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Julia E. Flaherty, David Stock, and Brian Lamb

Abstract

A 3D computational fluid dynamics study using Reynolds-averaged Navier–Stokes modeling was conducted and validated with field data from the Joint Urban 2003 dispersion study in Oklahoma City, Oklahoma. The modeled flow field indicated that the many short buildings in this domain had a relatively small effect on the flow field, whereas the few tall buildings considerably influenced the transport and diffusion of tracer gas through the domain. Modeled values were compared with observations along a vertical profile located about 500 m downwind of the source. The isothermal base case using the standard k–ε closure model was within 50% of the concentration measurements, and a convective case with ground and building surfaces 10°C hotter than ambient temperatures improved the modeled profile to within 30% of observations. Varying wind direction and source location had a marked effect on modeled concentrations at the vertical profile site. Ground-level concentrations were 6 times the observed values when the approach flow wind direction was changed by +15° and were nearly zero when the wind direction was changed by −15°. Similar results were obtained when the source was moved 50 m to the east and to the west, respectively. All cases underestimated wind speed and turbulent kinetic energy near the surface, although adding heat significantly improved the magnitude of the modeled turbulent kinetic energy. Model results based upon a Reynolds stress closure scheme were also compared with the vertical concentration profiles. Neither the isothermal case nor the thermal buoyancy case resulted in an improvement over the standard k–ε model.

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Liping Deng, Sally A. McFarlane, and Julia E. Flaherty

Abstract

Ground-based high temporal and vertical resolution datasets from observations during 2002–07 at the Atmospheric Radiation Measurement (ARM) tropical western Pacific (TWP) site on Manus Island are used to examine the characteristics of clouds and rainfall associated with the active phase of the Madden–Julian oscillation (MJO) passing over Manus. A composite MJO event at Manus is developed based on the NOAA MJO index 4 and precipitation using 13 events. The cloud characteristics associated with the active phase of the MJO at Manus show a two-phase structure as the wave passes over Manus. During the development phase, congestus plays an important role, and the enhanced convection is located between surface westerly and easterly wind anomalies (type-I structure). During the mature phase, deep convection is the dominant cloud type, and the enhanced convection is collocated with the westerly wind anomalies (type-II structure). Consistent with this two-phase structure, the heavy rainfall frequency also shows a two-peak structure during the MJO disturbance, while light rainfall does not show a clear relation to the intraseasonal disturbance associated with the MJO. In addition, a positive relationship between the precipitation rate and precipitable water vapor exists at Manus, and the atmospheric column is less moist after the passing of the MJO convection center than before.

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Julia E. Flaherty, Brian Lamb, K. Jerry Allwine, and Eugene Allwine

Abstract

An atmospheric tracer dispersion study known as Joint Urban 2003 was conducted in Oklahoma City, Oklahoma, during July of 2003. As part of this field program, vertical concentration profiles were measured at approximately 1 km from the downtown ground-level tracer gas release locations. These profiles showed that the urban landscape was very effective in mixing the plume vertically. In general, the lowest concentration measured along the profile was within 50% of the highest concentration in any given 5-min measurement period. The general slope of the concentration profiles was bounded by a Gaussian distribution with Briggs’s urban equations (stability classes D and E/F) for vertical dispersion. However, measured concentration maxima occurred at levels above the surface, which would not be predicted by Gaussian formulations. Variations in tracer concentration observed in the time series between different release periods were related to changes in wind direction as opposed to changes in turbulence. This was demonstrated using data from mobile analyzers that captured the width of the plume by traveling east to west along nearby streets. These mobile-van-analyzer data were also used to compute plume widths. Plume widths increased for wind directions at larger angles to the street grid, and a simple model comprising adjusted open-country dispersion coefficients and a street channeling component, were used to describe the measured widths. This dispersion dataset is a valuable asset not only for developing advanced tools for emergency-response situations in the event of a toxic release but also for refining air-quality models.

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