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C. R. Dickson and J. K. Angell

Abstract

Tetroon flights made at the National Reactor Testing Station, Idaho Falls, during the past few years have provided values of eddy velocity within the planetary boundary layer. In this desert-like terrain the root mean square (rms) values of longitudinal, lateral and vertical velocity tend to increase with height based on daytime observations throughout the year. The respective turbulence intensities, however, decrease with height. The mean values of rms velocity increase by factors of 2–3 between winter and summer.

On the basis of an extensive tetroon experiment in summer, when lapse-rate data were available, the rms vertical velocity increases markedly with increase in height, lapse rate and wind speed during the day, whereas the rms lateral velocity is proportional chiefly to wind speed. The average vertical turbulence intensity is more than twice as large as the year-average value obtained by Pasquill in England, but decreases with height in a similar manner. The variation with height, stability and wind speed of both the vertical momentum flux and the rate of production of eddy kinetic energy from the mean sheared flow are indicated.

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G. E. Start, C. R. Dickson, and L. L. Wendell

Abstract

An accelerated field measurements program was conducted to quantify atmospheric diffusion within a deep, steep-walled canyon in rough, mountainous terrain. Two principle objectives were pursued: impaction of plumes upon elevated terrain, and diffusion of gases within the canyon versus diffusion over flat, open terrain. Oil fog flow visualizations provided qualitative information; quantitative diffusion measurements were obtained using sulfur hexafluoride gas with analysis by highly sensitive gas phase coulometric techniques. Eleven 45 to 60 min gaseous tracer releases were conducted.

Stability-category-related differences in canyon diffusion versus flat terrain diffusion were found. Daytime lapse conditions showed little difference. Neutral stability tests showed five times greater dilution for canyon axial concentrations; strong inversion tests resulted in canyon plume centerline dilutions fifteen times greater than calculations using parameters derived for flat terrain. Plume effluents frequently impacted against elevated terrain.

Enhanced mechanical turbulence associated with gradient wind-flows near the mountain tops, density flows originating in side canyons, and turbulent wakes from pronounced terrain irregularities within the canyon are believed to be some of the additional physical mechanisms affecting plume dilutions in Huntington Canyon.

The present results should be relevant, at least qualitatively, to similar deep, steep-walled canyons. They should not be applied indiscriminantly to sites with less extreme topography. Additional measurements are needed at sites in less rugged terrain.

Highly buoyant plumes may require special study since buoyancy may be in competition with postulated effects from the enhanced mechanical turbulence observed within the rough terrain setting.

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J. K. Angell, D. H. Pack, and C. R. Dickson

Abstract

During July 1966, nearly 100 tetroon flights were made at the National Reactor Testing Station (NRTS), Idaho Falls, with the primary purpose of verifying the existence of longitudinal roll-vortices, or helices, in the planetary boundary layer. The transponder-equipped constant volume ballons (tetroons) were ballasted to float 300 m above the ground and were tracked by two M-33 radars. One radar tracked two tetroons released simultaneously from sites 500 m apart (in a direction normal to the mean flow) and the other radar tracked two tetroons released simultaneously from the same sites about one-half hour later.

In the flat, desert-like region of NRTS, there is evidence that counter-rotating helices of about 2 km diameter frequently exist during the afternoon. Basically, the helical motion appears to be one of solid rotation, with an average absolute value for the vorticity in the transverse plane of 4 × 10−3 sec−1, a magnitude similar to that derived from the vertical shear of the longitudinal wind. There is evidence that these helical structures move in a direction normal to the mean flow with a speed of about 1 m sec−1. During the afternoon, the average value of the tetroon-derived horizontal stress is nearly 6 dyn cm−2, and the average flux of kinetic energy from mean sheared flow to helix is nearly 6 cm2 sec−3. There is considerable agreement between the tetroon-derived data and the theoretical and laboratory work of Faller and Lilly on helical circulations, even though the evidence from this atmospheric experiment suggests that, during the afternoon, the longitudinal vortices are driven both by buoyancy and the vertical shear of the mean flow.

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J. K. Angell, C. R. Dickson, and W. H. Hoecker Jr.

Abstract

The Los Angeles Reactive Pollutant Project (LARPP) in the autumn of 1973 involved helicopter sampling of a volume of air “tagged” by means of three constant volume balloons (tetroons) released simultaneously from a point on the ground. Based on radar tracking of 35 tetroon triads at a mean height of 350 m above sea level, this paper considers the estimates of relative diffusion obtained from the rate of separation of the tetroons making up the triad. In the average, the median lateral standard deviation of tetroon position varies from 90 m after a travel time of 15 min to 800 m after 2 h, and from 140 m at a travel distance of 2 km to 1000 m at 20 km. The relative diffusion is indicated to be nearly twice as large in “neutral” as in “stable” conditions. Comparison with the results obtained by other investigators in other locations shows that the relative diffusion within the Los Angeles Basin is frequently unusually small, particularly with respect to travel time.

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J. K. Angell, C. R. Dickson, and W. H. Hoecker Jr.

Abstract

Tetroon trajectories within the Los Angeles Basin in the autumn of 1973 show that, on non-stagnation days, air located in the Los Angeles area in the morning can pass over the Puente Hills and reach the San Bernardino-Riverside area by mid-afternoon of the same day. However, the enhanced vertical mixing associated with these Hills would be expected to dilute any pollution present. Of perhaps more importance is the evidence that, on stagnation days when the atmosphere is stable, air from the Los Angeles area may drift southward in the early morning katabatic flow, stagnate for 2–3 h in the industrialized and high vehicle-density region north of Long Beach, and then move rapidly eastward with the sea breeze flow through Santa Ana Canyon, reaching the Riverside-San Bernardino area in late afternoon. In this case there would seem to be more potential for severe pollution in the latter area. However, the frequency of occurrence of this particular trajectory pattern is uncertain.

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J. K. Angell, W. H. Hoecker, and C. R. Dickson

Abstract

Pairs of constant volume balloons (tetroons) released ∼1 min apart from the same site at Haswell, Colo., and continuously tracked by two M-33 radars, are used to estimate the mutual consistency of tetroon vertical motions. In the strong well-organized vertical motion systems of midday and afternoon, it is possible unambiguously to relate the vertical oscillations of the two tetroons so long as the tetroon separation distance is less than about 3 km, implying that under these conditions the tetroons are indeed passing through the same systems (convection cells or thermals). At other times of day such a comparison becomes difficult, partly because of the smaller amplitude and period of oscillation. Where identification of the same vertical motion system is possible, the phase lag between vertical oscillations of adjacent tetroons is related in a meaningful way to the tetroon spacing and the relative position of the tetroon pair with respect to wind direction, and accordingly there is good evidence that tetroon vertical motions are basically consistent.

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J. K. Angell, D. H. Pack, G. C. Holzworth, and C. R. Dickson

Abstract

Tetroon flights at heights near 300 m within the Los Angeles Basin illustrate the useful results to be derived from the use of Lagrangian probes in an urban environment. The tetroon trajectories depict air-flow reversals which could only be obtained with difficulty from fixed-point data, and furthermore yield evidence for a diurnal recirculation of air within the Basin. Of importance from the viewpoint of vertical dispersion is the observation that tetroon-derived root mean square vertical velocities average 0.5 m sec−1 over the land and 0.2 m sec−1 over the water during daylight hours, while the period of vertical oscillation averages 17 min over land and 9 min over water. Broadly speaking, the period of vertical oscillation appears to be a function of lapse rate, in agreement with the Brunt-Väisälä formulation. Two flights released simultaneously indicate the existence of helical circulations within the marine air over the land. These helices have lateral and vertical dimensions of about 600 m and the tetroons complete a circuit in 20–30 min. Examples are presented of tetroon vertical motions over the Palos Verdes Hills.

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J. K. Angell, W. H. Hoecker, C. R. Dickson, and D. H. Pack

Abstract

Tetroon flights across Oklahoma City indicate the influence of an isolated urban area on the horizontal and vertical air velocity at heights near 400 m in relatively strong (13 m sec−1) daytime flow. The Lagrangian measurements so obtained are collated with fixed-point measurements of horizontal and vertical velocity on a 460 m television tower. Above the city in the morning there is a mean trajectory turning toward lower pressure of 10°. This turning, presumably fractionally induced, is noted only weakly in the afternoon and not all in the evening, but there is slight evidence for a bending of the trajectories around the city at these later times. During the day the city appears as the source of a plume of ascending air motion extending at least 30 km downwind of the city, with both tetroon and tower measurements indicating a mean upward velocity of almost 0.4 m sec−1 ten kilometers downwind of city-center at heights near 400 m. On the average the magnitude of the stress determined from the covariance of the eddy velocity components along the tetroon flights is about 70% of the magnitude measured on the tower, and there is a correlation of nearly 0.5 between individual measurements of stress by the two techniques. The magnitude of the tetroon stress is intimately related to building height and density, with a stress maximum of at least 3 dyn cm−2 located 10 km downwind of city-center in comparison with stress values near 1 dyn cm−2 beyond the city outskirts. The fraction of the stress associated with Lagrangian oscillations of 1–10 min period (in comparison with 1–30 min period) increases from 20% upwind of the city to 80% downwind of the city in the daytime average.

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J. K. Angell, D. H. Pack, L. Machta, C. R. Dickson, and W. H. Hoecker

Abstract

Between 9 September and 2 October 1969, 105 constant-volume balloons (tetroons) were released from various sites in the Los Angeles Basin for the purpose of obtaining three-dimensional air trajectories in the planetary boundary layer of the Basin. The tetroons, with transponders attached, were tracked by an M-33 radar positioned atop Mt. Thom, 5 km north of Glendale. In general, the tetroons at heights of a few hundred meters tended to move slowly westward between midnight and sunrise, slowly northward between sunrise and noon, and rapidly eastward between noon and sunset. Tetroons launched from Redondo Beach and the Long Beach area during the day distinctly show the convergence zone extending northeastward from the Palos Verdes Hills due to the confluence of sea breezes from the west and south. Tetroons released near the downtown area around sunrise tend to drift toward city center before moving northeastward into the Pasadena-Glendale area. During the day the tetroons oscillate through much of the depth of the mixed layer beneath the inversion, but at night the vertical oscillations are usually very small. The tetroon trajectories are compared with surface trajectories derived from the extensive surface wind network within the Basin, and it is shown that the ratio of trajectory separation distance to tetroon travel distance averages about 0.2 during the day and 0.4 at night. Oxidant readings obtained by a helicopter following along the tetroon trajectories are compared with fixed-point readings, and it appears that, downwind of the city center, most of the increase in oxidant during the day is due to photochemical effects on individual volumes of air and is not due to advection of previously-existing high values of oxidant into the area.

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J. K. Angell, D. H. Pack, C. R. Dickson, and W. H. Hoecker

Abstract

Constant-volume balloon (tetroon) flights tracked by radar across Columbus, Ohio, in March 1969, illustrate the effect of a city on the nighttime airflow at heights of 100–200 m. On the average, the urban influence on wind direction is small at a height of 100 m, but an anticyclonic turning of 10° is observed at 200 m. The anticyclonic turning is greater under inversion than under lapse conditions and greater after midnight than before; it appears to result both from an increase in the frictional force due to increased vertical mixing and from a mesoscale high pressure system formed aloft as the result of the warmer temperatures within the city. The decrease in wind speed across the city averages nearly 20% of the upwind speed under lapse conditions but is very small under inversion conditions. In both cases the region of maximum deceleration tilts downwind with height. The average upward air motion exceeds 4 cm sec−1 above the down-town area under light wind conditions, and increases to 1 m sec−1 as the wind speed approaches 20 m sec−1. In the case of strong winds, alternating regions of upward and downward motion occur downwind of the city.

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