Chih-Yue Jim Kao

Simulations of flow around a cubical building: comparison with towing-tank data and assessment of radiatively induced thermal effects

Abstract A three-dimensional (3-D) computational fluid dynamics (CFD) model, coupled with a meteorological radiation and surface physics package, is used to model the mean flow field and tracer dispersion in the vicinity of an idealized cubical building. We first compare the simulations with earlier numerical studies as well as towing-tank laboratory experiments, where radiation effects were not included. Our simulations capture most of the features revealed by the towing-tank data, including the variation of the flow reattachment point as a function of Froude number and the induction of a prominent lee wave in the low Froude number regime. The simulated tracer concentration also compares very favorably with the data. We then assess the thermal effects due to radiative heating on the ground and building including shading by the building, on the mean flow and tracer dispersion. Our simulations show that convergence within and beyond the cavity zone causes a substantial lofting of the air mass downstream from the building. This lofting results from the combination of thermal heating of the ground and building roof, and vortex circulation associated with the horseshoe eddy along the lateral sides of the building. The specific effect of shading on the flow field is isolated by comparing simulations for which the radiative heating and shading patterns are kept constant, but the environmental wind direction is altered. It is found that the shading exerts local cooling, which can be combined into the overall thermodynamic interaction, described above, to effectively alter the circulation downstream from the building.

Use of the CAPTEX Data for Evaluations of a Long-Range Transport Numerical Model with a Four-Dimensional Data Assimilation Technique

Abstract A “four-dimensional data assimilation” technique is employed in a time-dependent, three-dimensional mesoscale model to simulate long-range pollutant transport and diffusion in the eastern United States using the 1983 Cross-Appalachian Tracer Experiment (CAPTEX) data. CAPTEX deployed 19 rawinsonde stations to measure upper-air meteorological conditions four times daily and 86 automatic sequential air samplers to measure tracer concentrations from a point source. The total area coverage of the data network is approximately 1000 km (east-west) × 800 km (north-south). The assimilated wind fields and model-produced turbulence fields during a period of 2¼ days are used to simulate plume trajectories and surface concentrations through a random-particle statistical method. Two tracer releases in the CAPTEX are investigated: one was in a light-wind fair weather condition and produced a widely spread puff distribution; the other was associated with a surface cold front resulting in a rather narrow Puff dis...

Atmospheric Effects of the Emerging Mainland Chinese Transportation System at and Beyond the Regional Scale

Abstract Local surface travel needs in the People‘s Republic of China (mainlandChina) have traditionally been met largely by nonpolluting bicycles. A majorautomobile manufacturing/importing effort has begun in the country over thelast decade, and planning documents indicate that the Chinese may strive toacquire more than 100 million vehicles early in the next century. By analogywith large automotive fleets already existing in the western world, bothregional and global scale pollution effects are to be expected from theincrease. The present work adopts the latest projections of Chinese automobilemanufacture and performs some quantitative assessments of the extent ofpollution generation.Focus for the investigation is placed upon the oxidant ozone. Emissions of theprecursor species nitrogen oxides and volatile organics are constructed basedon data for the current automotive sector in the eastern portion of the UnitedStates. Ozone production is first estimated from measured values forcontinental/oceanic scale yields relative to precursor oxidation. Theestimates are then corroborated through idealized two dimensional modeling ofthe photochemistry taking place in springtime air flow off the Asian land massand toward the Pacific Ocean. The projected fleet sizes could increase coastaland remote oceanic ozone concentrations by tens of parts per billion (ppb)in the lower troposphere. Influences on the tropospheric aerosol system andon the major greenhouse gas carbon dioxide are treated peripherally. Nitrogenoxides created during the vehicular internal combustion process willcontribute to nitrate pollution levels measured in the open Pacific. Thepotential for soot and fugitive dust increases should be considered as theautomotive infrastructure develops. Since the emerging Chinese automotivetransportation system will represent a substantial addition to the globalfleet and all the carbon in gasoline is eventually oxidized completely, asignificant rise in global carbon dioxide inputs will ensue as well.Some policy issues are treated preliminary. The assumption is made thatalterations to regional oxidant/aerosol systems and to terrestrial climate areconceivable. The likelihood that the Chinese can achieve the latest vehiclefleet goals is discussed, from the points of view of new production, positivepollution feedbacks from a growing automobile industry, and known petroleumreserves. Vehicular fuel and maintenance options lying before the Chinese areoutlines and compared. To provide some perspective on the magnitude of theenvironmental changes associated with an Asian automotive buildup, recentestimates of the effects of future air traffic over the Pacific Rim aredescribed.

Numerical Simulations of Observed Arctic Stratus Clouds Using a Second-Order Turbulence Closure Model

Abstract A high-resolution one-dimensional version of a second-order turbulence closure radiative-convective model, developed at Los Alamos National Laboratory, is used to simulate the interactions among turbulence, radiation, and bulk cloud parameters in stratiform clouds observed during the Arctic Stratus Experiment conducted during June 1980 over the Beaufort Sea. The fidelity of the model to the underlying physics is assessed by comparing the modeled evolution of the cloud-capped boundary layer against data reported for two particular days of observations. Over the period encompassed by these observations, the boundary layer evolved from a well-mixed cloud-capped boundary layer overlying a stable cloudy surface layer to a shallower well-mixed boundary layer with a single upper cloud deck and a clear, diminished, stable surface layer. The model was able to reproduce the observed profiles of the liquid water content, cloud-base height, radiative heating rates, and the mean and turbulence variables over ...

Ventilation of liquefied petroleum gas components from the Valley of Mexico

The saturated hydrocarbons propane and the butane isomers are both indirect greenhouse gases and key species in liquefied petroleum gas (LPG). Leakage of LPG and its component alkanes/alkenes is now thought to explain a significant fraction of the volatile organic burden and oxidative potential in the basin which confines Mexico City. Propane and the butanes, however, are stable enough to escape from the basin. The gas Chromatographie measurements which have drawn attention to their sources within the urban area are used here to estimate rates of ventilation into the free troposphere. The calculations are centered on several well studied February/March pollution episodes. Carbon monoxide observations and emissions data are first exploited to provide a rough time constant for the removal of typical inert pollutant species from the valley. The timescale obtained is validated through an examination of meteorological simulations of three-dimensional flow. Heuristic arguments and transport modeling establish that propane and the butanes are distributed through the basin in a manner analogous to CO despite differing emissions functions. Ventilation rates and mass loadings yield outbound fluxes in a box model type computation. Estimated in this fashion, escape from the Valley of Mexico constitutes of the order of half of 1% of the northern hemispheric inputs for both propane and n-butane. Uncertainties in the calculations are detailed and include factors such as flow into the basin via surface winds and the size of the polluted regime. General quantification of the global propane and butane emissions from large cities will entail studies of this type in a variety of locales.

Airflow and air quality simulations over the western mountainous region with a four-dimensional data assimilation technique

Abstract We apply a three-dimensional meteorological model with a four-dimensional data assimilation (4-DDA) technique to simulate diurnal variations of wind, temperature, water vapor, and turbulence in a region extending from the west coast to east of the Rockies and from northern Mexico to Wyoming. The wind data taken during the 1985 SCENES ( S ubregional C ooperative E lectric Utility, Dept. of Defense, N ational Park Service, and E nvironmental Protection Agency S tudy on Visibility) field experiments are successfully assimilated into the model through the 4-DDA technique by ‘nudging’ the modeled winds toward the observed winds. The modeled winds and turbulence fields are then used in a Lagrangian random-particle statistical model to investigate how pollutants from potential sources are transported and diffused. Finally, we calculate the ground concentrations through a kernel density estimator. Two scenarios in different weather patterns are investigated with simulation periods up to 6 days. One is associated with the evolution of a surface cold front and the other under a high-pressure stagnant condition. In the frontal case, the impact of air-mass movement on the ground concentrations of pollutants released from the Los Angeles area is well depicted by the model. Also, the pollutants produced from Los Angeles can be transported to the Grand Canyon area within 24 h. However, if we use only the data that were obtained from the regular NWS rawinsonde network, whose temporal and spatial resolutions are coarser than those of the special network, the plume goes north-northeast and never reaches the Grand Canyon area. In the stagnant case, the pollutants meander around the source area and can have significant impact on local air quality.

Numerical Simulations of the Marine Stratocumulus-Capped Boundary Layer and Its Diurnal Variation

Abstract A high-resolution one-dimensional version of a second-order turbulence radiative–convective model, developed at Los Alamos National Laboratory, is used to simulate the diurnal cycle of the marine stratocumulus cloud-capped boundary layer. The fidelity of the model to the underlying physics is assessed by comparing the model simulation to data taken at San Nicolas Island during the intensive field observation (IFO) of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE), conducted during June and July 1987. The model is able to reproduce the observed diurnal cycle of the liquid water content, cloud-base height, radiative heating or cooling rates, and the mean and turbulence variables fairly well. The mechanisms that cause the diurnal variation and the decoupling of the boundary layer are examined. The possible role of an imposed diurnal cycle for the subsidence in inducing the cloud-top diurnal cycle observed during the FIRE IFO is also addressed. Three re...

Photochemical Numerics for Global-Scale Modeling: Fidelity and GCM Testing

Atmospheric photochemistry lies at the heart of global-scale pollution problems, but it is a nonlinear system embedded in nonlinear transport and so must be modeled in three dimensions. Total earth grids are massive and kinetics require dozens of interacting tracers, taxing supercomputers to their limits in global calculations. A matrix-free and noniterative family scheme is described that permits chemical step sizes an order of magnitude or more larger than time constants for molecular groupings, in the 1-h range used for transport. Families are partitioned through linearized implicit integrations that produce stabilizing species concentrations for a mass-conserving forward solver. The kinetics are also parallelized by moving geographic loops innermost and changes in the continuity equations are automated through list reading. The combination of speed, parallelization, and automation renders the programs naturally modular. Accuracy lies within 1% for all species in week-long fidelity tests. A 50-species, 150-reaction stratospheric module tested in a spectral GCM benchmarks at 10 min CPU time per day and agrees with lower-dimensionality simulations. Tropospheric nonmethane hydrocarbon chemistry will soon be added, and inherently three-dimensional phenomena will be investigated both decoupled from dynamics and in a complete chemical GCM

High-resolution modeling of LIDAR data mechanisms governing surface water vapor variability during SALSA.

An integrated tool that consists of a volume scanning high-resolution Raman water vapor LIDAR and a turbulence-resolving hydrodynamic model, called HIGRAD, is used to support the semi-arid land-surface‐atmosphere (SALSA) program. The water vapor measurements collected during SALSA have been simulated by the HIGRAD code with a resolution comparable with that of the LIDAR data. The LIDAR provides the required “ground truth” of coherent water vapor eddies and the model allows for interpretation of the underlying physics of such measurements and characterizes the relationships between surface conditions, boundary layer dynamics, and measured quantities. The model results compare well with the measurements, including the overall structure and evolution of water vapor plumes, the contrast of plume variabilities over the cottonwoods and the grass land, and the mid-day suppression of turbulent activities over the canopy. The current study demonstrates an example that such an integration between modeling and LIDAR measurements can advance our understanding of the structure of fine-scale turbulent motions that govern evaporative exchange above a heterogeneous surface.

A streamlined family photochemistry module reproduces major nonlinearities in the global tropospheric ozone system

Abstract Tropospheric photochemistry is central to terrestrial climate change and pollution effects and so will be modelled on global 3-D grids. The chemistry is also complex, numerically stiff and kinetically nonlinear. A packaged family based integrator has been developed specifically to combat the difficulties associated with computational modelling of atmospheric chemistry on the global scale. The present work describes its ability to reproduce the major nonlinear features of tropospheric kinetics— those relating the nitrogen oxides (NOx) and oxidizing organics to ozone. It is shown that the family modules can duplicate typical changes in ozone production as a function of NOx level while consuming a minimum number of mathematical operations. The tests are first performed in the box model mode for a variety of pristine and pollutant scenarios. Zero-dimensional runs are patterned largely after the nonlinearity investigations of Liu and coworkers. The testing is then extended to column representations for vertical mixing of ozone precursors in convective storms. Here the calculations follow the climatology of ozone production enhancements assembled by Pickering and colleagues. Benchmarking is reported for a mechanism containing full inorganic kinetics as well as decomposition sequences for six nonmethane hydrocarbons. Chemical species in the simulation number 92. The operations count is roughly 10,000 per cell step for time increments of 1 h or more. The coding should thus enable decadal scale runs on massively parallel processors. Scaling experiments indicate full vectorization has been achieved. The chemistry packages are optimized not only for speed but also for convenience. Modularity and routines automating setup of solutions to the kinetic continuity equations are outlined as incidentals.