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We present a proof-of-concept for the adaptive mesh refinement method applied to atmospheric boundary-layer simulations. Such a method may form an attractive alternative to static grids for studies on atmospheric flows that have a high degree of scale separation in space and/or time. Examples include the diurnal cycle and a convective boundary layer capped by a strong inversion. For such cases, large-eddy simulations using regular grids often have to rely on a subgrid-scale closure for the most challenging regions in the spatial and/or temporal domain. Here we analyze a flow configuration that describes the growth and subsequent decay of a convective boundary layer using direct numerical simulation (DNS). We validate the obtained results and benchmark the performance of the adaptive solver against two runs using fixed regular grids. It appears that the adaptive-mesh algorithm is able to coarsen and refine the grid dynamically whilst maintaining an accurate solution. In particular, during the initial growth of the convective boundary layer a high resolution is required compared to the subsequent stage of decaying turbulence. More specifically, the number of grid cells varies by two orders of magnitude over the course of the simulation. For this specific DNS case, the adaptive solver was not yet more efficient than the more traditional solver that is dedicated to these types of flows. However, the overall analysis shows that the method has a clear potential for numerical investigations of the most challenging atmospheric cases.

We demonstrate that the inference of galaxy stellar masses via spectral energy distribution (SED) fitting techniques for galaxies formed in the first billion years after the Big Bang carries fundamental uncertainties owing to the loss of star formation history (SFH) information from the very first episodes of star formation in the integrated spectra of galaxies. While this early star formation can contribute substantially to the total stellar mass of high-redshift systems, ongoing star formation at the time of detection outshines the residual light from earlier bursts, hampering the determination of accurate stellar masses. As a result, order-of-magnitude uncertainties in stellar masses can be expected. We demonstrate this potential problem via direct numerical simulation of galaxy formation in a cosmological context. In detail, we carry out two cosmological simulations with significantly different stellar feedback models, which span a significant range in SFH burstiness. We compute the mock SEDs for these model galaxies at z = 7 via calculations of 3D dust radiative transfer, and then backward fit these SEDs with prospector SED fitting software. The uncertainties in derived stellar masses that we find for z > 7 galaxies motivate the development of new techniques and/or priors for SFH to model star formation in the early Universe.

Based on the 1979–2018 datasets of Climate Prediction Center (CPC) daily maximum air temperature, HadISST, and NCEP-DOE II reanalysis, the impact of Pacific decadal oscillation (PDO) and Atlantic multidecadal oscillation (AMO) on the interdecadal variability in extreme high temperature (EHT) in North China (NC) is investigated through observational analysis and National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 5.3 (CAM5.3) numerical simulations. The observational results show an interdecadal shift in NC’s EHT in approximately 1996 with a cold period from 1983 to 1996 and a warm period from 1997 to 2014. The summer PDO and AMO are both closely related to NC’s EHT, of which AMO dominates. From the cold to warm period, the combination of PDO and AMO changed from a positive PDO (+ PDO) phase and a negative AMO (− AMO) phase to a negative PDO (− PDO) phase and a positive AMO (+ AMO) phase. The shift in the antiphase combination of PDO and AMO plays an important role in the interdecadal transition of NC’s EHT in 1996. PDO could impact NC’s EHT through the Pacific-East Asia teleconnection pattern, and AMO could influence the NC’s EHT through an atmospheric wave train in the midlatitudes of the Northern Hemisphere. During the warm period (− PDO and + AMO), warmer sea surface temperature anomalies (SSTA) in the northern North Pacific (NP) and North Atlantic (NA) could cause anticyclonic circulation anomalies over these two basins. The anticyclonic circulations anomalies over the NP could enhance the anticyclone over NC through the Pacific-East Asian (PEA) teleconnection pattern. It could also cause an easterly wind from the NP to NC which would weaken the upper westerly over NC. The anticyclonic anomalies over the NA, which were parts of the wave train, could affect other sectors of the wave train, resulting in anticyclonic anomalies over NC. The anticyclonic anomalies over NC could strengthen the continental high and weaken the upper zonal westerly, resulting in favorable EHT conditions. During the cold period (+ PDO and − AMO), because of the same atmospheric response mechanism, a westerly wind from NC to NP and a wave train with reversed anomaly centers could be found, causing a cyclonic anomaly over NC that is not conducive to the EHT. A series of numerical simulations using CAM5.3 confirm the above observational results and show that the combination of + PDO and − AMO changing to − PDO and + AMO has a great impact on the interdecadal shift in EHT in NC in 1996. The simulations also show that both + AMO and − PDO can lead the EHT in NC individually, and the impact of AMO on the EHT in NC is dominant.

Direct numerical simulations of three-dimensional compressible magnetohydrodynamic (MHD) turbulence have been performed in order to study the relation between wave modes and coherent structures and the consequent energization of test particles. Moreover, the question of which is the main mechanism of this particle energization is rigorously discussed. In particular, using the same initial conditions, we analyzed the nonlinear and linear evolution of a turbulent state along with the case of randomized phases. Then, the behaviors of the linear and nonlinear simulations were compared through the study of the time evolution of particle kinetic energy and preferential concentration. Also, spatiotemporal spectra were used to identify the presence of wave modes and quantify the fraction of energy around the MHD modes in linear and nonlinear simulations. Finally, the variation of the correlation time of the external forcing is studied in detail along with the effect on the particle energization (and clustering) and the presence of wave modes. More specifically, particle energization tends to decrease when the fraction of linear energy increases, supporting the idea that energization by structures is the dominant mechanism for particle energization instead of resonance with wave modes as suggested by Fermi energization theory.

Magnetized turbulence is ubiquitous in many astrophysical and terrestrial plasmas but no universal theory exists. Even the detailed energy dynamics in magnetohydrodynamic (MHD) turbulence are still not well understood. We present a suite of subsonic, super-Alfvénic, high plasma beta MHD turbulence simulations that only vary in their dynamical range, i.e., in their separation between the large-scale forcing and dissipation scales, and their dissipation mechanism (implicit large eddy simulation, ILES, and direct numerical simulation (DNS)). Using an energy transfer analysis framework we calculate the effective numerical viscosities and resistivities, and demonstrate that all ILES calculations of MHD turbulence are resolved and correspond to an equivalent visco-resistive MHD turbulence calculation. Increasing the number of grid points used in an ILES corresponds to lowering the dissipation coefficients, i.e., larger (kinetic and magnetic) Reynolds numbers for a constant forcing scale. Independently, we use this same framework to demonstrate that—contrary to hydrodynamic turbulence—the cross-scale energy fluxes are not constant in MHD turbulence. This applies both to different mediators (such as cascade processes or magnetic tension) for a given dynamical range as well as to a dependence on the dynamical range itself, which determines the physical properties of the flow. We do not observe any indication of convergence even at the highest resolution (largest Reynolds numbers) simulation at 20483 cells, calling into question whether an asymptotic regime in MHD turbulence exists, and, if so, what it looks like.

In the fracture‐conduit‐matrix system, distinct shape differences among water‐conducting channels (fractures and conduits) hinder developing an analytical model for flow characteristics and hydraulic conductivity assessment. To address this, we introduce the elliptic‐conduit‐matrix model (ECMM) under velocity slip interface conditions. This analytical model, adaptable to elliptical axis and matrix permeability variations, can be reduced to fracture‐matrix model, conduit‐matrix model, smooth elliptic (circular) pipes, and smooth fractures. Direct numerical simulations on flow in single, multi, and mixed channel‐matrix systems (varying channel counts and matrix permeabilities) validated the ECMM, demonstrating a higher precision compared to the previous models.

We present direct numerical simulations demonstrating deflagration-to-detonation transition (DDT) driven by oxygen flames in Type Ia supernova progenitors. Using the Castro hydrodynamics code coupled with the “aprox13” 13-isotope nuclear network, we simulate combustion in isolated fuel regions where oxygen flames trail carbon flames. In a fiducial 1D run at ρ0 = 3.5 × 107 g cm−3 we observe spontaneous DDT of the oxygen flame via the Zel’dovich gradient mechanism when the carbon–oxygen separation reaches ∼10 km. The oxygen detonation then captures the carbon flame and triggers a stable carbon detonation. Systematic 1D parameter scans show that successful carbon DDT requires upstream densities in the range (3.1–3.6) × 107g cm−3 and a minimum carbon-flame thickness of ≳20 m. 2D simulations confirm DDT and demonstrate that the multidimensional cellular structure of the oxygen detonation can promote carbon detonation at somewhat lower densities than in one dimension. These results provide direct numerical evidence that oxygen-flame-driven DDT is physically plausible in turbulent white-dwarf environments and underscore the importance of multidimensional effects for Type Ia supernova explosion modeling.

We investigate properties of solar-wind-like plasma turbulence using direct numerical simulations. We analyze the transition from large, magnetohydrodynamic (MHD) scales to the ion characteristic ones using two-dimensional hybrid (fluid electrons and kinetic ions) simulations. To capture and quantify turbulence properties, we apply the Karman–Howarth–Monin (KHM) equation for compressible Hall–MHD (extended by considering the plasma pressure as a tensor quantity) to the numerical results. The KHM analysis indicates that the transition from MHD to ion scales (the so-called ion break in the power spectrum) results from a combination of an onset of Hall physics and an effective dissipation owing to the pressure–strain energy-exchange channel and resistivity. We discuss the simulation results in the context of the solar wind.

We analyze direct numerical simulations of large-scale dynamos in inhomogeneous nonhelically driven rotating turbulence with and without shear. The forcing is modulated so that the turbulent intensity peaks in the middle of the computational domain and drops to nearly zero at the two ends above and below the midplane. A large-scale dynamo is driven by an α effect of opposite signs in the two hemispheres. In the presence of shear, the hemispheric magnetic helicity flux from small-scale fields becomes important and can even overcompensate for the magnetic helicity transferred by the α effect between large and small scales. This effect has not previously been observed in nonshearing simulations. Our numerical simulations show that the hemispheric magnetic helicity fluxes are nearly independent of the magnetic Reynolds number, but those between large and small scales, and the consequent dynamo effect, are still found to decrease with increasing Reynolds number—just like in nonshearing dynamos. However, in contrast to nonshearing dynamos, where the generated mean magnetic field declines with increasing magnetic Reynolds number, it is now found to remain independent of it. This suggests that catastrophic dynamo quenching is alleviated by the shear-induced hemispheric small-scale magnetic helicity fluxes that can even overcompensate the fluxes between large and small scales and thereby cause resistive contributions.

Fingering convection (also known as thermohaline convection) is a process that drives the vertical transport of chemical elements in regions of stellar radiative zones where the mean molecular weight increases with radius. Recently, Harrington & Garaud used three-dimensional direct numerical simulations (DNS) to show that a vertical magnetic field can dramatically enhance the rate of chemical mixing by fingering convection. Furthermore, they proposed a so-called “parasitic saturation” theory to model this process. Here, we test their model over a broad range of parameter space, using a suite of DNS of magnetized fingering convection, varying the magnetic Prandtl number, magnetic field strength, and composition gradient. We find that the rate of chemical mixing measured in the simulations is not always predicted accurately by their existing model, in particular when the magnetic diffusivity is large. We then present an extension of the Harrington & Garaud model which resolves this issue. When applied to stellar parameters, it recovers the results of Harrington & Garaud except in the limit where fingering convection becomes marginally stable, where the new model is preferred. We discuss the implications of our findings for stellar structure and evolution.