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Roland R. Draxler

Abstract

Some recent analyses of long-range transport and dispersion indicated conflicting results regarding the improvement in trajectory calculations when either the spatial or temporal density of the meteorological data are enhanced. Tests conducted with a variety of combinations of meteorological data, collected during CAPTEX, showed that increased temporal resolution did increase trajectory accuracy; however, it was not significantly different from enhancing the spatial coverage. Trajectory error was assumed to equal the distance between the centroid locations of the measured and calculated air concentration patterns. The most accurate trajectories were calculated when both spatial and temporal resolution were enhanced, such that the rms trajectory error was decreased from 180 km to 154 km for travel times of 24 to 42 h. Although the inclusion of surface observations did not improve calculated trajectories, the addition of vertical motion in the trajectory calculation methodology resulted in a further reduction of trajectory error to an average of 144 km.

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Roland R. Draxler

Abstract

Aircraft tracer measurements all made within 300 km of the release sites during, the Across North America Experiment (ANATEX) provided 30 separate trials to evaluate the error of back-trajectory calculations. The trajectory calculations used dynamic-model-output meteorological data from the NOAA prognostic Nested Grid Model (NGM) and from comparable gridded meteorological fields from 4-day rawinsonde or 2-day rawinsonde observations. Over all trials, no significant difference was discernable in trajectory accuracy using the different meteorological input data; absolute trajectory error ranged from 20% to 30% of the travel distance. When the trajectories were grouped into similar categories, the NGM data provided the but results when smaller-scale flow features were present; ones that could not be resolved in the rawinsonde network. Four-day rawinsonde data provided the best results when fronts or low pressure systems were present in the vicinity of the tracer release. Slower transport in more homogeneous zonal flow regimes resulted in some of the smallest errors of around 15% of the travel distance for all the methods. In 70% of the events, the 4-day and 2-day rawinsonde calculations were comparable, suggesting that linear temporal interpolation of the flow field is reasonable for most situations. The primary conclusion is that the NGM-generated meteorological data can be an adequate substitute for rawinsonde data in trajectory calculations.

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Roland R. Draxler

Abstract

Meteorological data fields from NOAA's Nested Grid Model (NGM) were archived at 2-hour intervals from initialization to the 12-hour forecast time, two forecasts per day, for January, February, and March 1987. The NGM predictions of the winds at the lowest level (175 m) and predictions of surface momentum and sensible heat flux were used in a separate boundary layer model (BLM) to derive values of wind speed and direction between the surface and the lowest NGM level. BLM results were compared with measurements made on Savannah River Laboratory's meteorologically instrumented 366 m TV tower on Beech Island, South Carolina, and the nearby Augusta, Georgia, weather station. The comparison between predicted and measured wind speeds and directions was quite favorable in that the forecast model was able to reproduce the diurnal variations at all levels of the tower. However, the BLM underpredicted the mean tower wind speed profile by about 20% (1 m s−1) at most levels of the tower. Diurnal variations of wind speed of 4 m s−1 at the top of the tower to less than 0.5 m s−1 at the bottom were well predicted at all levels, with the predicted maximums and minimums occurring at the appropriate times. Wind direction changes of 25 deg at night and less than 10 deg during the day were similarly predicted; however, predicted wind directions were biased by about +10 deg compared with the tower measured wind directions.

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Roland R. Draxler

Abstract

The data from a yearlong tracer dispersion experiment over Washington, D.C., in 1984 were used to evaluate Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) dispersion model calculations using coarse global meteorological reanalysis data [NCEP–NCAR and 40-Yr ECMWF Re-Analysis (ERA-40)] and calculations using meteorological data fields created by running a high-resolution meteorological model [fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5)]. None of the meteorological models were optimized for urban environments. The dispersion calculation using the ERA-40 data showed better performance than those using the NCEP–NCAR data and comparable performance to those using MM5 data fields. Calculations with MM5 data that used shorter-period forecasts were superior to calculations that used forecast data that extended beyond 24 h. Daytime dispersion model calculations using the MM5 data showed an underprediction bias not evident in calculations using the ERA-40 data or for nighttime calculations using either meteorological dataset. It was found that small changes in the wind direction for all meteorological model data resulted in dramatic improvements in dispersion model performance. All meteorological data modeled plume directions were biased 10°–20° clockwise to the measured plume direction. This bias was greatest when using the global meteorological data. A detailed analysis of the wind observations during the November intensive, which had the greatest difference between the model and measured plume directions, showed that only the very lowest level of observed winds could account for the transport direction of the measured plume. In the Northern Hemisphere, winds tend to turn clockwise with height resulting in positive directional transport bias if the lowest-level winds are not represented in sufficient detail by the meteorological model.

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Roland R. Draxler

Abstract

A three-dimensional primative equation model was used to simulate the low-level wind field, given the urban heat island as the lower temperature boundary condition. The specification of the average heat island bypassed the need to calculate the surface energy budget, considerably simplifying the model. This is especially desirable since temperature is a more accurate and easily obtainable measurement. The influence of the Washington, DC urban area on the local airflow was to enhance the vertical mixing due to the increased low-level instability as the air approached the warmer city center. This resulted in the lower-level winds turning anticyclonically (clockwise) from the upwind value. The anticyclonic turning, on the order of a degree per kilometer, is due to an increase in the downward transport of momentum and is accompanied by a wind speed increase of 17%. The upwind direction is quickly reestablished downwind of the city. The influence of the terrain upon the wind field could not be determined from the calculations since terrain effects were included in the surface temperature boundary condition. Observations from three instrumented towers around the perimeter of the urban center were not sufficient to deduce the complex nature of the flow. However, during southwest flow when two of the towers were downwind of the urban center, the mean wind direction between those towers was significantly different by 20°.

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Roland R. Draxler

Abstract

The recent solo transpacific balloon flight was used as a test case to evaluate multiple trajectory techniques to select different pathways based upon potential variations in balloon altitudes. Altitude changes between 3 and 8 km above ground resulted in predicted ending locations varying from the Hawaiian Islands to the Atlantic coast, after five days' travel. The method can be used to select optimum flight altitudes based upon forecast meteorological fields.

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Roland R. Draxler

Abstract

A Lagrangian transport and dispersion model was modified to generate multiple simulations from a single meteorological dataset. Each member of the simulation was computed by assuming a ±1-gridpoint shift in the horizontal direction and a ±250-m shift in the vertical direction of the particle position, with respect to the meteorological data. The configuration resulted in 27 ensemble members. Each member was assumed to have an equal probability. The model was tested by creating an ensemble of daily average air concentrations for 3 months at 75 measurement locations over the eastern half of the United States during the Across North America Tracer Experiment (ANATEX). Two generic graphical displays were developed to summarize the ensemble prediction and the resulting concentration probabilities for a specific event: a probability-exceed plot and a concentration-probability plot. Although a cumulative distribution of the ensemble probabilities compared favorably with the measurement data, the resulting distribution was not uniform. This result was attributed to release height sensitivity. The trajectory ensemble approach accounts for about 41%–47% of the variance in the measurement data. This residual uncertainty is caused by other model and data errors that are not included in the ensemble design.

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Roland R. Draxler and Albion D. Taylor

Abstract

A computer model using actual meteorological data is developed to simulate the effects of wind shear on an instantaneous pollutant puff. The effect of wind shear on dispersion is obtained by subdividing, during the nocturnal phase, the previous daytime pollutant mixed layer into 300 m layers. Each layer is tracked as a separate trajectory. Vertical mixing is resumed during the next daytime phase. Further subdivisions occur each nocturnal phase. The computer simulation of puff growth as a result of shear confirmed the proportionality of puff spread to geostrophic wind speed during homogeneous conditions as derived by Taylor (1982). Both Taylor's and the computer simulation results are consistent with published experimental long-range dispersion data representative of average conditions.

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Roland R. Draxler and Barbara J. B. Stunder

Abstract

Perfluorocarbon tracer concentration profiles measured by aircraft 600–900 km downwind of the release locations during CAPTEX are discussed and compared with some model results. In general, the concentrations decreased with height in the upper half of the boundary layer where the aircraft measurements were made. The results of a model sensitivity study suggested that the shape of the profile was primarily due to winds increasing with height and relative position of the sampling with respect to the upwind and downwind edge of the plume. Further modeling studies showed that relatively simple vertical mixing parameterizations could account for the complex vertical plume structure when the model had sufficient vertical resolution. In general, the model performed better with slower winds and corresponding longer transport times.

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Glenn D. Rolph and Roland R. Draxler

Abstract

Initialization and forecast fields from the National Weather Service's (NWS) Nested Grid Model (NOM) were archived on the 90 km calculational grid at 2-hour intervals out to 12 hours twice per day, for the 3-month period of January–March 1987. The resulting time series of meteorological data were used to determine the sensitivity of calculated trajectories to changes in temporal and spatial density of meteorological data during a wide range of synoptic conditions. Trajectories were started from 63 evenly spaced locations, twice per day, for a duration of 4 days each over the 74-day period. The 9324 separate trajectories were computed using the meteorological data at 90, 180, and 360 km grid spacing and at 2-, 4-, 6-, and 12-hour time intervals. Calculated trajectories were compared with the base “truth” case of 2-hour data on the 90 km grid.

Trajectories were most sensitive to changes in temporal resolution when the grid resolution was 90 and 180 km. Trajectories computed on the coarser 360 km grid had substantially larger deviations from the base case and were no longer sensitive to changes in temporal resolution. Relative horizontal transport deviations ranged from 5–25% of the travel distance at 96 hours depending upon the spatial and temporal resolution. Results suggest that it rawinsonde observations are the primary source of meteorological data (400 km spacing every 12 hours), then the greatest improvement in trajectory accuracy can be achieved by enhancing the temporal frequency of observations to 6-hour intervals. Results were not different when trajectories were categorized by cyclonic or anticyclonic conditions. However, horizontal deviations during cyclonic conditions were as much as 30% larger than those during anticyclonic conditions. This was attributed primarily to stronger wind speeds in cyclonic systems.

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