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Philip L. Haagenson and Alvin L. Morris

Abstract

Forecasts of the position and structure of the St. Louis pollutant plume became necessary during the NCAR Fate of Atmospheric Pollutants Study (FAPS). The purpose of the project was to obtain estimates of the mean lifetimes of a number of short-lived pollutants by taking measurements at distances of 80 and 120 km from the Gateway Arch. Determining where and when to make such measurements at a reasonable cost involved assessing the interaction of the pollutants from the city with the atmosphere. This paper describes the concepts used by the meteorologist to help anticipate the variation of the plume in space and time, including its diurnal variation, in weather conditions considered favorable for sampling. The plume models that evolved from these concepts are presented, and a comparison is made of the actual behavior of the plume as determined from measured concentrations of the pollutants with the behavior predicted by the models. The comparison shows good agreement.

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Philip L. Haagenson and Paul D. Sperry

Abstract

A spring season sample of isentropic back trajectories for the North Atlantic region is presented. The sample data, derived from select time periods during four spring seasons, are used to investigate the relationship of three-dimensional trajectories with observed wind direction and synoptic type. The trajectory data show a predominance of transport from the west and suggest that three-dimensional air motions in the midlatitude regions have a pronounced vertical component whose direction and magnitude are significantly related to the transport direction. Statistical results indicate that the observed wind direction and synoptic type can be used to derive a reasonably good estimate of the mean transport direction for an ensemble of 36-h or 72-h trajectories. However, neither of them provides a good estimate of the transport direction for individual trajectories. The results also show that the mean vertical and horizontal displacements of the trajectory air parcels are not strongly related to observed wind direction or synoptic type.

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Richard A. Brost, Philip L. Haagenson, and Ying-Hwa Kuo

Abstract

We compared observed and model-simulated surface concentration on a spatial wale of 1100 km and a temporal scale of 36 h. The Eulerian tracer model calculated advection by the mean winds and gradient transport for subgrid-scale turbulent transfer. The simulations were evaluated by the maximum concentrations, spatial correlations between concentrations, plume sizes, and trajectory errors. We examined the following inputs and model parameters: 1) different meteorologies, including simulated, observed, and combinations of the two; 2) the spatial and temporal resolution of the observations; 3) the spatial resolution of the meteorological model; and 4) the spatial resolution of the tracer model. The best meteorology was observed horizontal winds, enhanced with additional, nonstandard rawinsondes and model-simulated eddy diffusivities. The spatial resolution of the tracer model was more important than that of the meteorological model. Meteorology from a mesoscale model could be competitive with that from standard observations. We found consistency between different measures of tracer simulation quality.

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Philip L. Haagenson, Kun Gao, and Ying-Hwa Kuo

Abstract

Perfluorocarbon tracer data collected during the Across North America Tracer Experiment (ANATEX) are used to evaluate different meteorological analysis, simulations, and long-range transport calculations. Three basic types of meteorological analysis and simulation are evaluated: objective analysis of observed data, prognostic simulation with observed lateral boundary conditions, and four-dimensional data assimilation (FDDA). The evaluation is based on 1) the root-mean-square separation between two-dimensional meteorological trajectories (constructed from different analyses or simulations) and surface tracer trajectories and 2) the relationship between the upward displacement of three-dimensional trajectories and the maximum value of the surface tracer concentration. The root-mean-square data indicate that the optimum value of the Newtonian nudging coefficient for the FDDA wind field in the lower troposphere is 6 × 10−4 s−1, and the quality of the prognostic simulation is lower than that for FDDA or the objective analysis, particularly when surface fronts are present. These data also show that trajectory errors, with respect to transport distance, are larger in low-speed wind regimes than in medium- to high-speed regimes, and suggest that the rate of increase of trajectory error decreases with time, but the uncertainty of the rate of increase is quite large during the first 18 h of transit. The three-dimensional trajectories indicate that large-scale upward motion is a mechanism for removal of tracer from the boundary layer, and the strongest correlation between the upward displacement of the trajectory air parcels and the surface tracer concentration is obtained using the FDDA dataset. The overall results suggest that when both the vertical and horizontal components of the wind fields are considered, FDDA (using an appropriate value for the nudging coefficient) is better than the other methodologies.

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Philip L. Haagenson, Ying-Hwa Kuo, Marina Syumanich, and Nelson L. Seaman

Abstract

Perfluorocarbon tracer data collected during the Cross Appalachian Tracer Experiment (CAPTEX '83) are used to determine the accuracy of three trajectory models: an isentropic, an isobaric, and a dimensional sigma model. The root-mean-square separation between model trajectories and trajectories derived from the surface tracer concentration is used to evaluate the models and assess the validity of isobaric, isentropic, isosigma, and mean transport vector assumptions. The root-mean-square data suggest that wind flow corresponding approximately to the low to middle boundary layer is the most appropriate for simulating the transport of boundary layer pollutants, and that the isentropic and isosigma transport assumptions are more realistic than the isobaric assumption, The results also indicate that synoptic type and the diurnal variation of mixing and wind shell within the boundary layer can affect the magnitude of root-mean-square separation between tracer trajectory and transport model trajectories. The uncertainty of the trajectory error suggested by the root-mean- square separation is approximately 50 km. Comparison of the tracer study with a theoretical study suggests that surface tracer data are useful for quantifying the magnitude of error in trajectory model calculations of boundary layer transport.

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Ying-Hwa Kuo, Marina Skumanich, Philip L. Haagenson, and Julius S. Chang

Abstract

Fourteen observing system simulation experiments (OSSE) wore conducted using the results from a mesoscale model on the Oxidation and Scavenging Characteristics of April Rains(OSCAR) experiment to test the accuracy of trajectory models. Our results indicate that the current synoptic network and observational frequency over North America are inadequate for accurate computation of long-range transport of episodic events. It appears that improving the Observational frequency would be more cost effective than improving the spatial resolution for the existing network.

Reducing the three-dimensional air flow to two dimensions leads to a substantial amount of error for air parcel trajectories. Among the three simplifying assumptions—isobaric, isosigma, and isentropic—the isentropic model gives considerably better results than the isobaric or isosigma models, especially for the vertical transport.

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