C. David Whiteman

Observations of Thermally Developed Wind Systems in Mountainous Terrain

Slope and valley wind systems are local thermally driven circulations that form frequently in complex terrain areas. Recent research has focused on the temperature structure along the slope and valley axes that leads to the wind systems. Two new tools being used in these analyses include the topographic amplification factor, which quantifies the role of the topography in producing bulk temperature gradients along a valley’s axis, and atmospheric heat budgets, which identify key physical processes leading to changes in temperature structure. Both tools are in an early stage of development, are being applied primarily to steady-state nighttime periods, and are leading to new concepts and understanding.

Low-Level Jet Climatology from Enhanced Rawinsonde Observations at a Site in the Southern Great Plains

Abstract A climatology of the Great Plains low-level jet (LLJ) is developed from 2 yr of research rawinsonde data obtained up to eight times per day at a site in north-central Oklahoma. These data have better height and time resolution than earlier studies, and show that jets are stronger than previously reported and that the heights of maximum wind speed are closer to the ground. LLJs are present in 47% of the warm season soundings and 45% of the cold season soundings. More than 50% of the LLJs have wind maxima below 500 m above ground level (AGL). Because the 404-MHz radar profiler network in the central United States has its first data points at 500 m AGL, it is likely to miss some LLJ events and will have inadequate vertical resolution of LLJ wind structure. Previous studies have identified LLJs on the basis of a wind speed profile criterion. This criterion fails to separate the classical southerly LLJs from the less frequent northerly jets, which differ in both structure and evolution. Classical sout...

THE TERRAIN-INDUCED ROTOR EXPERIMENT : A Field Campaign Overview Including Observational Highlights

Abstract The Terrain-Induced Rotor Experiment (T-REX) is a coordinated international project, composed of an observational field campaign and a research program, focused on the investigation of atmospheric rotors and closely related phenomena in complex terrain. The T-REX field campaign took place during March and April 2006 in the lee of the southern Sierra Nevada in eastern California. Atmospheric rotors have been traditionally defined as quasi-two-dimensional atmospheric vortices that form parallel to and downwind of a mountain ridge under conditions conducive to the generation of large-amplitude mountain waves. Intermittency, high levels of turbulence, and complex small-scale internal structure characterize rotors, which are known hazards to general aviation. The objective of the T-REX field campaign was to provide an unprecedented comprehensive set of in situ and remotely sensed meteorological observations from the ground to UTLS altitudes for the documentation of the spatiotem-poral characteristics ...

Cold-Air-Pool Structure and Evolution in a Mountain Basin: Peter Sinks, Utah

Abstract The evolution of potential temperature and wind structure during the buildup of nocturnal cold-air pools was investigated during clear, dry, September nights in Utahs Peter Sinks basin, a 1-km-diameter limestone sinkhole that holds the Utah minimum temperature record of −56°C. The evolution of cold-pool characteristics depended on the strength of prevailing flows above the basin. On an undisturbed day, a 30°C diurnal temperature range and a strong nocturnal potential temperature inversion (22 K in 100 m) were observed in the basin. Initially, downslope flows formed on the basin sidewalls. As a very strong potential temperature jump (17 K) developed at the top of the cold pool, however, the winds died within the basin and over the sidewalls. A persistent turbulent sublayer formed below the jump. Turbulent sensible heat flux on the basin floor became negligible shortly after sunset while the basin atmosphere continued to cool. Temperatures over the slopes, except for a 1–2-m-deep layer, became war...

Wintertime Evolution of the Temperature Inversion in the Colorado Plateau Basin

The Colorado Plateau, surrounded by a ring of mountains, has the meteorological characteristics of a basin. Deep, persistent potential temperature inversions form in this basin in winter. The formation, maintenance, and dissipation of these inversions are investigated using two to four times daily radiosonde data from the winter and early spring of 1989‐90. In winter, inversion evolution is forced primarily by synoptic-scale events. The buildup takes place over one or more days as warm air advection occurs above the basin with the approach of high pressure ridges. The breakup, which occurs with cold air advection above the basin as troughs approach, can occur over periods less than 12 h. Many approaching troughs modulate inversion strength and depth but are too weak to destroy the persistent inversion. Later in the winter and spring, the radiation-induced nocturnal inversion is destroyed nearly every day by the daytime growth of convective boundary layers from the basin floor and sidewalls.

The Persistent Cold-Air Pool Study

The Persistent Cold-Air Pool Study (PCAPS) was conducted in Utahs Salt Lake valley from 1 December 2010 to 7 February 2011. The field campaigns primary goal was to improve understanding of the physical processes governing the evolution of multiday cold-air pools (CAPs) that are common in mountain basins during the winter. Meteorological instrumentation deployed throughout the Salt Lake valley provided observations of the processes contributing to the formation, maintenance, and destruction of 10 persistent CAP episodes. The close proximity of PCAPS field sites to residences and the University of Utah campus allowed many undergraduate and graduate students to participate in the study. Ongoing research, supported by the National Science Foundation, is using the PCAPS dataset to examine CAP evolution. Preliminary analyses reveal that variations in CAP thermodynamic structure are attributable to a multitude of physical processes affecting local static stability: for example, synoptic-scale processes impact ...

Breakup of Temperature Inversions in Deep Mountain Valleys: Part II. Thermodynamic Model

Abstract A thermodynamic model is developed to simulate the evolution of vertical temperature structure during the breakup of nocturnal temperature inversions in mountain valleys. The primary inputs to the model are the valley floor width, sidewall inclination angles, characteristics of the valley inversion at sunrise, and an estimate of sensible heat flux obtained from solar radiation calculations. The outputs, obtained by a numerical integration of the model equations, are the time-dependent height of a convective boundary layer that grows upward from the valley floor after sunrise, the height of the inversion top, and vertical potential temperature profiles of the valley atmosphere. The model can simulate the three patterns of temperature structure evolution observed in deep valleys of western Colorado. The well-known inversion breakup over flat terrain is a special case of the model, for which valley floor width becomes infinite. The characteristics of the model equations are investigated for several ...

Diurnal Mountain Wind Systems

Diurnal mountain wind systems are local thermally driven wind systems that form over mountainous terrain and are produced by the buoyancy effects associated with the diurnal cycle of heating and cooling of the lower atmospheric layers. This chapter reviews the present scientific understanding of diurnal mountain wind systems, focusing on research findings published since 1988. Slope flows are examined first, as they provide a good introduction to the many factors affecting diurnal mountain wind systems. The energy budgets governing slope flows; the effects of turbulence, slope angle, ambient stability, background flows and slope inhomogeneities on slope flows; and the methods used to simulate slope flows are examined. Then, valley winds are reviewed in a similar manner and the diurnal phases of valley and slope winds and their interactions are summarized. Recent research on large-scale mountain-plain wind systems is reviewed, with an emphasis on the Rocky Mountains and the Alps. Winds occurring in closed basins and over plateaus are then discussed, and analogies between the two wind systems are outlined. This is followed by a discussion of forecasting considerations for diurnal mountain wind systems. Finally, the chapter concludes with a summary of open questions and productive areas for further research.

Single-station integral measures of atmospheric stagnation, recirculation and ventilation

Abstract Mathematical definitions of integral quantities used to characterize the stagnation, recirculation and ventilation potential of various airsheds are proposed. These integral quantities can be calculated from wind data collected at fixed time intervals and at fixed heights in the atmosphere, and could be calculated, for example, from data from ground-based remote wind profilers. These integral quantities, since they are calculated from data at single stations, provide useful characterizations of the flow at individual measurement points, but are true measures of the transport of a plume only under idealized homogenous wind conditions. The utility of these single-station measures for characterizing the air pollution transport potential of an airshed is illustrated using three months of hourly surface and radar profiler measurements of horizontal wind speed and direction collected at three locations in the Colorado Plateaus Basin region of Arizona during the winter of 1990. A surface station at Bullfrog Basin, located on a sheltered basin floor and exposed to diurnal wind systems, experienced stagnations 62% of the time, recirculations 34% of the time, and ventilations 8% of the time. A surface station at Desert View, located on the south rim of the Grand Canyon and exposed to synoptic-scale wind systems, experienced stagnations 8% of the time, recirculations 4% of the time, and ventilations 35% of the time. A radar profiler station at Page, Arizona, experienced stagnations about 20% of the time and recirculations about 25% of the time during the winter at heights below ∼ 400 m a.g.l.; above this height, to levels near 1100 m a.g.l. (the approximate height of surrounding plateaus), the frequency of stagnations and recirculations dropped rapidly, and the frequency of ventilations ranged from 40 to 70%.

Katabatic Flow Mechanisms on a Low-Angle Slope

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.