Entire Model Domain Research Articles

On‐line simulations of passive chemical tracers in the University of California, Los Angeles, atmospheric general circulation model: 1. CFC‐11 and CFC‐12

Long‐term simulations of the response of atmospheric CFC‐11 and CFC‐12 to standard emission scenarios have been carried out using the University of California, Los Angeles (UCLA) atmospheric general circulation model (AGCM) coupled on‐line with the UCLA atmospheric chemistry model. For both compounds, photochemical loss rates are computed interactively over the entire model domain at each time step of the integration. Using industrial‐based emission estimates, the simulations for CFC‐12 closely track the long‐term trends recorded in both hemispheres by the Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment/Advanced Global Atmospheric Gases Experiment (AGA) and Climate Monitoring and Diagnostics Laboratory monitoring networks. The agreement between simulations and observations is best when AGA‐deduced emissions are employed. The predicted surface mixing ratios of CFC‐11, on the other hand, are somewhat overestimated by the model. Because the transport and loss processes, as well as source distributions, are roughly similar for these halocarbons, the divergence in surface concentrations points to the possibility that emissions of CFC‐11 may be overestimated for the period extending from the late 1980s through the early 1990s, and perhaps even at earlier times. As for CFC‐12, the best agreement is achieved using AGA emissions. The simulated interhemispheric exchange time constant for these CFCs is about 0.6 year. In the annual cycle, maximum transport occurs from the Northern to Southern Hemisphere within the lowest atmospheric layers during northern winter. Our best estimates of the annually averaged mean global lifetimes of CFC‐11 and CFC‐12 are about 55 and 100 years, respectively. The simulations indicate that both the mean residence time and interhemispheric exchange rate depend on the assumed model vertical domain. For the mass balance analysis, when the upper boundary of the AGCM is artificially fixed below ∼35 km for CFC‐11, or ∼43 km for CFC‐12, there is a tendency for the timescales (lifetimes and interhemispheric exchange times) to be overestimated. Comparisons between CFC distributions and trends calculated using low and high spatial resolution show relatively small differences in the present case. These results, especially regarding CFC persistence and interhemispheric exchange, suggest that the present model accurately represents the global dispersion of long‐lived chemical tracers.

A Hybrid Eulerian–Lagrangian Model for Spectral Wave Evolution with Application to Bottom Friction on the Continental Shelf

Abstract A hybrid Eulerian–Lagrangian wave model is presented that solves the spectral energy balance equation for surface gravity waves in varying depth. The energy of each spectral component is advected along (Lagrangian) ray trajectories. The source terms in the energy balance equation (e.g., interactions between wave components and nonconservative processes) are computed on a fixed Eulerian grid and interpolated onto the ray trajectories. The source terms are integrated in time along the rays. This integration is performed in parallel over the entire model domain. The main advantage of this new model, named CREST (Coupled Rays with Eulerian Source Terms), is that refraction of waves by subgrid-scale depth variations is evaluated accurately using precomputed rays, and thus the model can be applied with relatively coarse source term grids to large coastal areas. Hindcasts of swell evolution across the North Carolina continental shelf are presented for a source term restricted to energy dissipation in th...

Hydraulic Gradient Comparison Method to Estimate Aquifer Hydraulic Parameters Under Steady‐State Conditions

AbstractThe hydraulic gradient comparison method is an inverse method for estimation of aquifer hydraulic conductivity (or trans‐missivity) and boundary conductance for a ground water flow model under steady‐state conditions. This method, following formal optimization techniques, defines its objective function to minimize differences between interpreted (observed) and simulated hydraulic gradients, which results in minimization of differences between observed and simulated hydraulic heads. The key features of this method are that (1) the derived optimality conditions have an explicit form with a clear hydrology concept that is con‐sistent with Darcy's law, and (2) the derived optimality conditions are spatially independent as they are a function of only local hydraulic conductivity and local hydraulic gradient. This second feature allows a multidimensional optimization problem to be solved by many one‐dimensional optimization procedures simultaneously, which results in a substantial reduction in computation time. The results of the numerical performance testing on a heterogeneous hypothetical case confirm that minimizing gradient residuals in the entire model domain leads to minimizing head residuals. Application of the method in real‐world projects requires rigorous conceptual model development, use of a global calibration target, and an iterative calibration proess. The conceptual model development includes interpretation of a potentiometric surface and estimation of other hydrologic parameters. This method has been applied to a wide range of real‐world modeling projects, including the Rocky Mountain Arsenal and Rocky Flats sites in Colorado, which demonstrates that the method is efficient and practical.

A continental outbreak of air during the Second Aerosol Characterization Experiment (ACE 2): A Lagrangian experiment

A mesoscale meteorological model is used to simulate the continental outbreak of air that occured over North Atlantic Ocean during the Second Aerosol Characterization Experiment (ACE 2). Comparison with Meteosat 6 visible images shows that the model is able to reproduce most of the observed cloud features in the entire modeling domain, including a frontal system, orographic clouds over the Pyrenees and the Iberian Peninsula, boundary layer clouds in the ACE 2 region, and cloud‐free areas in the dry air outflow off the Portuguese coast. The model reproduces correctly the temperature, humidity, wind speed and direction, cloud water content, and light precipitation events observed by the aircraft during the Lagrangian experiment, while following a tagged boundary layer air parcel over a 28‐hour long period. A passive tracer, introduced in the model at the time corresponding to the balloon launch, followed the balloon track with an error in position that is smaller than 1° after 24‐hours with about 20% higher speed. An estimate of the boundary layer entrainment velocity is obtained, based on observed boundary layer growth rates and modeled vertical wind speed. The average entrainment velocity of 1.28 cm/s for the 28‐hour Lagrangian period agrees with experimentally derived entrainment velocities.

Changes of total CO2 and pCO2 in the surface ocean during the mixed layer development in the northern North Pacific

A main objective is to simulate the seasonal variations in a carbon cycle during the mixed layer development from a fall through winer over the northern North Pacific. A bulk mixed layer model is employed, which is initialized with the historical data and forced by the observed atmospheric data. The model retains the following mechanisms: cooling and freshwater flux through the sea surface, kinetic energy input due to wind stresses, and air‐sea CO2 flux as well as turbulent vertical mixing of heat, salt, total CO2, and carbonate alkalinity at the bottom of the mixed layer. Total CO2, carbonate alkalinity, and pCO2 are related to each other under the assumption of chemical equilibrium. Since biological processes are secondary for the pCO2 evolution during winter, they are omitted. The evolution of pCO2 in the model mixed layer shows a rapid increase of pCO2 in the subpolar region due to mixing of deep water rich in CO2 before exceeding the atmospheric pCO2, and then the CO2 flux stays almost constant during the winter. In the subtropical region the CO2 is transported always from the atmosphere to the ocean, as the pCO2 decreases from cooling. The ocean is a CO2 sink (−0.18 GtC half‐year−1) over the entire model domain north of 30°N. However, in the subpolar region the ocean is a CO2 source (0.04 GtC half‐year−1). The CO2 flux varies from year to year, while there is no qualitative difference in the flux distribution.

Mean circulation and transports in the South Atlantic Ocean: Combining model and drifter data

Numerical experiments with a medium‐resolution primitive equation model of the South Atlantic mean circulation are described. The results from the standard model realization indicate that the model succeeds in representing the large‐scale transport and circulation features. However, a comparison with a velocity field derived from surface drifter data reveals discrepancies of the modeled velocities from the observations in magnitude as well as direction of the flow field. In order to diminish the model deviations from the data, an attempt is made to couple the model to the observations through a simple data assimilation technique. The assimilated model succeeds in improving the subtropical gyre circulation. Only a minor effect on the basin‐scale integrated quantities is observed. However, the density field may be deformed as a response to the assimilation of velocity data without simultaneously adapting a corresponding density structure. The influence of the disturbance of the density structure is most prominent at the edges of the observed data set, which does not cover the entire model domain, and is confined to the upper ocean and balanced above the thermocline. We calculated a meridional heat transport that is generally in accordance with estimates from other sources. The analysis of heat and salt fluxes suggests that the model features both the so‐called “warm water path” and “cold water path” in closing the global thermohaline circulation. While heat is mainly imported in surface and thermocline waters with the Agulhas Current around South Africa, it is the Antarctic Intermediate Water that compensates for more than 50% of the salt loss by the outflowing North Atlantic Deep Water.

The Project for Intercomparison of Land-surface Parameterization Schemes (PILPS) Phase 2(c) Red–Arkansas River basin experiment:: 1. Experiment description and summary intercomparisons

Sixteen land-surface schemes participating in the Project for the Intercomparison of Land-surface Schemes (PILPS) Phase 2(c) were run using 10 years (1979–1988) of forcing data for the Red–Arkansas River basins in the Southern Great Plains region of the United States. Forcing data (precipitation, incoming radiation and surface meteorology) and land-surface characteristics (soil and vegetation parameters) were provided to each of the participating schemes. Two groups of runs are presented. (1) Calibration–validation runs, using data from six small catchments distributed across the modeling domain. These runs were designed to test the ability of the schemes to transfer information about model parameters to other catchments and to the computational grid boxes. (2) Base-runs, using data for 1979–1988, designed to evaluate the ability of the schemes to reproduce measured energy and water fluxes over multiple seasonal cycles across a climatically diverse, continental-scale basin. All schemes completed the base-runs but five schemes chose not to calibrate. Observational data (from 1980–1986) including daily river flows and monthly basin total evaporation estimated through an atmospheric budget analysis, were used to evaluate model performance. In general, the results are consistent with earlier PILPS experiments in terms of differences among models in predicted water and energy fluxes. The mean annual net radiation varied between 80 and 105 W m −2 (excluding one model). The mean annual Bowen ratio varied from 0.52 to 1.73 (also excluding one model) as compared to the data-estimated value of 0.92. The run-off ratios varied from a low of 0.02 to a high of 0.41, as compared to an observed value of 0.15. In general, those schemes that did not calibrate performed worse, not only on the validation catchments, but also at the scale of the entire modeling domain. This suggests that further PILPS experiments on the value of calibration need to be carried out.

Fine grid tidal modeling of the Yellow and East China Seas

A fine grid tidal modeling experiment is carried out in order to investigate the tidal regimes for major five tidal constituents, the nonlinear tidal phenomena in terms of M 4 and MS 4 generation, and the independent tide by the tide generating force in the Yellow and East China Seas (YECS). In this study a two-dimensional numerical model based upon a subgrid-scale (SGS) stress modeling technique is used with the tide generating force included. The model was validated with recently observed tide and current data. The calculated tidal charts for tidal elevation show a generally good agreement with existing ones, with more accurate feature of the M 2 cotidal chart in comparison with both the observed data and the existing tidal charts. A careful comparison of the computed diurnal amplitude with observations suggests that the diurnal constituents seem to be overdamped especially in the Kyunggi Bay region, for the case when quadratic bottom friction law is used. Propagation features of the M 4( MS 4) tides are discussed in the YECS, based upon the analyses of the observed and calculated results. The amphidromic system of the M 4 is quite complicated and one noticeable characteristic is that the propagation direction of the M 4 tidal wave along the west coast of Korean peninsula is opposite to that of the M 2 tidal wave. This result coincides with observations. The propagation feature of the MS 4 is almost similar to that of the M 4, but with lesser amplitude. The responses of the M 4 tidal features to momentum diffusion term and depth-dependent form of the friction coefficient are also discussed. It is also shown that when the independent tide ( Defant, 1960) arising from tide generating force (TGF) coexists with tidal waves (co-oscillating tide) arising from external boundary forcing, the TGF tidal waves are dissipated and their amphidromes tend to move westward. This may be interpreted as a process whereby the incident and reflected TGF tidal waves are damped by co-oscillating tide arising from external force at open boundaries. The TGF amplitude is found to be up to 10 cm and 4 cm in the Kyunggi Bay for the M 2 and S 2 constituents while those for all diurnal constituents are less than 1 cm over the entire model domain.

Using statistical methods in combination with FEM modelling for parameter studies in the field of materials technology

FEM models are generally expensive, time consuming and yielding a large amount of information, much of which of secondary interest. In many cases the information sought can be given as one or a few key figures although complete fields of stresses, velocities or temperatures are computed throughout the entire model domain. If FEM analysis is used for parameter studies, the goal is generally to determine some trends in the data. An efficient way to do this is to apply statistical experimental design for selecting the cases to be simulated and use multivariate regression analysis to fit a simple model for the relationship between the parameters and the key figures of interest. Based on this model, it is possible to estimate the magnitude of the key figures for any combination of parameter values within the range studied. This approach has been applied for flow simulations of various metal forming processes at RDM-HA, Karmφy. Valuable results have been obtained for twin-roll strip casting, extrusion and wire rolling. A problem frequently encountered when using experimental design in an industrial scale is that the process limits the control of the free variables, giving possible co-variance in the parameters. This will impair the applicability of the regression analysis. For FEM studies this is rarely a problem - the exception being closed systems where constraints in e.g. geometry or composition are present. This approach also allows the combination of data from FEM analysis and on-site experiments. Depending of the nature of the data different concepts for data point weighting may be selected when combining simulated and measured data in the regression analysis.

The determination of the large‐scale circulation of the Pacific Ocean from satellite altimetry using model Green's functions

A Green's function method for obtaining an estimate of the ocean circulation using both a general circulation model and altimetric data is demonstrated. The fundamental assumption is that the model is so accurate that the differences between the observations and the model‐estimated fields obey a linear dynamics. In the present case, the calculations are demonstrated for model/data differences occurring on a very large scale, where the linearization hypothesis appears to be a good one. A semi‐automatic linearization of the Bryan/Cox general circulation model is effected by calculating the model response to a series of isolated (in both space and time) geostrophically balanced vortices. These resulting impulse responses or “Green's functions” then provide the kernels for a linear inverse problem. The method is first demonstrated with a set of “twin experiments” and then with real data spanning the entire model domain and a year of TOPEX/POSEIDON observations. Our present focus is on the estimate of the time‐mean and annual cycle of the model. Residuals of the inversion/assimilation are largest in the western tropical Pacific, and are believed to reflect primarily geoid error. Vertical resolution diminishes with depth with 1 year of data. The model mean is modified such that the subtropical gyre is weakened by about 1 cm/s and the center of the gyre shifted southward by about 10°. Corrections to the flow field at the annual cycle suggest that the dynamical response is weak except in the tropics, where the estimated seasonal cycle of the low‐latitude current system is of the order of 2 cm/s. The underestimation of observed fluctuations can be related to the inversion on the coarse spatial grid, which does not permit full resolution of the tropical physics. The methodology is easily extended to higher resolution, to use of spatially correlated errors, and to other data types.

Atmosphere-ocean interaction

An oceanic model with an Eulerian vertical coordinate and an explicit vertical advection scheme is subject to the Courant–Friedrichs–Lewy (CFL) limitation. Depending on the horizontal grid spacing, the horizontal-to-vertical grid resolution ratio and the flow pattern this limitation may easily become the most restrictive factor in choosing model time step, with the general tendency to become more severe as horizontal resolution becomes finer. Using terrain-following coordinate makes local vertical grid spacing depend on topography, ultimately resulting in very fine resolution in shallow areas in comparison with other models, z-coordinate, and isopycnic, which adds another factor in restricting time step. At the same time, terrain-following models are models of choice for the fine-resolution coastal modeling, often including tides interacting with topography resulting in large amplitude baroclinic vertical motions. In this article we examine the possibility of mitigating vertical CFL restriction, while at the same time avoiding numerical inaccuracies associated with standard implicit advection schemes. In doing so we design a combined algorithm which acts like a high-order explicit scheme when Courant numbers are small enough to allow explicit method (which is usually the case throughout the entire modeling domain except just few “hot spots”), while at the same time has the ability to adjust itself toward implicit scheme should it became necessary to avoid stability limitations. This is done in a seamless manner by continuously adjusting weighting between explicit and implicit components.

Efficacy of nitrogen oxides and hydrocarbons emissions controls in ozone attainment strategies as predicted by the Urban Airshed Model

The primary object of this paper is to provide a preliminary assessment of the effectiveness of NO x vs Volatile Organic Compounds (VOC) emissions control options in improving O3 air quality over the New York metropolitan area. To this end, we have applied the Urban Airshed Model (UAM) with the Carbon Bond IV (CB-IV) chemical mechanism utilizing the results of the Regional Oxidant Model (ROM) for the specification of initial/boundary concentrations and wind fields to the UAM. After examining the sensitivity of the predicted O3 concentrations to initial/boundary conditions and biogenic emissions, we have evaluated the impact of various hypothetical emissions reduction options on O3 air quality. Nested ROM/UAM simulations with an across-the-board reduction of 75% in the NO x and VOC emissions from sources located within the New York metropolitan area indicate that the option of VOC-only control is superior to the NO x -only control in reducing not only peak O3 levels over the entire modeling domain but also population exposure to unhealthy O3 levels. The model predicts that the combined 75% NO x and VOC control option also reduces the peak O3 concentration, but the improvement in O3 air quality is less than that predicted for the 75% VOC-only control strategy. Additional modeling analyses with different mix and levels of emissions control and meteorological conditions are needed to confirm these preliminary findings.

Modeling sulfur concentrations and depositions in the United States during ANATEX

Abstract The Hybrid Single-Particle Lagrangian Integrated Trajectory (HY-SPLIT) long-range transport, personal-computer-based model was incorporated with a non-linear chemistry module that includes gas- and aqueous-phase oxidation of sulfur dioxide (SO2) and dry and wet removal of SO2 and sulfate (SO42−) particles. The model considers multiple area and point sources (∼ 500) and calculates simultaneously 24-h averages of several species: air concentrations of SO2 and SO42− particles and SO42− wet deposition on each grid cell of the meteorological domain. Three-month averages of modeled air concentrations of SO2 and SO42− were compared with measurements from five National Oceanic and Atmospheric Administration/Air Resources Laboratory dry deposition sites. The model tended to underpredict air concentrations of SO2 (ratios of model-predicted to measured concentrations between 0.6 and 1.2) and overpredict air concentrations of SO42− (ratios between 1.0 and 1.6). Modeled SO42− wet deposition values were calculated, averaged for the 3 months, and compared with the measured values at 137 combined UAPSP, MAP3S, and NADP precipitation chemistry sites. For the majority of these sites, the agreement was good; the ratios between modeled and measured depositions were between 0.5 and 1.6. However, a consistent underprediction was observed across the southern and extreme eastern U.S. This underprediction was primarily attributed to the proximity of the edge of the model domain to the eastern and southern U.S., which caused trajectories from the continent that would have curved back into the south-eastern U.S. to terminate prematurely. When SO2 emissions were reduced by 50% over the entire model domain, approximately 50–55% reductions were observed in SO2 air concentrations over the entire model domain and at all levels. However, sulfate air concentrations and depositions were only reduced 35–50% due to non-linear reactions involving SO2. When emissions of SO2 were reduced by 50% only in the Ohio Valley, reductions in SO2 and SO42− air concentrations to the north and south of the Ohio Valley were generally less than 15%. These small reductions are attributed to local emissions of SO2 and long-range transport of SO42− from other regions not affected by the reduction in emissions. SO42− wet deposition, however, was reduced by 15–23% to the north of the Ohio Valley, indicating some long-range transport from the Ohio Valley. In the Ohio Valley, reductions of SO2 air concentrations were between 10 and 42%, depending on the proximity to major sources, and reductions of SO42− air concentrations were between 10 and 30%. However, SO42− wet deposition was reduced less than 10%, consistent with the assumption that its primary contribution is from long-range transport.