Vertical Grid Resolution Research Articles

Is the Coarse-Grid Global Climate Model a Useful Tool for Regional Paleoclimate Reconstruction?

Is the Coarse-Grid Global Climate Model a Useful Tool for Regional Paleoclimate Reconstruction? This study considers the climate at the time of the Weichselian ice sheet maximum in Central Europe, especially in Poland, in respect of the NASA Educational version of Global Climate Model (EdGCM). The final results of the EdGCM simulations for the Last Glacial Maximum (LGM) are presented here and these have been analysed to determine whether or not they are useful for regional scale modelling; the problem being that, in a global climate model, both horizontal and vertical grid resolution is very low. Also, an attempt has been made to define the role of the ice megalobe which, in terms of the numerical coarse-grid model, covered the northern parts of Germany and Poland. The broad details of the climate during the Weichselian are defined and our results are compared with previous opinion. The influence of ice megalobe on regional climate is demonstrated and it is also concluded that a coarse-grid global climate model could be a helpful tool in a regional climate recognition. However, whether at a regional or local scale, it is recommended that a specially-configured version of a Regional Climate Model (RCM) based on Global Climate Model (GCM) boundary conditions be used in similar reconstructions.

Numerical effects on organic-matter sedimentation and remineralization in biogeochemical ocean models

We investigate the effects of different vertical grid resolutions and algorithms for the calculation of particle sinking on the sedimentation and remineralization of particulate organic matter. Simulations carried out with an idealized 1D model of detritus sinking show that a coarse vertical resolution, such as used in many global biogeochemical models, tends to enhance the particle flux through numerical mixing within the vertical boxes, and thereby simulates deeper remineralization, compared to a model with a fine vertical resolution. This effect can be ameliorated by assuming a distribution of detritus within the individual grid boxes that corresponds to the prescribed sinking and remineralization parameters. Experiments of the different flux algorithms, carried out with 3D global biogeochemical models of different vertical grid resolution reveal impacts on simulated biogeochemical tracer distributions that are similar to those obtained by substantial variations in biogeochemical model parameters. Our results indicate that numerical schemes have to be considered when comparing biogeochemical parameter values of different models and also when porting biogeochemical models among different circulation models.

On the Impact of WRF Model Vertical Grid Resolution on Midwest Summer Rainfall Forecasts

Abstract Weather Research and Forecast (WRF) model exploratory sensitivity simulations were performed to determine the impact of vertical grid resolution (VGR) on the forecast skill of Midwest summer rainfall. Varying the VGR indicated that a refined VGR, while adopting the widely used North America Regional Reanalysis (NARR) for initial and lateral boundary conditions, does not necessarily result in a consistent improvement in quantitative precipitation forecasts (QPFs). When averaged over a variety of microphysical schemes in an illustrative case, equitable threat score (ETS) and bias values actually worsened with a greater overpredicted rainfall for half of the rainfall thresholds when the VGR was refined. Averaged over strongly forced cases, ETS values worsened for all rainfall thresholds while biases mostly increased, indicating a further overprediction of rainfall when the number of levels was increased. Skill improved, however, for all rainfall thresholds when the resolution above the melting level was increased. Skill also improved for most rainfall thresholds when the resolution in the surface layer was increased, which is attributed to better-resolved surface turbulent momentum and thermal fluxes. Likewise, a refined VGR resulted in improvements in weakly forced cases, which are governed mostly by thermodynamic forcing and are sensitive to vertical profiles of temperature and moisture. Application of the factor separation method suggested that the refined VGR more frequently had a negative impact on skill through the interaction between lower-atmospheric processes and microphysical processes above the melting level.

Large‐Eddy Simulation of Maritime Deep Tropical Convection

This study represents an attempt to apply Large‐Eddy Simulation (LES) resolution to simulate deep tropical convection in near equilibrium for 24 hours over an area of about 205 × 205 km2, which is comparable to that of a typical horizontal grid cell in a global climate model. The simulation is driven by large‐scale thermodynamic tendencies derived from mean conditions during the GATE Phase III field experiment. The LES uses 2048 × 2048 × 256 grid points with horizontal grid spacing of 100 m and vertical grid spacing ranging from 50 m in the boundary layer to 100 m in the free troposphere. The simulation reaches a near equilibrium deep convection regime in 12 hours. The simulated vertical cloud distribution exhibits a tri‐modal vertical distribution of deep, middle and shallow clouds similar to that often observed in Tropics. A sensitivity experiment in which cold pools are suppressed by switching off the evaporation of precipitation results in much lower amounts of shallow and congestus clouds. Unlike the benchmark LES where the new deep clouds tend to appear along the edges of spreading cold pools, the deep clouds in the no‐cold‐pool experiment tend to reappear at the sites of the previous deep clouds and tend to be surrounded by extensive areas of sporadic shallow clouds. The vertical velocity statistics of updraft and downdraft cores below 6 km height are compared to aircraft observations made during GATE. The comparison shows generally good agreement, and strongly suggests that the LES simulation can be used as a benchmark to represent the dynamics of tropical deep convection on scales ranging from large turbulent eddies to mesoscale convective systems. The effect of horizontal grid resolution is examined by running the same case with progressively larger grid sizes of 200, 400, 800, and 1600 m. These runs show a reasonable agreement with the benchmark LES in statistics such as convective available potential energy, convective inhibition, cloud fraction, precipitation rates, and surface latent and sensible fluxes. All runs reveal a tri‐model cloud distribution in the vertical. However, there are differences in the updraft‐core cloud statistics, and convergence of statistical properties is found only between the LES benchmark and the run with 200 m grid size. The effect of vertical grid resolution is also investigated with another run that uses a typical cloud‐resolving model (CRM) horizontal grid size on the order of 1 km and only 64 vertical levels. A comparison to the run with 256 vertical levels shows different vertical cloud distributions. It is concluded that representation of the often observed tri‐modal vertical distribution of clouds requires a vertical grid spacing in the range of 50‐100 m in mid‐to‐low troposphere.

Modeling Supercritical Carbon Dioxide Injection in Heterogeneous Porous Media

We investigate the physical processes that occur during the sequestration of carbon dioxide (CO2) in liquid-saturated, brine-bearing geologic formations using the numerical simulator TOUGH2. CO2 is injected in a supercritical state that has a much lower density and viscosity than the liquid brine it displaces. In situ, the supercritical CO2 forms a gas-like phase, and also partially dissolves in the aqueous phase, creating a multi-phase, multi-component environment that shares many important features with the vadose zone. The flow and transport simulations employ an equation of state package that treats a two-phase (liquid, gas), three-component (water, salt, CO2) system. Chemical reactions between CO2 and rock minerals that could potentially contribute to mineral trapping of CO2 are not included. The geological setting considered is a fluvial/deltaic formation that is strongly heterogeneous, making preferential flow a significant effect, especially when coupled with the strong buoyancy forces acting on the gas-like CO2 plume. Key model development issues include vertical and lateral grid resolution, grid orientation effects, and the choice of characteristic curves.

Modeling basin‐scale internal waves in a stratified lake

Basin—scale internal waves provide the driving forces for vertical and horizontal fluxes in a stratified lake below the wind‐mixed layer. Thus, correct modeling of lake mixing and transport requires accurate modeling of basin‐scale internal waves: examining this capability with a hydrostatic, z‐coordinate three‐dimensional (3D) numerical model at coarse grid resolutions is the focus of this paper. It is demonstrated that capturing the correct thermocline forcing with a 3D mixed‐layer model for surface dynamics results in a good representation of low‐frequency internal wave dynamics. The 3D estuary and lake computer model ELCOM is applied to modeling Lake Kinneret, Israel, and is compared with field data under summer stratification conditions to identify and illustrate the spatial structure of the lowest‐mode basin‐scale Kelvin and Poincaré waves that provide the largest two peaks in the internal wave energy spectra. The model solves the unsteady Reynolds‐averaged Navier‐Stokes equations using a semi‐implicit method similar to the momentum solution in the TRIM code with the addition of quadratic Euler‐Lagrange dis‐cretization, scalar (e.g., temperature) transport using a conservative flux‐limited approach, and elimination of vertical diffusion terms in the governing equations. A detailed description is provided of turbulence closure for the vertical Reynolds stress terms and vertical turbulent transport using a 3D mixed‐layer model parameterized on wind and shear energy fluxes instead of the convential eddy viscosity/diffusivity assumption. This approach gives a good representation of the depth of the mixed‐layer at coarse vertical grid resolutions that allows the internal waves to be energized correctly at the basin scale.

Sensitivity of ozone to model grid resolution — II. Detailed process analysis for ozone chemistry

Most of the current air quality models used to simulate ozone (O3) formation predict only the concentrations of O3 without the capabilities of understanding and explaining the formation processes of O3. In this paper we present a process analysis method developed to understand and quantify the chemical and physical processes that lead to formation of O3 in Eulerian grid models. In a previous study we used a high-resolution version of regional acid deposition model (HR-RADM) to simulate O3 formation at different grid resolutions. In this study we further applied this detailed process analysis method to the HR-RADM simulations to determine the roles of individual mechanistic processes contributing to O3 formation, as well as to examine the effects of grid resolution on these regulating processes. We first selected several source areas and examined the processes that lead O3 formation in these areas. The “OH-cycle” and “NO-cycle” pathways derived from the process analysis method appear as important measures that can significantly enhance our ability to quantify and explain the formation processes of O3. We also compared O3 processes between two different grid resolutions over an equal source area with nearly equal emissions. The results suggest that (1) the effects of grid resolution on the chemistry of NOx are far more important than that on the chemistry of VOC; (2) grid resolution significantly influences the competing rates of chemistry and vertical transport processes for the emitted NOx, causing the differences in O3 predictions between two different grid resolutions. Because the balance of chemistry and vertical transport controls the model predictions, correct representation is needed for both. This leads to a conclusion that to improve model accuracy in predicting O3 formation, it is not only necessary to have adequate horizontal grid resolution, but also necessary to have adequate vertical grid resolution and accurate representation of the vertical transport process.

Modeling of CSO-Induced Pollutant Transport in Mt. Hope Bay

Combined sewage overflow (CSO) discharges due to a storm event create strongly stratified pollutant plumes on the surface of Mt. Hope Bay, of Rhode Island and Massachusetts. To examine the CSO-induced pollutant impact, a three-dimensional numerical model was used to simulate the transport processes in the bay resulting from a dye release at CSO discharges during a storm event on September 22, 1990. The model used a vertical γ-coordinate system, with higher grid resolution near surface, to describe the constituent transport. Model predictions reasonably showed that the CSO plume remained concentrated near the surface and was transported down bay along the eastern shore. Agreement of predictions of sea surface elevation, currents, salinity, and dye concentrations with available observations was good. Time series of predicted dye concentrations were used to estimate the flushing time, which was approximately 2 days at midbay. Sensitivity studies were performed for several vertical-grid resolutions. The use o...

ICARO: a package for wind field studies over complex terrain

ICARO is a software package developed to construct, examine, and compare wind fields in complex terrain. The user may interactively run mathematical models to evaluate three dimensional wind fields from available experimental measurements. Graphics and post-processing programs are implemented in ICARO to analyze and intercompare wind field results, as well as verify results with measurements. These features may be extended to results from other fluid-dynamic models. Different vertical grid resolutions are possible for verification of models performance. ICARO has proved to be a very quick and useful tool for both case studies and field experiments. The package is written in FORTRAN 77 and runs on DEC-VAX computers with a VMS operating system using NCAR graphics routines.

The effect of spatial resolution on the simulation of upper-tropospheric frontogenesis using a sigma-coordinate primitive equation model

The effects of varying horizontal and vertical grid resolution on the numerical simulation of upper-tropospheric frontal structures are examined using a, two-dimensional, dry, hydrostatic sigma-coordinate primitive equation model. These effects are illustrated with the results of 72 h model integrations in which frontogenesis is forced solely by confluence. Four different horizontal grid spacings, ranging from 100 km to 12.5 km, and four different vertical, grid resolutions, varying from 10 to 70 layers (∼90 mb to 13 mb), are considered. The intensity and integrity of the frontogenesis as a function of grid resolution are diagnosed in terms of time histories and spatial distributions of frontal parameters, such as the speed of the along-front jet, maxima of absolute vorticity and potential temperature gradient, and ageostrophic motions. The time histories show that, provided the vertical resolution is sufficient, increasing horizontal resolution leads to better-defined frontal structure due to the decrease in cross-frontal scale. They also indicate that for a given horizontal resolution there exists an optimal vertical resolution beyond which frontal parameters change only slightly. This optimal vertical resolution increases with increasing horizontal resolution, and apparently is related to the horizontal resolution through the slope of the frontal zone. The time histories for simulations combining low vertical resolution with high horizontal resolution exhibit substantial high-frequency variability. Cross sections show that this temporal variability appears to be manifested spatially in the form of gravity waves characterized by wavelengths on the order of 200 km and periods of 12 h, over the range of horizontal and vertical resolution that is considered. Although the source mechanism for these waves cannot be established definitively, it likely involves a grid-induced ageostrophic component of the along-front wind which disrupts thermal wind balance. This results thus demonstrates the potential risk of flawed frontal simulations, when incompatible combinations of horizontal and vertical resolution are used.

An Improved Numerical Approximation of the Horizontal Gradients in a Terrain-Following Coordinate System

Abstract The use of a fine vertical grid resolution near the surface may lead to a numerically inconsistent approximation of the horizontal gradient terms in a terrain-following coordinate system. This occurs when the distance between two vertical grid points is smaller than the elevation difference between two horizontally adjacent (in the terrain-following coordinate system) points. In this paper an improved numerical procedure is proposed which eliminates this inconsistency and significantly increases the accuracy of the numerical approximation. Results are compared with those obtained with the conventional forward and centered schemes.