Simple Layer Model Research Articles

Numerical studies of plasma edge in W7-X with 3D FINDIF code

Abstract Modelling of the plasma transport for inherently three-dimensional (3D) problems as in stellarators requires dedicated complex codes. FINDIF is a 3D multifluid plasma edge transport code that has been previously successfully used for the analysis of energy transport in the TEXTOR-DED tokamak [1], where 3D perturbations led to an ergodic structure of field lines in the plasma edge. The ongoing efforts to apply it meaningfully to Wendelstein 7-X (W7-X) plasma problems resulted in advancements in the main model and accompanying tools for mesh generation and post-processing. In order to verify the applicability of the code and to compare with the reported simulation (EMC3-EIRENE) and experimental (OP1.1) results, a series of simulations for varying plasma density, temperature and anomalous transport coefficients as well as for fixed input power were performed. The connection length pattern of FINDIF traced magnetic field lines on the limiter was reproduced and its impact on heat loads was confirmed. An increase in the peak heat load on the limiter with a rise in plasma density, temperature and anomalous plasma transport coefficients was observed. The decay lengths of density, electron temperature and heat flux did not change with density, and were decreasing with temperature and increasing with anomalous plasma transport coefficient, which was compared to the simple scrape-off layer (SOL) model.

Experimental study of the edge radial electric field in different drift configurations and its role in the access to H-mode at ASDEX Upgrade

The formation of the equilibrium radial electric field (Er) has been studied experimentally at ASDEX Upgrade (AUG) in L-modes of “favorable” (ion ∇ B-drift toward primary X-point) and “unfavorable” (ion ∇ B-drift away from primary X-point) drift configurations, in view of its impact on H-mode access, which changes with drift configurations. Edge electron and ion kinetic profiles and impurity velocity and mean-field Er profiles across the separatrix are investigated, employing new and improved measurement techniques. The experimental results are compared to local neoclassical theory as well as to a simple 1D scrape-off layer (SOL) model. It is found that in L-modes of matched heating power and plasma density, the upstream SOL Er and the main ion pressure gradient in the plasma edge are the same for either drift configurations, whereas the Er well in the confined plasma is shallower in unfavorable compared to the favorable drift configuration. The contributions of toroidal and poloidal main ion flows to Er, which are inferred from local neoclassical theory and the experiment, cannot account for these observed differences. Furthermore, it is found that in the L-mode, the intrinsic toroidal edge rotation decreases with increasing collisionality and it is co-current in the banana-plateau regime for all different drift configurations at AUG. This gives rise to a possible interaction of parallel Pfirsch–Schlüter flows in the SOL with the confined plasma. Thus, the different H-mode power threshold for the two drift configurations cannot be explained in the same way at AUG as suggested by LaBombard et al. [Phys. Plasmas 12, 056111 (2005)] for Alcator C-Mod. Finally, comparisons of Er profiles in favorable and unfavorable drift configurations at the respective confinement transitions show that also the Er gradients are all different, which indirectly indicates a different type or strength of the characteristic edge turbulence in the two drift configurations.

Modeling seismic wave propagation in the Loess Plateau using a viscoacoustic wave equation with explicitly expressed quality factor

The thick Quaternary loess on the Loess Plateau of China produces strong seismic attenuation, resulting in weak reflections from subsurface exploration targets. Accurately simulating seismic wavefield in the Loess Plateau is important for guiding subsequent data processing and interpretation. We present a 2D/3D wavefield simulation method for the Loess Plateau using a viscoacoustic wave equation with explicitly expressed quality factor. To take into account the effect of irregular surface, we utilize a vertically deformed grid to represent the topography, and solve the viscoacoustic wave equation in a regular computational domain that conforms to topographic surface. Grid deformation introduces the partial derivatives such as ∂vx/∂z and ∂vy/∂z in the wave equation, which is difficult to be accurately computed using traditional staggered-grid finite-difference method. To mitigate this issue, a finite-difference scheme based on a fully staggered-grid is adopted to solve the viscoacoustic wave equation. Numerical experiments for a simple layer model and 2D/3D realistic Loess Plateau models demonstrate the feasibility and adaptability of the proposed method. The 3D modeling results show comparable amplitude and waveform characteristics to the field data acquired from the Chinese Loess Plateau, suggesting a good performance of the proposed modeling method.

Imaging Steeply Dipping Faults Using Angle-Controlled Decoupled Elastic Reverse-Time Migration of Multicomponent Seismic Data

High-angle and steeply-dipping faults are important for geothermal fluid flow and production. However, imaging such faults is challenging for conventional seismic imaging methods using acoustic waves because of the limited illumination of compressional waves. We develop a novel imaging method to use elastic waves including both compressional-to-compressional (P-P) waves and compressional-to-shear (P-to-S) converted waves to image the high-angle faults. P-S converted waves provide additional seismic illumination for high-angle faults because they have smaller reflection angles than those of the P-P reflection waves at interfaces. Our new imaging method employs the decoupled wave equations in the context of elastic-wave reversetime migration (ERTM) to perform imaging using both P-P and P-S waves. Our decoupled ERTM does not require the polarity correction of S waves. In addition, we use the Poynting vector, measuring the wave propagation direction by computing the wave energy flux, to control the image dip angles thus improving the ability to image high-angle faults. Based on the angle-controlled P-P and P-S images, we further enhance the structural images of high-angle faults by extracting the significant structures that coexist in both P-P and P-S images. We apply our angle-controlled decoupled ERTM (ADEM) to synthetic multi-component seismic data for a simple layer model with two vertical faults and a modified Marmousi2 model with artificially embedded vertical faults. Our numerical results demonstrate that our ADEM method can effectively image high-angle faults in complex structures using both P-P and P-S waves.

Evaluating the Response of Diurnal Variations in Surface and Air Temperature to Evaporative Conditions across Vegetation Types in FLUXNET and ERA5

Abstract The diurnal variations of surface and air temperature are closely related, but their different responses to evaporative conditions can inform us about land–atmosphere interactions. Here, we evaluate the responses of the diurnal ranges in surface (ΔTs) and air (ΔTa) temperature to evaporative fraction at 160 FLUXNET sites and in the ERA5 reanalysis. We show that the sensitivity of ΔTs to evaporative fraction depends on vegetation type, whereas ΔTa does not. On days with low evaporative fraction, ΔTs in FLUXNET is enhanced by up to ∼20 K (∼30 K in ERA5) in short vegetation, but only by up to ∼10 K (∼10 K in ERA5) in forests. Note that ΔTa responds rather similarly to evaporative fraction irrespective of vegetation type (∼5 K in FLUXNET, ∼10 K in ERA5). We find a systematic bias in ERA5’s ΔT response to evaporative conditions, showing a stronger sensitivity to evaporative fraction than in FLUXNET. We then demonstrate with a simple atmospheric boundary layer (SABL) model that the weak response of ΔTa to evaporative fraction can be explained by greater boundary layer growth under dry conditions, which increases the heat storage capacity and reduces the response of air temperature to evaporative fraction. Additionally, using a simplified surface energy balance (SSEB) model we show that ΔTs mainly responds to solar radiation, evaporative fraction, and aerodynamic conductance. We conclude that the dominant patterns of diurnal temperature variations can be explained by fundamental physical concepts, which should help us to better understand the main controls of land–atmosphere interactions. Significance Statement Generally, air temperature is used more widely than the surface temperature, and often they are assumed to be equivalent. However, we show that their responses to changes in vegetation type and evaporative conditions are quite different. Using FLUXNET observations, ERA5 reanalysis, and two simple physical models, we found that these responses are much stronger in surface temperature, especially in short vegetation, and relatively weaker in air temperature. Despite being measured just 2 m above the surface, air temperature carries lesser imprints of evaporation and vegetation than the surface temperature because of boundary layer dynamics. These findings suggest the importance of coupled land–atmosphere processes in shaping surface and air temperature differently and provide insights on their distinctive responses to global changes.

Reducing Systematic Biases Over the Indian Region in CFS V2 by Dynamical Downscaling

AbstractThe usefulness of dynamical downscaling of seasonal reforecasts of Indian Monsoon is explored to address the seasonal mean biases in the reforecasts. Almost all the current generation global coupled models, including the Climate Forecast System version 2 (CFSv2, T126 ∼110 km), exhibit systematic mean dry bias over the central Indian region during the summer monsoon season. Cold sea surface temperature (SST) biases in the Indian Ocean and a weak monsoon circulation due to a colder tropospheric temperature contribute to this dry bias. Such systematic biases restrict the use of skillful forecasts from these models in climate applications (such as agriculture or hydrology). Dynamical downscaling of seasonal forecasts (∼110 km resolution) using the Weather Research and Forecasting (WRF) model coupled to a simple ocean mixed layer model (OML; WRFOML) at 38 km resolution significantly reduces the majority of the systematic biases reported earlier. The seasonal mean dry bias reduces to 16% in WRFOML as compared to 44% (33%) in the CFSv2‐T126 (WRFCTL) over the Indian land region. Warmer SSTs in the Indian Ocean and a more robust monsoon circulation emanating from a realistic simulation of the tropospheric temperature reduced the systematic biases in WRFOML compared to CFSv2‐T126 and WRFCTL. Additionally, category‐wise rainfall distributions are also improved drastically in the downscaled simulations (WRFOML). Downscaled reforecasts with reduced systematic biases have better suitability for climate applications.

Convolution-based multi-scale envelope inversion

Envelope inversion (EI) is an efficient tool to mitigate the nonlinearity of conventional full waveform inversion (FWI) by utilizing the ultralow-frequency component in the seismic data. However, the performance of envelope inversion depends on the frequency component and initial model to some extent. To improve the convergence ability and avoid the local minima issue, we propose a convolution-based envelope inversion method to update the low-wavenumber component of the velocity model. Besides, the multi-scale inversion strategy (MCEI) is also incorporated to improve the inversion accuracy while guaranteeing the global convergence. The success of this method relies on modifying the original envelope data to expand the overlap region between observed and modeled envelope data, which in turn expands the global minimum basin of misfit function. The accurate low-wavenumber component of the velocity model provided by MCEI can be used as the migration model or an initial model for conventional FWI. The numerical tests on simple layer model and complex BP 2004 model verify that the proposed method is more robust than EI even when the initial model is coarse and the frequency component of data is high.

Integrated modeling of ASDEX Upgrade plasmas combining core, pedestal and scrape-off layer physics

The design of future fusion reactors and their operational scenarios requires an accurate prediction of the plasma confinement. We have developed a new model that integrates different elements describing the main physics phenomena which determine plasma confinement. In particular, we are coupling a new pedestal transport model, based on empirical observations, to the ASTRA transport code, which, together with the TGLF turbulent transport model and the NCLASS neoclassical transport model, allows us to describe transport from the magnetic axis to the separatrix. We also coupled a simple scrape-off layer model to ASTRA, which provides the boundary conditions at the separatrix, which are a function of the main engineering parameters. By this way no experimental data of the kinetic profiles is needed, and the only inputs of the model are the magnetic field, the plasma current, the heating power, the fueling rate, the seeding rate, the plasma boundary, and the effective charge. In the modeling work-flow, first a scan in pedestal pressure is performed, by changing the pedestal width. Then the pedestal top pressure is determined using the MISHKA MHD stability code. This modeling framework is tested by simulating ASDEX Upgrade discharges. We show comparisons with experimental fueling, power, and current scans. The changes of the pedestal structure and the gradients in the plasma core, due to the different combinations of fueling, heating power, and plasma current, are well captured by the model. We also show that the predicted pedestal and global stored energies are in good agreement with the experimental measurements, and more accurate with respect to the prediction of the IPB98(y,2) scaling law. The long term goal is to obtain a robust and complete model which can be used to identify important hidden dependencies affecting global plasma confinement, which are difficult to capture by statistical regressions on global parameters.

Finite Element Analysis and Experimental Characterization of Soil Electrical Resistivity at El Roque de los Muchachos Observatory

This paper presents a study of the soil electrical resistivity of El Roque de los Muchachos Observatory, located in La Palma Island (Spain). This work is mainly motivated by the current plans of building an array of Cherenkov Telescopes (CTA) as well as other scientific installations, which demand low earth resistances for the operation of sensitive instruments, prevention of damage due to electrostatic discharges and protection against lightning strikes. Despite the top quality of the sky, the terrain is mostly filled of hard rocks and materials with high resistivity and hardness. No reliable data of resistivities could be found in available literature, therefore a dedicated resistivity survey onsite like the one presented here is essential to optimize the earth resistance of future installations. In this work, we present measurements done in six different locations of an area covering around 250 m × 275 m and centered on coordinates 28˚45'42.9N, 17˚53'28.5W. Low resistivity (Ωm) layers have been found at specific places and depths. The resistivity at the sites has been simulated with COMSOL Multiphysics software using two different models: a simple single layer model and a three-layer model. Agreement with measurements within 10% discrepancies was obtained in all cases. The main contributions of this work are the presentation of reliable values of soil resistivity at ORM, together with the accurate simulation of the soil profiles.

Mapping daily evapotranspiration over a large irrigation district from MODIS data using a novel hybrid dual-source coupling model

Accurate knowledge of spatially distributed evapotranspiration (ET) is essential for better understanding the availability and utilization of water resources. Over the past several decades, remote sensing (RS) technique has been widely applied to acquire indispensable input data for the estimation of ET at different spatial and temporal scales. Several dual-source ET models that can partition evaporation and transpiration have been developed by utilizing remote sensing data and ancillary meteorological observations. However, due to the significant spatial variability of air temperature over heterogeneous land surface, these models typically cannot be applied in heterogeneous regions with scarce meteorological observations. In the present study, a new model, referred to as the HTEM-ABL model, is developed by coupling the Hybrid dual-source scheme and Trapezoid framework-based ET Model (HTEM) with a simple atmospheric boundary layer (ABL) model for ET estimation without ancillary air temperature observations. This model uses Moderate Resolution Imaging Spectroradiometer (MODIS) data as the main input and is evaluated with field observations over a large irrigation district in North China. Results show that the HTEM-ABL model can be well applied to the study area, and the accuracy of HTEM-ABL model is slightly superior to the original HTEM model. The root mean square errors between the calculated and measured ET are 0.63 mm/day and 29.6 mm/year at the field scale and the regional scale, respectively. Furthermore, the estimated air temperature has been proved to be accurate qualitatively and quantitatively, and provides another aspect to evaluate the HTEM-ABL model. The spatial and temporal distributions of ET produced by the HTEM-ABL model are in good agreement with both the distribution of land use types and the variations of crop growth stages. In addition, the HTEM-ABL model shows less sensitivity to land surface temperature compared with the original HTEM model.

Generation of micro-droplet arrays by dip-coating of biphilic surfaces; the dependence of entrained droplet volume on withdrawal velocity

Droplet array chips were realized using an alignment-free fabrication process in silicon. The chips were textured with a homogeneous nano-scale surface roughness but were partially covered with a self-assembled monolayer of perfluorodecyltrichlorosilane (FDTS), resulting in a super-biphilic surface. When submerged in water and withdrawn again, microliter sized droplets are formed due to pinning of water on the hydrophilic spots. The entrained droplet volumes were investigated under variation of spot size and withdrawal velocity. Two regimes of droplet formation were revealed: at low speeds, the droplet volume achieved finite values even for vanishing speeds, while at higher speeds the volume was governed by fluid inertia. A simple 2D boundary layer model describes the behavior at high speeds well. Entrained droplet volume could be altered, post-fabrication, by more than a factor of 15, which opens up for more applications of the dip-coating technique due to the significant increase in versatility of the micro-droplet array platform.

Surface Wave Effects on the Wind-Power Input to Mixed Layer Near-Inertial Motions

AbstractOcean surface waves play an essential role in a number of processes that modulate the momentum fluxes through the air–sea interface. In this study, the effects of evolving surface waves on the wind-power input (WPI) to near-inertial motions (NIMs) are examined by using momentum fluxes from a spectral wave model and a simple slab ocean mixed layer model. Single-point numerical experiments show that, without waves, the WPI and the near-inertial kinetic energy (NI-KE) are overestimated by about 20% and 40%, respectively. Globally, the overestimate in WPI is about 10% during 2005–08. The largest surface wave effects occur in the winter storm-track regions in the midlatitude northwestern Atlantic, Pacific, and in the Southern Ocean, corresponding to large inverse wave age and rapidly varying strong winds. A relatively low frequency of occurrence of wind sea is found in the midlatitudes, which implies that the influence of evolving surface waves on WPI is intermittent, occurring less than 10% of the total time but making up the dominant contributions to reductions in WPI. Given the vital role of NIMs in diapycnal mixing at the base of the mixed layer and the deep ocean, the present study suggests that it is necessary to include the effects of surface waves on the momentum flux, for example, in studies of coupled ocean–atmosphere dynamics or climate models.

A “Cold Path” for the Gulf Stream–Troposphere Connection

Abstract The mechanism by which the Gulf Stream sea surface temperature (SST) front anchors a band of precipitation on its warm edge is still a matter of debate, and little is known about how synoptic activity contributes to the mean state. In the present study, the influence of the SST front on precipitation is investigated during the course of a single extratropical cyclone using a regional configuration of the Met Office Unified Model. The comparison of a control run with a simulation in which SST gradients were smoothed brought the following conclusions: a band of precipitation is reproduced for a single extratropical cyclone, and the response to the SST gradient is dominated by a change of convective precipitation in the cold sector of the storm. Several climatological features described by previous studies, such as surface wind convergence on the warm edge or a meridional circulation cell across the SST front, are also reproduced at synoptic time scales in the cold sector. Based on these results, a simple boundary layer model is proposed to explain the convective and dynamical response to the SST gradient in the cold sector. In this model, cold and dry air parcels acquire more buoyancy over a sharp SST gradient and become more convectively unstable. The convection sets a pressure anomaly over the entire depth of the boundary layer that drives wind convergence. This case study offers a new pathway by which the SST gradient can anchor a climatological band of precipitation.

3-D Quasi-Atomistic Model for Line Edge Roughness in Nonplanar MOSFETs

As the physical sizes of devices have been scaled down, the negative impact of process-induced random variation on device performance has increased; therefore, there is an urgent demand for advanced simulation methods for variation. In this paper, a 3-D quasi-atomistic simulation methodology for line edge roughness (LER) in nonplanar devices, such as FinFETs and gate-all-around (GAA) FETs, is proposed. In addition, a simple gate oxide layer model is proposed to analyze the impact of LER on device performance while excluding the impact of oxide thickness variation. To verify the importance of the quasi-atomistic 3-D LER model and to compare the LER-induced performance variation in a FinFET to that in a GAA FET, the case studies using the 3-D quasi-atomistic LER model for FinFETs and GAA FETs are performed.

Analysis of Observed Streaming in Field Measurements

AbstractAnalysis of velocity and suspended sediment concentration data from field measurements at Pearl Beach in New South Wales, Australia, reveals the existence of onshore currents in the close vicinity of the rippled bed, whereas the velocity is offshore directed farther up in the water column. This might be caused by wave-induced streaming beneath irregular waves over ripples. To test this hypothesis, a simple one-dimensional vertical bottom boundary layer model capable of capturing streaming was applied, yielding a qualitatively fair agreement between the predicted and measured mean velocity and suspended sediment concentration profiles, although the predicted suspended sediment concentration was 1 order of magnitude smaller. Overall, these model results support the hypothesis of the mean near-bed onshore velocity caused by wave-induced streaming over ripples.

Measurements of the double layer capacitance for electrodes in supercritical CO2/acetonitrile electrolytes

Electrochemical impedance measurements were made in the single supercritical phase of CO2 with 11wt% acetonitrile as the co-solvent at temperatures and pressures between 306 and 316K and 15.5 and 20.2MPa over the potential range from −0.45 to +0.75V vs. Pt. Gold, platinum and glassy carbon electrodes were studied together with the effects of electrolyte concentration for [NBu4n][BF4]. Two other electrolytes, [NBu4n][B{3,5-C6H3(CF3)2}4] and [NMe4][BF4], were also studied. We find that the impedance data can be described by a simple RC equivalent circuit where the uncompensated solution resistance is independent of the electrode potential and is consistent with earlier measurements for the electrolyte conductivity. The results for the double layer capacitance can be described by a simple Helmholtz layer model and are very similar to those found for similar electrolytes in non-aqueous solvents such as propylene carbonate. For glassy carbon the double layer capacitances show a parabolic like potential dependence which we attribute to the lower density of states near the Fermi level.

Monitoring the thin film formation during sputter deposition of vanadium carbide.

The thin film formation of magnetron sputtered polycrystalline coatings was monitored by in situ X-ray reflectivity measurements. The measured intensity was analyzed using the Parratt algorithm for time-dependent thin film systems. Guidelines for the on-line interpretation of the data were developed. For thick coatings, the experimental resolution needs to be included in the data evaluation in order to avoid misinterpretations. Based on a simple layer model, the time-dependent mean electron density, roughness and growth velocity were extracted from the data. As an example, the method was applied to the hard coating material vanadium carbide. Both instantaneous and slowly varying changes of the coating could be detected. It was shown that the growth velocity is proportional to the DC power. Significant changes of the microstructure induced by the working gas pressure are mainly driven by the chemical composition.

Meteorological forcing data for urban outdoor thermal comfort models from a coupled convective boundary layer and surface energy balance scheme

Site-specific meteorological forcing appropriate for applications such as urban outdoor thermal comfort simulations can be obtained using a newly coupled scheme that combines a simple slab convective boundary layer (CBL) model and urban land surface model (ULSM) (here two ULSMs are considered). The former simulates daytime CBL height, air temperature and humidity, and the latter estimates urban surface energy and water balance fluxes accounting for changes in land surface cover. The coupled models are tested at a suburban site and two rural sites, one irrigated and one unirrigated grass, in Sacramento, U.S.A. All the variables modelled compare well to measurements (e.g. coefficient of determination=0.97 and root mean square error=1.5°C for air temperature). The current version is applicable to daytime conditions and needs initial state conditions for the CBL model in the appropriate range to obtain the required performance. The coupled model allows routine observations from distant sites (e.g. rural, airport) to be used to predict air temperature and relative humidity in an urban area of interest. This simple model, which can be rapidly applied, could provide urban data for applications such as air quality forecasting and building energy modelling, in addition to outdoor thermal comfort.

Molecular alignment in α-CuPc films probed by reflection anisotropy spectroscopy

Reflection anisotropy spectroscopy (RAS) is generally recognized to be very sensitive to small in-plane anisotropies in thin films, but RA spectra modeling is scarce in literature. Here we propose a simple optical layer model to extract the intrinsic in-plane optical anisotropy of CuPc films from the measured RA spectra recorded as a function of azimuthal sample angle and using the knowledge of the uniaxial optical anisotropy of the film determined from ellipsometry. In addition, the orientation of the transition dipole moments of the first absorption band is determined and the results support the crystallographic model in which the CuPc molecules are aligned parallel in two neighboring stacks in the α-phase.

Q-compensated reverse-time migration

Reduced amplitude and distorted dispersion of seismic waves caused by attenuation, especially strong attenuation, always degrades the resolution of migrated images. To improve image resolution, we evaluated a methodology of compensating for attenuation ([Formula: see text]) effects in reverse-time migration ([Formula: see text]-RTM). The [Formula: see text]-RTM approach worked by mitigating the amplitude attenuation and phase dispersion effects in source and receiver wavefields. Source and receiver wavefields were extrapolated using a previously published time-domain viscoacoustic wave equation that offered separated amplitude attenuation and phase dispersion operators. In our [Formula: see text]-RTM implementation, therefore, attenuation- and dispersion-compensated operators were constructed by reversing the sign of attenuation operator and leaving the sign of dispersion operator unchanged, respectively. Further, we designed a low-pass filter for attenuation and dispersion operators to stabilize the compensating procedure. Finally, we tested the [Formula: see text]-RTM approach on a simple layer model and the more realistic BP gas chimney model. Numerical results demonstrated that the [Formula: see text]-RTM approach produced higher resolution images with improved amplitude and phase compared to the noncompensated RTM, particularly beneath high-attenuation zones.