depending on whether the computational resources are configured to maximize throughput or efficiency. A pair of regionally refined cases are also shown to highlight the refinement capability. Plain Language Summary Most atmospheric models use the same grid for dynamics (e.g., advection) and physics (e.g., clouds). For spectral element models the grid uses irregularly spaced points and the treatment of element edges can lead to grid imprinting bias. Previous studies have shown that using a regularly spaced physics grid in a spectral element model can alleviate the grid imprinting biases. This alternative physics grid can also reduce the computational cost of the model if the physics grid is coarser than the dynamics grid. This study presents a new approach for using a regularly spaced physics grid in a global spectral element model that additionally allows mesh refinement for regionally focused simulations. The use of a relatively coarse physics grid is shown to make the model faster without qualitatively degrading the simulated climate. Key Points A method is presented for defining a finite volume physics grid in a spectral element model that allows for regional refinement The new method is shown to qualitatively preserve the model solution and effective resolution A relatively coarse physics grid increases the speed of physics by roughly 60%–120% depending on how the computational resources are configured

Slow slip events (SSE) in many subduction zones incrementally stress the adjacent locked megathrust, suggesting that they could potentially either trigger or evolve into damaging earthquakes. We explore this with 2D quasi‐dynamic simulations with rate‐state friction, dilatancy, and coupled 1D pore‐fluid and heat transport. Steady‐state weakening friction allows transient slip to nucleate, but is inhibited by dilatant strengthening and destabilized by thermal pressurization. SSE spontaneously nucleate in Low Effective‐Stress Velocity‐Weakening (LESVW) regions. If the dimension of the LESVW is relatively small the SSE are trapped at its updip end, imparting a strong stress concentration in the locked zone. After several centuries SSE penetrate into the region of higher effective stress, where thermal pressurization eventually leads to dynamic rupture. For larger LESVW regions SSE tend to increase in length with time; ultimately higher slip speeds enhance thermal weakening, leading to dynamic instability within the SSE zone. In both cases the onset of the ultimate SSE is essentially indistinguishable from preceding events. Key Points There are two distinct mechanisms by which ETS can evolve into dynamic ruptures No change in behavior prior to dynamic event that could serve as warning If occurs in nature, probability of damaging quake is elevated during ETS

This work documents version two of the Department of Energy's Energy Exascale Earth System Model (E3SM). E3SMv2 is a significant evolution from its predecessor E3SMv1, resulting in a model that is nearly twice as fast and with a simulated climate that is improved in many metrics. We describe the physical climate model in its lower horizontal resolution configuration consisting of 110 km atmosphere, 165 km land, 0.5° river routing model, and an ocean and sea ice with mesh spacing varying between 60 km in the mid‐latitudes and 30 km at the equator and poles. The model performance is evaluated with Coupled Model Intercomparison Project Phase 6 Diagnosis, Evaluation, and Characterization of Klima simulations augmented with historical simulations as well as simulations to evaluate impacts of different forcing agents. The simulated climate has many realistic features of the climate system, with notable improvements in clouds and precipitation compared to E3SMv1. E3SMv1 suffered from an excessively high equilibrium climate sensitivity (ECS) of 5.3 K. In E3SMv2, ECS is reduced to 4.0 K which is now within the plausible range based on a recent World Climate Research Program assessment. However, a number of important biases remain including a weak Atlantic Meridional Overturning Circulation, deficiencies in the characteristics and spectral distribution of tropical atmospheric variability, and a significant underestimation of the observed warming in the second half of the historical period. An analysis of single‐forcing simulations indicates that correcting the historical temperature bias would require a substantial reduction in the magnitude of the aerosol‐related forcing. Plain Language Summary The U.S. Department of Energy recently released version two of its Energy Exascale Earth System Model (E3SM). E3SMv2 experienced a significant evolution in many of its model components (most notably the atmosphere and sea ice models), and its supporting software infrastructure. In this work, we document the computational performance of E3SMv2 and analyze its ability to reproduce the observed climate. To accomplish this, we utilize the standard Diagnosis and Evaluation and Characterization of Klima experiments augmented with historical simulations for the period 1850–2015. We find that E3SMv2 is nearly twice as fast as its predecessor and more accurately reproduces the observed climate in a number of metrics, most notably clouds and precipitation. We also find that the model's simulated response to increasing carbon dioxide (the equilibrium climate sensitivity) is much more realistic. Unfortunately, E3SMv2 underestimates the global mean surface temperature compared to observations during the second half of historical period. Using sensitivity experiments, where forcing agents (carbon dioxide, aerosols) are selectively disabled in the model, we determine that correcting this problem would require a strong reduction in the impact of aerosols. Key Points E3SMv2 is nearly twice as fast as E3SMv1 with a simulated climate that is improved in many metrics (e.g., precipitation and clouds) Climate sensitivity is substantially lower with a more plausible equilibrium climate sensitivity of 4.0 K (compared to an unlikely value of 5.3 K in E3SMv1) E3SMv2 underestimates the warming in the late historical period due to excessive aerosol‐related forcing

Advection of trace species, or tracers, also called tracer transport, in models of the atmosphere and other physical domains is an important and potentially computationally expensive part of a model's dynamical core. Semi-Lagrangian (SL) advection methods are efficient because they permit a time step much larger than the advective stability limit for explicit Eulerian methods without requiring the solution of a globally coupled system of equations as implicit Eulerian methods do. Thus, to reduce the computational expense of tracer transport, dynamical cores often use SL methods to advect tracers. The class of interpolation semi-Lagrangian (ISL) methods contains potentially extremely efficient SL methods. We describe a finite-element ISL transport method that we call the interpolation semi-Lagrangian element-based transport (Islet) method, such as for use with atmosphere models discretized using the spectral element method. The Islet method uses three grids that share an element grid: a dynamics grid supporting, for example, the Gauss–Legendre–Lobatto basis of degree three; a physics parameterizations grid with a configurable number of finite-volume subcells per element; and a tracer grid supporting use of Islet bases with particular basis again configurable. This method provides extremely accurate tracer transport and excellent diagnostic values in a number of verification problems.

This paper provides an overview of the United States (US) Department of Energy's (DOE's) Energy Exascale Earth System Model version 2 (E3SMv2) fully coupled regionally refined model (RRM) and documents the overall atmosphere, land, and river results from the Coupled Model Intercomparison Project 6 (CMIP6) DECK (Diagnosis, Evaluation, and Characterization of Klima) and historical simulations – a first-of-its-kind set of climate production simulations using RRM. The North American (NA) RRM (NARRM) is developed as the high-resolution configuration of E3SMv2 with the primary goal of more explicitly addressing DOE's mission needs regarding impacts to the US energy sector facing Earth system changes. The NARRM features finer horizontal resolution grids centered over NA, consisting of 25→100 km atmosphere and land, a 0.125∘ river-routing model, and 14→60 km ocean and sea ice. By design, the computational cost of NARRM is ∼3× of the uniform low-resolution (LR) model at 100 km but only ∼ 10 %–20 % of a globally uniform high-resolution model at 25 km.A novel hybrid time step strategy for the atmosphere is key for NARRM to achieve improved climate simulation fidelity within the high-resolution patch without sacrificing the overall global performance. The global climate, including climatology, time series, sensitivity, and feedback, is confirmed to be largely identical between NARRM and LR as quantified with typical climate metrics. Over the refined NA area, NARRM is generally superior to LR, including for precipitation and clouds over the contiguous US (CONUS), summertime marine stratocumulus clouds off the coast of California, liquid and ice phase clouds near the North Pole region, extratropical cyclones, and spatial variability in land hydrological processes. The improvements over land are related to the better-resolved topography in NARRM, whereas those over ocean are attributable to the improved air–sea interactions with finer grids for both atmosphere and ocean and sea ice. Some features appear insensitive to the resolution change analyzed here, for instance the diurnal propagation of organized mesoscale convective systems over CONUS and the warm-season land–atmosphere coupling at the southern Great Plains. In summary, our study presents a realistically efficient approach to leverage the fully coupled RRM framework for a standard Earth system model release and high-resolution climate production simulations.

We present an architecture-portable and performant implementation of the atmospheric dynamical core (High-Order Methods Modeling Environment, HOMME) of the Energy Exascale Earth System Model (E3SM). The original Fortran implementation is highly performant and scalable on conventional architectures using the Message Passing Interface (MPI) and Open MultiProcessor (OpenMP) programming models. We rewrite the model in C++ and use the Kokkos library to express on-node parallelism in a largely architecture-independent implementation. Kokkos provides an abstraction of a compute node or device, layout-polymorphic multidimensional arrays, and parallel execution constructs. The new implementation achieves the same or better performance on conventional multicore computers and is portable to GPUs. We present performance data for the original and new implementations on multiple platforms, on up to 5400 compute nodes, and study several aspects of the single- and multi-node performance characteristics of the new implementation on conventional CPU (e.g., Intel Xeon), many core CPU (e.g., Intel Xeon Phi Knights Landing), and Nvidia V100 GPU.

We present a new evaluation framework for implicit and explicit (IMEX) Runge–Kutta time-stepping schemes. The new framework uses a linearized nonhydrostatic system of normal modes. We utilize the framework to investigate the stability of IMEX methods and their dispersion and dissipation of gravity, Rossby, and acoustic waves. We test the new framework on a variety of IMEX schemes and use it to develop and analyze a set of second-order low-storage IMEX Runge–Kutta methods with a high Courant–Friedrichs–Lewy (CFL) number. We show that the new framework is more selective than the 2-D acoustic system previously used in the literature. Schemes that are stable for the 2-D acoustic system are not stable for the system of normal modes.

This paper describes the atmospheric component of the US Department of Energy's Energy Exascale Earth System Model (E3SM) version 3. Significant updates have been made to the atmospheric physics compared to earlier versions. Specifically, interactive gas chemistry has been implemented, along with improved representations of aerosols and dust emissions. A new stratiform cloud microphysics scheme more physically treats ice processes and aerosol‐cloud interactions. The deep convection parameterization has been largely improved with sophisticated microphysics for convective clouds, making model convection sensitive to large‐scale dynamics, and incorporating the dynamical and physical effects of organized mesoscale convection. Improvements in aerosol wet removal processes and parameter re‐tuning of key aerosol and cloud processes have improved model aerosol radiative forcing. The model's vertical resolution has increased from 72 to 80 layers with the extra eight layers added in the lower stratosphere to better simulate the Quasi‐Biennial Oscillation. These improvements have enhanced E3SM's capability to couple aerosol, chemistry, and biogeochemistry and reduced some long‐standing biases in simulating tropical variability. Compared to its predecessors, the model shows a much stronger signal for the Madden‐Julian Oscillation, Kelvin waves, mixed Rossby‐gravity waves, and eastward inertia‐gravity waves. Aerosol radiative forcing has been considerably reduced and is now better aligned with community best estimates, leading to significantly improved skill in simulating historical temperature records. Its simulated mean‐state climate is largely comparable to E3SMv2, but with some notable degradation in shortwave cloud radiative effect, precipitable water, and surface wind stress, which will be addressed in future updates. Plain Language Summary This study is part of a series describing the newly released version 3 of the US Department of Energy's Energy Exascale Earth System Model (E3SMv3), focusing on updates to its atmospheric component model (EAMv3). Substantial improvements have been made in representing atmospheric chemistry, aerosols, clouds, convective processes, and their interactions in the model. The model's vertical resolution in the lower stratosphere has increased to better simulate the Quasi‐Biennial Oscillation. These updates strengthen E3SM's ability to model aerosol, chemistry, and biogeochemistry, and reduce biases in tropical variability. The model now shows stronger signals for phenomena like the Madden‐Julian Oscillation and Kelvin waves. Aerosol radiative forcing is better aligned with community estimates, improving the model's skill in simulating historical temperatures. The model's simulated mean‐state climate is largely comparable to its predecessor model EAMv2. Key Points Significant updates were made to Earth System Model version 3 atmospheric physics, including gas phase chemistry, aerosols, clouds, and convection Improved cloud, convection, and vertical resolution largely improved tropical variability simulation in troposphere and stratosphere Improved aerosol representation and aerosol‐cloud interactions have led to a much‐reduced and realistic aerosol radiative forcing