Increasing Grid Resolution Research Articles

Single porosity model: Exploring the spatial resolution limits in complex urban patterns

When modeling large-scale urban floods, the Porosity Nonlinear Shallow Water Equations (PNSWE) emerge as an interesting and consistent subgrid approach to reduce computational time while preserving the solution structure, allowing for computational efficiency at the cost of some loss in the accuracy of the results. Porosity accounts for changes in storage and exchanges due to obstacles in urban areas, and it introduces an extra source term proportional to the porosity gradient into the momentum equations. However, no systematic analyses on the effects of grid size have been performed in real domains, when this model is used to represent fine-scale topographic information at a coarser scale. In this study, we analyze how accuracy is affected by gradually increasing grid resolution in a generalized porosity approach computed at the cell-level. The Single Porosity model (SP) in Cartesian coordinates is employed to simulate a real-world urban flooding event, with resolution transitioning from fine- to macro-scale. At an intermediate scale, the meso-scale, where cell size approximates street width and computational time is significantly reduced, the model captures main preferential flow paths by means of the porosity gradient within the urban area. Good agreement with refined classical model solutions is observed at this scale for flood extension and hazard maps, providing valuable information for early-warning systems. Numerical results underscore the importance of porosity models in rapidly assessing flow properties during an event, enhancing real-time decision-making with reliable information.

Cross-code verification of non-neutral ABL and single wind turbine wake modelling in LES

Large Eddy Simulations (LES) of atmospheric boundary layer (ABL) flow with the actuator disc (AD) for turbine modelling is a widely used method of simulating wind farm flows. Hence, it is important to understand the requirements for achieving a good comparison between ABL flow and turbine wakes between different research group setups, despite unavoidable differences which include LES numerical framework, sub-grid scale (SGS) model, and turbine modelling, and at grid resolutions achievable in large wind farm simulations. In this work, conventionally neutral (CNBL), stable (SBL) and convective (CBL) boundary layers, and single turbine wakes under these different conditions, are compared between the EPFL pseudo-spectral code WIRE LES and the DTU finite-volume code EllipSys3D. ABL profiles largely agree well with hub height velocity magnitudes agreeing to 0.7%, 3.8% and 0.5% for the CNBL, SBL and CBL respectively. The scale-dependent SGS model of WIRE LES results in reduced grid dependency, while EllipSys3D required higher grid resolution in the SBL. Wake flows show improved wake recovery and greater added turbulence intensity with increasing grid resolution, and good agreement is achieved with a radius R to cell size ratio of R/(dxdydz)1/3 ≥ 6.5. Trends in wake flow with different stability conditions, such as the influence of inflow turbulence intensity or shear, are well replicated between codes. Likewise, wake deficit and added TI profiles, and distributions of turbine power and thrust also agree well. Mean power output predictions match to 4.3%, 7.2% and 3.8% in the CNBL, SBL and CBL respectively between the two codes. Overall, these results demonstrate that good agreement is possible with aligned turbine data and sufficient grid resolution.

Can One Predict Coronal Mass Ejection Arrival Times With Thirty‐Minute Accuracy?

AbstractJ. Schmidt and Cairns (2019, https://doi.org/10.48550/arXiv.1905.08961) have recently claimed that they can predict Coronal Mass Ejection (CME) arrival times with an accuracy of 0.9 ± 1.9 hr for four separate events. They also stated that the accuracy gets better with increased grid resolution. Here, we show that combining their results with the Richardson extrapolation (Richardson & Gaunt, 1927, https://doi.org/10.1098/rsta.1927.0008), which is a standard technique in computational fluid dynamics, could predict the CME arrival time with 0.2 ± 0.26 hr accuracy. The CME arrival time errors of this model would lie in a 95% confidence interval [−0.21, 0.61] hr. We also show that the probability of getting these accurate arrival time predictions with a model with a standard deviation exceeding 2 hr is less than 0.1%, indicating that these results cannot be due to random chance. This unprecedented accuracy is about 20 times better than the current state‐of‐the‐art prediction of CME arrival times with an average error of about ±10 hr. Based on our analysis there are only two possibilities: the results shown by J. Schmidt and Cairns (2019, https://doi.org/10.48550/arXiv.1905.08961) were not obtained from reproducible numerical simulations, or their method combined by the Richardson extrapolation is in fact providing CME arrival times with half an hour accuracy. We believe that this latter interpretation is very unlikely to hold true. We also discuss how the peer‐review process apparently failed to even question the validity of the results presented by J. Schmidt and Cairns (2019, https://doi.org/10.48550/arXiv.1905.08961).

Impacts of solid wall boundary conditions in the lattice Boltzmann method on turbulent outdoor flow: A case study of a single 1:1:2 building model

Solid wall boundary condition is an important topic in outdoor wind environment simulation represented by single building based on lattice Boltzmann method (LBM), but there is a lack of quantitative comparative analysis of simulation results under these boundary conditions. This paper conducted LBM-based large-eddy simulation (LBM-LES) of turbulent flow around a 1:1:2 single building, applying both half-way bounce-back and interpolation-based bounce-back (the Bouzidi–Firdaouss–Lallemand scheme, BFL) to solid wall boundaries. The simulation accuracy of different solid wall boundary conditions was examed with consideration of grid resolutions and relative positions between grid points and solid boundaries. Data obtained via experiments and the finite volume method-based large-eddy simulation (FVM-LES) technique were used to examine the flow structure and distribution of the time-averaged velocity and turbulent kinetic energy (TKE). The computational costs were also discussed. Results demonstrated that when the all the boundaries fell on the grid and orthogonal to the grid, the half-way bounce-back scheme attained a high accuracy. On the contrary, the BFL scheme achieved a higher accuracy if the solid wall boundaries did not fall on the grid. Under both boundary schemes, the grid resolutions of b/32 and b/16 (b: building width) could satisfy the acceptable range, but the b/32 grid resolution facilitated a better performance near the wall. The BFL scheme was more sensitive to the grid resolution and the accuracy improved faster with increasing grid resolution than half-way. As for the calculation costs, the calculation time of BFL is more than twice that of half-way bounce-back. However, BFL is still recommended if the wall boundaries are curved and complex in the actual built environment.

Large‐eddy simulation of very stable boundary layers. Part I: Modeling methodology

AbstractThe large‐eddy simulation of very stable boundary layers (SBLs) remains challenging due to the large anisotropy of turbulent motions resulting from buoyancy suppression effects, and to the general reduction of its energetic scales. Here, we develop a novel large‐eddy simulation model setup to enable the modeling of the very SBL, avoiding the use of the Monin–Obukhov similarity theory at its lower boundary condition, as it breaks down in these conditions in agreement with previous studies. The new approach requires very fine grids in the vertical direction, in the order of a few centimeters. A series of Ekman‐layer‐type boundary layers under weak‐wind conditions ( and 2 ms), and strong surface cooling rates (1 and 3 Khr) at high latitude is studied using isotropic grids of 0.05 and 0.10 m resolution. The low‐order statistics show some sensitivity to the grid resolution; for example, both the SBL height and the low‐level jet position decrease with increasing grid resolution. In general, some differences are spotted regarding the shape of the wind speed and temperature vertical profiles, compared with weakly to moderately stratified conditions, suggesting that the structure and characteristics of the SBL evolve with increasing stratification. Moreover, the wind turning is significant from the surface, where the surface wind angle relative to the geostrophic wind is about 50. Lastly, two regimes within very stable conditions can be qualitatively identified primarily based on the gradient Richardson number () profiles: In the less stable regime, increases almost linearly with height and a quasi‐steady state is achieved; however, in the most stable regime, sharply increases near the surface and remains constant throughout the SBL while increasing with time up to . Thus, a quasi‐steady state is not achieved; and although turbulence does not collapse, it remains weak.

Re-examining the partially saturated-cells method for incompressible flows with stationary and moving bodies

We revisit the Partially-Saturated-Cells method (PSC) in the context of incompressible single-phase flows past stationary and moving bodies. This methodology adds a solid collision operator to the Lattice Boltzmann Equation, resulting in a unified evolution equation everywhere in the domain that suitably accounts for the boundary conditions. The unified evolution equation, which also allows for a smooth transition from solid cells to fluid cells, is dictated by the solid volume fraction, obtained using a simple geometric approach. We prove that the PSC approach exhibits nominal second-order accuracy in presence of straight and curved boundaries for pressure, although spatial order of accuracy in velocity degrades to first-order. Our analysis also reveals that PSC is approximately discretely conservative in the incompressible limit with the mass conservation errors being dependent on the lattice size and relaxation time. For simulations involving moving body problems, the method is found to introduce acceptable level of spurious force oscillations, which rapidly decrease with increasing grid resolution. To the best of the authors' knowledge, the present investigations pertaining to discrete conservation and spurious force oscillations in the PSC approach have never been discussed in literature. Extensive numerical experiments are performed involving both stationary and moving bodies, with imposed and induced motion, to assess the applicability of the PSC approach. Our findings illustrate that the inclusion of added mass effects are important for accurate force and torque computations in case of accelerating solids. Numerical investigations, that include unsteady flows featuring three degrees-of-freedom fluid-particle interactions clearly highlight the ability of the PSC approach in resolving complex hydrodynamical phenomena. This in-depth evaluation of the PSC approach conclusively demonstrates its efficacy as a promising computational framework for efficient and robust computations of incompressible fluid flows over a range of flow scenarios.

Ocean Modeling in the Makassar Strait and Balikpapan Bay Using Online Nesting Method

Nur, A.A.; Suprijo, T.; Mandang, I.; Radjawane, I.M.; Park, H., and Khadami, F., 2021. Ocean modeling in the Makassar Strait and Balikpapan Bay using online nesting method. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 206–210. Coconut Creek (Florida), ISSN 0749-0208. This study had applied offline nesting method (one-way interaction) by using Regional Ocean Modeling System (ROMS) to simulate currents circulation in the Balikpapan Bay and showing good results with a high agreement between model output and observational measurement for water level data. It proves that even using the offline nesting method is the better option than increasing grid resolution on the entire model domain that will be demanding more computational cost, while the area of interest is a small-scale application. Therefore, this study aims to improve the model solution by using the online nesting method (two-way interaction) with a refinement ratio of 1:5 from the larger domain to a smaller domain. Three domains which include larger domain (L1) with spatial resolution 2.5 km that encompasses almost the entire Makassar Strait and acted as parent grid for the model. Coarser model of domain L2 with 500 m resolution becomes the buffer zone between the parent model (L1) and smaller domain (L3) in the Balikpapan Bay that has 100 m resolution. Initial and boundary conditions (IC/BC) for the L1 domain consisting of water levels, currents, temperature, and salinity were derived from the Hybrid Coordinate Ocean Model (HyCOM) re-analysis dataset. By using this method, there is only IC needed to be prepared for the other domain by interpolating IC data from the L1 domain and the BC would be provided directly from their larger domain during the simulation. Surface forcing for all domains extracted from the European Centre for Medium-Range Weather Forecast (ECMWF) ERA5 hourly dataset. Statistical methods such as Root Mean Square Error (RMSE) and model skill were used to validate model results for each model domain and found the improvement of the model solution when comparing observational data by model results for the larger and smaller domain at the same location, respectively.

Influence of river cross-section data resolution on flood inundation modeling: Case study of Kashkan river basin in western Iran

In this study, a coupling of a hydrologic and hydraulic model was utilized to assess the impacts of river geometry data resolution on the flood inundation characteristics in a data-scarce environment. Hydrological modeling incorporates soil conservation service curve-number (SCS-CN) and the geomorphologic based instantaneous unit hydrograph model (GIUH) to compute the direct runoff hydrograph in Kashkan river basin located in western Iran. 1D HEC-Geo-RAS model was used and performed to simulate inundation extent of 100-yr floods (~1800 m3/s) along 40 km reach of Kashkan river with a ground survey of river cross-section (2000 cross-sections, each including 500 data-points). The effect of cross-section spacing (20 m to 5000 m) and dataset resolution along cross-section (5–500 points) on 100-yr (~1800 m3/s) flood inundation extent area (FIA), average flow depth (FD), and velocity (FV) in river centerline were scrutinized. Results indicate that coarser cross-section spacing (20–5000 m) showed an overestimating tendency to simulate FIA (~5.75%), FD (391.34%), and FV (31.2%). While increasing grid resolution (5–500 points) on cross-section tends to underestimate of FIA (17.84%),FD (84.13%), and FV (49.12%). The predominant effect of cross-section spacing on FIA, FD, and FV was found compared to the number of data-points on a cross-section. Results of this study could potentially be considered as a baseline for mapping of flood inundation extent in the environment that ground or remotely sensed high-resolution topographic data of river cross-section are scarce while making a trade-off between model accuracy, computational time, and cost of data compilation is crucial.

Sub-kilometer dispersion simulation of a CO tracer for an inter-Andean urban valley

Air quality planning and management can benefit from the ability of numerical models to produce skillful forecasts. However, obtaining these forecasts is a continuing challenge, particularly in complex terrain. Here we examine the dispersion of traffic emissions in the Aburrá valley (located in the Colombian Andes) during an episode of severe air pollution, using the WRF-Chem and a Lagrangian model at sub-kilometer resolution. We focus on the identification of areas with higher CO concentrations (the pollutant analyzed), and examine the role of local and regional airflows in the distribution of CO inside the valley. The meteorological model performance, considering different planetary boundary layer parameterizations and grid sizes, is evaluated using available observations. Overall, the meteorological model performance is within recommended benchmarks for complex terrain. Model performance improves with increasing grid resolution for surface temperature and wind direction, but not for wind speed. Both dispersion models reproduce important features of the spatial and diurnal variability of CO in the valley, but underestimate CO concentrations throughout the simulation period. The south and southeast areas of the valley present the largest CO concentrations, associated with a prevalent northerly transport along the valley axis and reduced transport on the eastern slope. The representation of CO improves when the model adequately reproduces observed rainfall, due to effects of rainfall on boundary layer height and stability, which condition dispersion. Better simulation of transport processes is crucial for informing decisions on air quality management in critical areas such as urban valleys.

Grid-based diffusion Monte Carlo for fermions without the fixed-node approximation.

A diffusion Monte Carlo algorithm is introduced that can determine the correct nodal structure of the wave function of a few-fermion system and its ground-state energy without an uncontrolled bias. This is achieved by confining signed random walkers to the points of a uniform infinite spatial grid, allowing them to meet and annihilate one another to establish the nodal structure without the fixed-node approximation. An imaginary-time propagator is derived rigorously from a discretized Hamiltonian, governing a non-Gaussian, sign-flipping, branching, and mutually annihilating random walk of particles. The accuracy of the resulting stochastic representations of a fermion wave function is limited only by the grid and imaginary-time resolutions and can be improved in a controlled manner. The method is tested for a series of model problems including fermions in a harmonic trap as well as the He atom in its singlet or triplet ground state. For the latter case, the energies approach from above with increasing grid resolution and converge within 0.015E_{h} of the exact basis-set-limit value for the grid spacing of 0.08 a.u. with a statistical uncertainty of 10^{-5}E_{h} without an importance sampling or Jastrow factor.

Space syntax visibility graph analysis is not robust to changes in spatial and temporal resolution

Space syntax is an influential framework for quantifying the relationship between environmental geometry and human behavior. Although many studies report high syntactic–behavioral correlations, previous pedestrian data were collected at low spatiotemporal resolutions, and data transformations and sampling strategies vary widely; here, we systematically test the robustness of space syntax’s predictive strength by examining how these factors impact correlations. We used virtual reality and motion tracking to correlate 30 syntactic measures with high resolution walking trajectories downsampled at 10 grid resolutions and subjected to various log transformations. Overall, correlations declined with increasing grid resolution and were sensitive to data transformations. Moreover, simulations revealed spuriously high correlations (e.g. R2 = 1) with sparsely sampled data (<23 locations). These results strongly suggest that syntactic–behavioral correlations are not robust to changes in spatiotemporal resolution, and that high correlations obtained in previous studies could be inflated due to transformations, data resolution, or sampling strategies.

Identifying the appropriate spatial resolution for the analysis of crime patterns.

BackgroundA key issue in the analysis of many spatial processes is the choice of an appropriate scale for the analysis. Smaller geographical units are generally preferable for the study of human phenomena because they are less likely to cause heterogeneous groups to be conflated. However, it can be harder to obtain data for small units and small-number problems can frustrate quantitative analysis. This research presents a new approach that can be used to estimate the most appropriate scale at which to aggregate point data to areas.Data and methodsThe proposed method works by creating a number of regular grids with iteratively smaller cell sizes (increasing grid resolution) and estimating the similarity between two realisations of the point pattern at each resolution. The method is applied first to simulated point patterns and then to real publicly available crime data from the city of Vancouver, Canada. The crime types tested are residential burglary, commercial burglary, theft from vehicle and theft of bike.FindingsThe results provide evidence for the size of spatial unit that is the most appropriate for the different types of crime studied. Importantly, the results are dependent on both the number of events in the data and the degree of spatial clustering, so a single ‘appropriate’ scale is not identified. The method is nevertheless useful as a means of better estimating what spatial scale might be appropriate for a particular piece of analysis.

Stochastic modeling of discontinuous dynamic recrystallization at finite strains in hcp metals

We present a model that aims to describe the effective, macroscale material response as well as the underlying mesoscale processes during discontinuous dynamic recrystallization under severe plastic deformation. Broadly, the model brings together two well-established but distinct approaches – first, a continuum crystal plasticity and twinning approach to describe complex deformation in the various grains, and second, a discrete Monte-Carlo-Potts approach to describe grain boundary migration and nucleation. The model is implemented within a finite-strain Fast Fourier Transform-based framework that allows for efficient simulations of recrystallization at high spatial resolution, while the grid-based Fourier treatment lends itself naturally to the Monte-Carlo approach. The model is applied to pure magnesium as a representative hexagonal closed packed metal, but is sufficiently general to admit extension to other material systems. Results demonstrate the evolution of the grain architecture in representative volume elements and the associated stress–strain history during the severe simple shear deformation typical of equal channel angular extrusion. We confirm that the recrystallization kinetics converge with increasing grid resolution and that the resulting model captures the experimentally observed transition from single- to multi-peak stress–strain behavior as a function of temperature and rate.

Classification of Tsunami Hazard Area along the Eastern Coast of South Korea based on Improved Tsunami Scenario Database

We classified the areas in the eastern coast of South Korea which are highly likely to be affected by the tsunami that occurred along the northwestern coast of Japan using the improved tsunami scenario database of the Korea Meteorological Administration. To improve the accuracy of database, we used the latest data on bathymetry, coastline, and land information and increased grid resolution for numerical simulation. When tsunami occurred along the northwestern coast of Japan, the maximum wave height appeared in the around Imwon. And the wave height of Gangwon coastal area was higher than those of Gyeongsang. The tsunami height increased as the magnitude of the earthquake increased and decreased as it moved southward along the eastern coast. In conclusion, attention should be paid when an earthquake of magnitude 7.0 or greater occurs in the plate boundary region of the East Sea. And when a tsunami occurs in this region, we need to focus on the area near Imwon. Keywords: Tsunami, Improved Scenario Database, Tsunami Hazard Area, East Sea Coast

Solving the linearized forward-speed radiation problem using a high-order finite difference method on overlapping grids

The linearized potential flow approximation for the forward speed radiation problem is solved in the time domain using a high-order finite difference method. The finite-difference discretization is developed on overlapping, curvilinear body-fitted grids. To ensure numerical stability, the convective derivatives in the free-surface boundary conditions are treated using an upwind-biased stencil. Instead of solving for the radiation impulse response functions, a pseudo-impulsive Gaussian type displacement is employed in order to tailor the frequency-content to the discrete spatial resolution. Frequency-domain results are then obtained from a Fourier transform of the force and motion signals. In order to make a robust Fourier transform, and capture the response around the critical frequency, the tail of the force signal is asymptotically extrapolated assuming a linear decay rate. Fourth-order convergence of the calculations on simple geometries is demonstrated, along with a nearly linear scaling of the solution effort with increasing grid resolution. The code is validated by comparison with analytical and semi-analytical solutions using submerged and floating closed-form geometries. Calculations are also made for a modern bulk carrier, and good agreement is found with experimental measurements.

Updates in the NCEP GFS Cumulus Convection Schemes with Scale and Aerosol Awareness

Abstract The current operational NCEP Global Forecast System (GFS) cumulus convection schemes are updated with a scale-aware parameterization where the cloud mass flux decreases with increasing grid resolution. The ratio of advective time to convective turnover time is also taken into account for the scale-aware parameterization. In addition, the present deep cumulus convection closure using the quasi-equilibrium assumption is no longer used for grid sizes smaller than a threshold value. For the shallow cumulus convection scheme, the cloud-base mass flux is modified to be given by a function of mean updraft velocity. A simple aerosol-aware parameterization where rain conversion in the convective updraft is modified by aerosol number concentration is also included in the update. Along with the scale- and aerosol-aware parameterizations, more changes are made to the schemes. The cloud-base mass-flux computation in the deep convection scheme is modified to use convective turnover time as the convective adjustment time scale. The rain conversion rate is modified to decrease with decreasing air temperature above the freezing level. Convective inhibition in the subcloud layer is used as an additional trigger condition. Convective cloudiness is enhanced by considering suspended cloud condensate in the updraft. The lateral entrainment in the deep convection scheme is also enhanced to more strongly suppress convection in a drier environment. The updated NCEP GFS cumulus convection schemes display significant improvements especially in the summertime continental U.S. precipitation forecasts.

Self-similarity of a Rayleigh–Taylor mixing layer at low Atwood number with a multimode initial perturbation

ABSTRACTHigh-fidelity large eddy simulation (LES) of a low-Atwood number (A = 0.05) Rayleigh–Taylor mixing layer is performed using the 10th-order compact difference code Miranda. An initial multimode perturbation spectrum is specified in Fourier space as a function of mesh resolution such that a database of results is obtained in which each successive level of increased grid resolution corresponds approximately to one additional doubling of the mixing layer width, or generation. The database is then analysed to determine approximate requirements for self-similarity, and a new metric is proposed to quantify how far a given simulation is from the limit of self-similarity. It is determined that mixing layer growth reaches a high degree of self-similarity after approximately 4.5 generations. Statistical convergence errors and boundary effects at late time, however, make it impossible to draw similar conclusions regarding the self-similar growth of more sensitive turbulence parameters. Finally, self-similar turbulence profiles from the LES database are compared with one-dimensional simulations using the k-L-a and BHR-2 Reynolds-averaged Navier–Stokes models. The k-L-a model, which is calibrated to reproduce a quadratic turbulence kinetic energy profile for a self-similar mixing layer, is found to be in better agreement with the LES than BHR-2 results.

Sensitivities of the hydrologic cycle to model physics, grid resolution, and ocean type in the aquaplanet Community Atmosphere Model

AbstractPrecipitation distributions and extremes play a fundamental role in shaping Earth's climate and yet are poorly represented in many global climate models. Here, a suite of idealized Community Atmosphere Model (CAM) aquaplanet simulations is examined to assess the aquaplanet's ability to reproduce hydroclimate statistics of real‐Earth configurations and to investigate sensitivities of precipitation distributions and extremes to model physics, horizontal grid resolution, and ocean type. Little difference in precipitation statistics is found between aquaplanets using time‐constant sea‐surface temperatures and those implementing a slab ocean model with a 50 m mixed‐layer depth. In contrast, CAM version 5.3 (CAM5.3) produces more time mean, zonally averaged precipitation than CAM version 4 (CAM4), while CAM4 generates significantly larger precipitation variance and frequencies of extremely intense precipitation events. The largest model configuration‐based precipitation sensitivities relate to choice of horizontal grid resolution in the selected range 1–2°. Refining grid resolution has significant physics‐dependent effects on tropical precipitation: for CAM4, time mean zonal mean precipitation increases along the Equator and the intertropical convergence zone (ITCZ) narrows, while for CAM5.3 precipitation decreases along the Equator and the twin branches of the ITCZ shift poleward. Increased grid resolution also reduces light precipitation frequencies and enhances extreme precipitation for both CAM4 and CAM5.3 resulting in better alignment with observational estimates. A discussion of the potential implications these hydrologic cycle sensitivities have on the interpretation of precipitation statistics in future climate projections is also presented.

Limits on the ability of global Eulerian models to resolve intercontinental transport of chemical plumes

Abstract. Quasi-horizontal chemical plumes in the free troposphere can preserve their concentrated structure for over a week, enabling transport on intercontinental scales with important environmental impacts. Global Eulerian chemical transport models (CTMs) fail to preserve these plumes due to fast numerical dissipation. We examine the causes of this dissipation and how it can be cured. Goddard Earth Observing System (GEOS-5) meteorological data at 0.25° × 0.3125° horizontal resolution and ∼ 0.5 km vertical resolution in the free troposphere are used to drive a worldwide ensemble of GEOS-Chem CTM plumes at resolutions from 0.25° × 0.3125° to 4° × 5°, in both 2-D (horizontal) and 3-D. Two-dimensional simulations enable examination of the sensitivity of numerical dissipation to grid resolution. We show that plume decay is driven by flow divergence and shear, filamenting the plumes until GEOS-Chem's high-order advection scheme cannot resolve gradients and fast numerical diffusion ensues. This divergence can be measured by the Lyapunov exponent (λ) of the flow. Dissipation of plumes is much faster at extratropical latitudes than in the tropics and this can be explained by stronger divergence. The plume decay constant (α) is linearly related to λ, and increasing grid resolution provides only modest benefits toward plume preservation. Three-dimensional simulations show near-complete dissipation of plumes within a few days, independent of horizontal grid resolution and even in the tropics. This is because vertical grid resolution is inadequate in all cases to properly resolve plume gradients. We suggest that finer vertical grid resolution in the free troposphere is essential for models to resolve intercontinental plumes, while current horizontal resolution in these models (∼ 1°) is sufficient.

Impact of operational model nesting approaches and inherent errors for coastal simulations

A region of freshwater influence (ROFI) under hypertidal conditions is used to demonstrate inherent problems for nested operational modelling systems. Such problems can impact the accurate simulation of freshwater export within shelf seas, so must be considered in coastal ocean modelling studies. In Liverpool Bay (our UK study site), freshwater inflow from 3 large estuaries forms a coastal front that moves in response to tides and winds. The cyclic occurrence of stratification and remixing is important for the biogeochemical cycles, as nutrient and pollutant loaded freshwater is introduced into the coastal system. Validation methods, using coastal observations from fixed moorings and cruise transects, are used to assess the simulation of the ROFI, through improved spatial structure and temporal variability of the front, as guidance for best practise model setup. A structured modelling system using a 180m grid nested within a 1.8km grid demonstrates how compensation for error at the coarser resolution can have an adverse impact on the nested, high resolution application. Using 2008, a year of typical calm and stormy periods with variable river influence, the sensitivities of the ROFI dynamics to initial and boundary conditions are investigated. It is shown that accurate representation of the initial water column structure is important at the regional scale and that the boundary conditions are most important at the coastal scale. Although increased grid resolution captures the frontal structure, the accuracy in frontal position is determined by the offshore boundary conditions and therefore the accuracy of the coarser regional model.