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C. David Whiteman

Abstract

Three sulfur hexafluoride atmospheric tracer experiments were conducted during the post-sunrise temperature inversion breakup period in the deep, narrow Brush Creek Valley of Colorado. Experiments were conducted under clear, undisturbed weather conditions.

A continuous elevated tracer plume was produced along the axis of the valley before sunrise and the behavior of the plume during the inversion breakup period was detected down-valley from the release point using an array of radio-controlled sequential bag samplers, a vertical SF6 profiling system carried on a tethered balloon, two portable gas chromatographs operated on a sidewall of the valley, and a continuous real-time SF6 monitor operated from a research aircraft. Supporting meteorological data came primarily from tethered balloon profilers. The nocturnal elevated plume was carried and diffused in down-valley flows. After sunrise, convective boundary layers grew upward from the sunlit valley surfaces, fumigating the elevated plume onto the valley floor and sidewalls. Upslope flow developed in the growing convective boundary layers, carrying fumigated SF6 up the sidewalls and causing a compensating subsidence over the valley center. High post-sunrise SF6 concentrations were experienced on the northeast-facing sidewall of the northwest–southeast oriented valley as a result of cross-valley flow, which developed due to differential solar heating of the sidewalls. Reversal of the down-valley wind system brought air with lower SF6 concentrations into the lower valley.

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C. David Whiteman

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C. David Whiteman

Abstract

The breakup of temperature inversions in the deep mountain valleys of western Colorado has been studied by means of tethered balloon observations of wind and temperature structure on clear weather days in different seasons. Vertical potential temperature structure profiles evolve following one of three patterns. Two of the patterns are special cases of the third pattern, in which inversions are destroyed by two continuous processes-upward growth of a convective boundary layer (CBL) into the base of the valley inversion, and descent of the inversion top. The three idealized patterns are described and 21 case studies of inversion breakup following the patterns are summarized. Inversion breakup begins at sunrise and is generally completed in 3½–5 h, unless the valley is snow covered or the ground is wet. Warming of the inversion layer is consistent with subsidence heating. An hypothesis is offered to explain the observations, stressing the role of the sensible heat flux in causing the CBL to grow and an upslope flow to develop over the sidewalls. As mass is removed from the base of the inversion layer in the upslope flows, the inversion sinks and warms.

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David C. Bader and C. David Whiteman

Abstract

A two-dimensional dynamical model was used to simulate the daytime boundary-layer evolution and resulting plume dispersion in a cross-valley section of a northwest–southeast oriented narrow valley in the first 4 h after sunrise. Two cases were simulated, one using a summertime heating distribution and a second with a wintertime heating distribution. In each case, additional conservation equations were added to simulate the dispersion of two plumes released 150 m and 650 m above the valley floor. In the summer case, the lower plume migrated to the more strongly heated southwest sidewall in the first 90 min after sunrise, and was then advected up the sidewall in the slope flow for the remainder of the simulation. This result is consistent with observations. The upper plume diffused slowly in the remnants of the nocturnal inversion layer until it was entrained by the growing convective boundary layer 3 h after sunrise. The boundary layer's thermodynamic structure remained nearly symmetric about the valley axis throughout the transition period. The asymmetric dispersion characteristics seen in the summer case were not found in the winter simulation. The seasonal change in solar illumination reduced the differences in surface heat flux between the two sidewalls that gave rise to the asymmetry observed in the summer case.

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C. David Whiteman and Shiyuan Zhong

Abstract

Thermally driven downslope flows were investigated on a low-angle (1.6°) slope on the west side of the floor of Utah’s Salt Lake Valley below the Oquirrh Mountains using data from a line of four tethered balloons running down the topographic gradient and separated by about 1 km. The study focused on the evolution of the temperature and wind structure within and above the slope flow layer and its variation with downslope distance. In a typical situation, on clear, undisturbed October nights a 25-m-deep temperature deficit of 7°C and a 100–150-m-deep downslope flow with a jet maximum speed of 5–6 m s−1 at 10–15 m AGL developed over the slope during the first 2 h following sunset. The jet maximum speed and the downslope volume flux increased with downslope distance. The downslope flows weakened in the late evening as the stronger down-valley flows expanded to take up more of the valley atmosphere and as ambient stability increased in the lower valley with the buildup of a nocturnal temperature inversion. Downslope flows over this low-angle slope were deeper and stronger than has been reported previously by other investigators, who generally investigated steeper slopes and, in many cases, slopes on the sidewalls of isolated mountains where the downslope flows are not subject to the influence of nighttime buildup of ambient stability within valleys.

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C. David Whiteman and Sumner Barr

Abstract

Hourly tethered-balloon wind soundings from the 650-m deep, narrow, Brush Creek Valley of Colorado are analyzed to determine the nocturnal atmospheric mass (or volume) budget of the valley. Under the assumption that the volume flux on an entire valley cross section can be approximated from balloon soundings over the valley center, volume fluxes are calculated from tethered balloon profiles taken on 30–31 July 1982 at several points along the valley's longitudinal axis in a 7-km long segment of the valley.

Down-valley volume fluxes increased in the 3 h following sunset to levels that were basically maintained through the night. Down-valley volume fluxes increased with distance down the valley axis from 0.9 million m3 s−1 at the upper end of the segment to 2.8 million m3 s−1 at the lower end, producing an average volume flux divergence of 271 m2 s−1. If we assume that the volume flux divergence is supported entirely by subsidence of air into the valley, a peak sinking rate of 0.10 m s−1 is obtained at the level of the valley's rim. Mean vertical velocity profiles through the valley's depth are calculated, and an error analysis is performed.

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C. David Whiteman and Rolando Garibotti

Rime mushrooms, commonly called ice mushrooms, are large bulbous or mushroom-shaped accretions of hard rime that build up on the upwind side of mountain summits and ridges and on windward rock faces. This paper reviews the characteristics of rime mushrooms; the topographical, geographical, and meteorological conditions under which they form; and the significant challenge they pose to climbers. Photographs and descriptions from Southern Patagonia, where rime mushrooms are well known, illustrate the phenomenon.

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C. David Whiteman and Xindi Bian

A short review of solar semidiurnal atmospheric tides is presented. Semidiurnal atmospheric tides have been documented in the troposphere primarily through analyses of long time series of surface pressure measurements, although the winds produced by these tides have, by now, been well documented in the middle and upper atmosphere. Recent research using UHF and very-high-frequency radar wind profilers has now identified tidal wind perturbations in tropospheric data. This review focuses on the tidal wind characteristics and the distinctive signature of this wind system in radar profiler data analyses.

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Thomas Haiden and C. David Whiteman

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Momentum and heat budget equations for katabatic flows on sloping surfaces are revisited. Terms in these equations are evaluated using wind and potential temperature data from four tethered-balloon data collection systems on a 3-km line running down a 1.6° slope at the foot of the Oquirrh Mountains in Utah’s Great Salt Lake valley. The analyses focus on the development with downslope distance of the katabatic flow and the associated negatively buoyant layer under synoptically undisturbed conditions. With strong ambient stratification, the katabatic flow shows little variation between sites, suggesting a state close to local equilibrium. When the ambient stratification is weaker, the acceleration of the katabatic flow between two tethersonde sites is systematically larger than what would be predicted based on observed buoyancy. Comparison of observed flow direction with the local topographic gradient indicates that slope curvature, associated with small deviations from the basically planar slope, may be responsible for the anomalous increase. It is concluded that the cross-slope homogeneity of the flow, which is assumed in simplified katabatic flow models, may be significantly disturbed even on slopes that appear to be planar to the observer.

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Pirmin Kaufmann and C. David Whiteman

Abstract

Twelve typical wintertime wind patterns for the Grand Canyon region were derived from a two-stage cluster analysis wind-field classification scheme. The wind measurements were collected by a surface network of 15 stations deployed for a period of approximately three months. The wind patterns are strongly influenced by the complex terrain of the region. The analyses relate the wind patterns to meteorological conditions, providing insight into the physical processes generating the wind fields. Most patterns have a distinct diurnal cycle, caused by thermally induced winds near the ground. They provide evidence that thermally forced flows are important in winter and are not easily overridden by ambient flows. Some patterns differ primarily in the ratio of high- and low-elevation site wind speeds, indicating the importance of decoupling of the low-elevation winds in this basin area from the stronger ambient winds.

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