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K. Jerry Allwine

Abstract

During September and October 1984, a major meteorological and tracer study was conducted in Colorado's Brush Creek valley. The characteristics of atmospheric dispersion during the nighttime and morning transition periods are discussed in this paper. Tracer released near the valley floor did not reach the ridgetops (escape from the valley) during the nighttime but was confined to the valley, being carried in down-valley flows. After sunrise, with the onset of convective boundary-layer growth and initiation of upslope flows, the tracer within the valley was carried into the upper elevations of the valley atmosphere and ventilated from the valley. This was confirmed by the ridgetop tracer samplers and by a tracer mass budget applied to a valley atmosphere control volume. The ventilation rate of tracer from the valley atmosphere to the above-ridgetop flows was calculated from the tracer mass budget. A dimensionless form of the ventilation rate is proposed. The Gaussian plume equation adequately represented (16% average deviation) the average nighttime plume centerline concentration, out to 8 km from the release, when the plume was fully contained in down-valley flows. This agreement was attained by accounting for plume reflections from the valley sidewalls and using measured turbulence statistics in the calculation of the dispersion coefficients. Beyond 8 km down valley from the release, the Brush Creek valley merged with the Roan Creek valley and the two airstreams mixed, resulting in a sudden dilution of the tracer plume. The Gaussian plume equation was not valid after the two airstreams merged.

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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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K. Jerry Allwine, Brian K. Lamb, and Robert Eskridge

Abstract

During January 1989, five nighttime SF6 tracer experiments were conducted in Roanoke, Virginia. The experiments were designed to help identify and understand the dispersion characteristics of a basin atmosphere during winter stagnation conditions. The basin studied was the Roanoke basin located on the eastern slopes of the Appalachian Mountains. This paper documents this tracer study and gives results from the experiment conducted on the night of 16–17 January 1989. A cold-air pool formed in the basin beginning after the evening transition period and filled to near the elevation of the lowest mountain barrier. A simple model of the ascent rate of the top of this cold-air pool is proposed. A sharp potential temperature jump was present at the top of this fully developed cold-air pool. Vertical measurements of tracer showed the initial ground-level plume to become elevated and ride over the top of the cold-air pool. Horizontal plume spread was enhanced over that expected from turbulent diffusion alone, by shear in wind-direction vertical profiles. The tracer concentrations within the cold-air pool increased slowly with time, even after the release was terminated. After sunrise, the elevated plume appeared to fumigate to the ground.

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Lisa S. Darby, K. Jerry Allwine, and Robert M. Banta

Abstract

Differences in nighttime transport and diffusion of sulfur hexafluoride (SF6) tracer in an urban complex-terrain setting (Salt Lake City, Utah) are investigated using surface and Doppler lidar wind data and large-scale surface pressure differences. Interacting scales of motion, as studied through the URBAN 2000 field program combined with the Vertical Transport and Mixing (VTMX) experiment, explained the differences in the tracer behavior during three separate intensive operating periods. With an emphasis on nighttime stable boundary layer conditions, these field programs were designed to study flow features responsible for the nighttime transport of airborne substances. This transport has implications for air quality, homeland security, and emergency response if the airborne substances are hazardous. The important flow features investigated included thermally forced canyon and slope flows and a low-level jet (LLJ) that dominated the basin-scale winds when the surface pressure gradient was weak. The presence of thermally forced flows contributed to the complexity and hindered the predictability of the tracer motion within and beyond the city. When organized thermally forced flows were present, the tracer tended to stay closer to the city for longer periods of time, even though a strong basin-scale LLJ did develop. When thermally forced flows were short lived or absent, the basin-scale low-level jet dominated the wind field and enhanced the transport of tracer material out of the city.

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C. David Whiteman, K. Jerry Allwine, Leo J. Fritschen, Montie M. Orgill, and James R. Simpson

Abstract

Surface energy budget measurements were made concurrently at five sites located on the valley floor, sidewalls and ridgetop of Colorado's 650-m deep Brush Creek Valley (39°32′N, 108°24′W) on the nearly clear day of 25 September 1984 using the Bowen ratio energy budget technique.

Daily average surface heat flux values for a natural sagebrush ecosystem on the floor of the semiarid valley included an input of 109 W m−2 net all-wave radiation and 15 W m−2 ground heat flux, and a loss of 48 W m−2 latent heat flux and 76 W m−2 sensible heat flux. Significant differences in instantaneous, daily, and daytime fluxes occurred from site to site as a function of slope aspect and inclination angles and surface properties, including vegetation cover and soil moisture.

Strong contrasts in instantaneous latent and sensible heat fluxes occurred between the opposing northeast-and southwest-facing sidewalls of the valley as solar insolation varied through the course of the day and as shadows propagated across the valley. This differential heating and moistening of the air above the opposing slopes produces cross valley circulations and the resulting moisture and heat transports observed by other investigators.

The ridgetop site, with a nearly unobstructed view of the sky and the longest daytime period, received the highest daily total of net radiation (12.12 MJ m−2) and lost the highest sensible heat flux total (8.49 MJ m−2). The dry southwest-facing slope produced a nearly equivalent daily total sensible heat flux, despite the later sunrise and earlier sunset at this site, because of the dry soil, lack of vegetation, and intense afternoon radiation on the sloping surface. One of the valley floor sites, located in a wheatgrass meadow, produced a daily total latent heat flux (7.37 MJ m−2) over four times larger than the dry southwest-facing sidewall. Mean daytime Bowen ratios varied from 0.86 at the valley floor meadow site to 7.60 on the southwest-facing sidewall.

Daily total sensible heat fluxes in the valley were much larger than required to destroy typical nocturnal temperature inversions, and the excess is available on clear fall days to grow deep convective boundary layers over the region. Hodographs show clockwise turning of the winds above the northeast-facing sidewall during the course of the day, counterclockwise turning on the southwest-facing sidewall, and clockwise turning on the floor of the narrow valley as the cycle of down-slope, down-valley, up-slope and up-valley winds is executed. The times of reversal of the slope and valley wind systems at the individual energy budget sites were closely related to the time of sign reversal of sensible heat flux, within the time resolution of the sensible heat flux data.

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Dennis Finn, Kirk L. Clawson, Roger G. Carter, Jason D. Rich, and K. Jerry Allwine

Abstract

The URBAN 2000 experiments were conducted in the complex urban and topographical terrain in Salt Lake City, Utah, in stable nighttime conditions. Unexpected plume dispersion often arose because of the interaction of complex terrain and mountain–valley flow dynamics, drainage flows, synoptic influences, and urban canopy effects, all within a nocturnal boundary layer. It was found that plume dispersion was strongly influenced by topography, that dispersion can be significantly different than what might be expected based upon the available wind data, and that it is problematic to rely on any one urban-area wind measurement to predict or anticipate dispersion. Small-scale flows can be very important in dispersion, and their interaction with the larger-scale flow field needs to be carefully considered. Some of the anomalies observed include extremely slow dispersion, complicated recirculation dispersion patterns in which plume transport was in directions opposed to the measured winds, and flow decoupling. Some of the plume dispersion anomalies could only be attributed to small-scale winds that were not resolved by the existing meteorological monitoring network. The results shown will make clear the difficulties in modeling or planning for emergency response to toxic releases in a nocturnal urban boundary layer within complex terrain.

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C. David Whiteman, K. Jerry Allwine, Leo J. Fritschen, Montie M. Orgill, and James R. Simpson

Abstract

Solar and longwave radiation data are presented for five sites in Colorado's 650 m deep semiarid Brush Creek Valley (39°32′N, 108°24′W) during September 1984.

During the sunlit period of a nearly clear day, individual sites received 0.73–0.81 of the theoretical extraterrestrial solar radiation. Incoming solar radiation increased with elevation in the valley. Direct radiation made up 0.86– 0.88 of the downward shortwave flux. On average, 0.12–0.21 of the incoming shortwave radiation was reflected at the individual sites. Strong variations in reflected solar radiation and outgoing longwave radiation occurred from site to site. Because of the large direct beam component, aspect and inclination angles of the valley surfaces had a strong effect on the solar radiation received. Contrasts between a southwest- and northeast-facing sidewall were significant. Shading from surrounding topography produced inter-site differences in both instantaneous and daily total radiation. Inter-site differences in most daily totals on a clear day were larger than standard deviations of the daily totals at a valley floor site computed over a 16-day period of variable weather. The ridgetop site, on account of its unobstructed view of the sky, had a higher average positive net radiation during the day and a higher average negative net radiation during the night than the valley stations.

Observations averaged over a 15-day period of variable weather illustrated the general effect of cloudy weather in reducing contrasts in radiation climate among sites.

A simple theoretical correction for radiation measured on a horizontal surface provided a useful estimate of net and global radiation on the underlying sloping surface.

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K. Jerry Allwine, Xindi Bian, C. David Whiteman, and Harold W. Thistle

Abstract

VALDRIFT (valley drift) is a valley atmospheric transport, diffusion, and deposition model. The model is phenomenological—that is, the dominant meteorological processes governing the behavior of the valley atmosphere are formulated explicitly in the model, although in a highly parameterized fashion. The key meteorological processes treated are 1) nonsteady and nonhomogeneous along-valley winds and turbulent diffusivities, 2) convective boundary layer growth, 3) inversion descent, and 4) nocturnal temperature inversion breakup. The model is applicable under relatively cloud-free, undisturbed synoptic conditions in which the winds in the valley are predominantly along the valley’s axis. The model is configured to operate through one diurnal cycle for a single narrow valley. The inputs required are the valley topographic characteristics, pollutant release rate as a function of time and space, wind speed and direction as functions of time measured at one height, lateral and vertical turbulent eddy diffusivities as functions of stability, and the valley temperature inversion characteristics at sunrise. The outputs are three-dimensional concentration fields and ground-level deposition fields as functions of time. The scientific foundations of VALDRIFT are given in this paper along with a brief discussion of the model inputs and outputs. Air concentrations estimated by VALDRIFT compare favorably with results from a tracer experiment conducted in a deep mountain valley.

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Jerome D. Fast, K. Jerry Allwine, Russell N. Dietz, Kirk L. Clawson, and Joel C. Torcolini

Abstract

Six perfluorocarbon tracer experiments were conducted in Salt Lake City, Utah, during October 2000 as part of the Vertical Transport and Mixing (VTMX) field campaign. Four tracers were released at different sites to obtain information on dispersion during stable conditions within down-valley flow, canyon outflow, and interacting circulations in the downtown area. Some of the extensive tracer data that were collected are presented in the context of the meteorological field campaign measurements. Tracer measurements at building-top sites in the downtown area and along the lower slopes of the Wasatch Front indicated that vertical mixing processes transported material up to at least 180 m above the valley floor, although model simulations suggest that tracers were transported upward to much higher elevations. Tracer data provided evidence of downward mixing of canyon outflow, upward mixing within down-valley flow, horizontal transport above the surface stable layer, and transport within horizontal eddies produced by the interaction of canyon and down-valley flows. Although point meteorological measurements are useful in evaluating the forecasts produced by mesoscale models, the tracer data provide valuable information on how the time-varying three-dimensional mean and turbulent motions over urban and valley spatial scales affect dispersion. Although the mean tracer transport predicted by the modeling system employed in this study was qualitatively similar to the measurements, improvements are needed in the treatment of turbulent vertical mixing.

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Jerome D. Fast, Rob K. Newsom, K. Jerry Allwine, Qin Xu, Pengfei Zhang, Jeffrey Copeland, and Juanzhen Sun

Abstract

Two entirely different methods for retrieving 3D fields of horizontal winds from Next Generation Weather Radar (NEXRAD) radial velocities have been evaluated using radar wind profiler measurements to determine whether routine wind retrievals would be useful for atmospheric dispersion model applications. The first method uses a physical algorithm based on four-dimensional variational data assimilation, and the second simpler method uses a statistical technique based on an analytic formulation of the background error covariance. Both methods can be run in near–real time, but the simpler method was executed about 2.5 times as fast as the four-dimensional variational method. The observed multiday and diurnal variations in wind speed and direction were reproduced by both methods below ∼1.5 km above the ground in the vicinity of Oklahoma City, Oklahoma, during July 2003. However, wind retrievals overestimated the strength of the nighttime low-level jet by as much as 65%. The wind speeds and directions obtained from both methods were usually similar when compared with profiler measurements, and neither method outperformed the other statistically. Within a dispersion model framework, the 3D wind fields and transport patterns were often better represented when the wind retrievals were included along with operational data. Despite uncertainties in the wind speed and direction obtained from the wind retrievals that are higher than those from remote sensing radar wind profilers, the inclusion of the wind retrievals is likely to produce more realistic temporal variations in the winds aloft than would be obtained by interpolation using the available radiosondes, especially during rapidly changing synoptic- and mesoscale conditions.

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