William E. Clements

Mean Structure of the Nocturnal Drainage Flow in a Deep Valley

Abstract Wind and temperature data collected by an instrumented tethered balloon and a Doppler lidar in a deep valley are used to investigate the mean properties of the nocturnal drainage flow down the valley on four nights when the wind at ridgetop had an up-valley component. We examine the vertical structure of temperature and the vertical and horizontal structure of the drainage wind. An empirical description of the wind field is derived and used to estimate the mass flux resulting from the drainage flow. Mean properties of the flow are presented and relationships among some of the parameters are examined.

Experimental Design of the 1984 ASCOT Field Study

Abstract During September and October of 1984 the Department of Energys Atmospheric Studies in Complex Terrain program conducted an intensive field study in the Brush Creek Valley of western Colorado. The overall objective of the study was to enhance the understanding of pollutant transport and diffusion associated with valley flows. Data collections were designed to investigate nocturnal and morning transition wind, turbulence, and temperature fields in the valley, in its tributaries, and on its side-slopes, and how these are affected by the free stream conditions above the valley. The release and sampling of atmospheric tracers were used to study transport and diffusion. The experimental design of this study is presented.

Characteristics of the atmospheric state and the surface radiation budget at the tropical western Pacific ARM Site

The Department of Energys Atmospheric Radiation Measurement (ARM) program has been operating a radiation and cloud observing station in the tropical western Pacific (TWP) since October, 1996. This station includes instruments for measuring the surface radiation budget, characteristics of the atmospheric column that affect that budget, and surface meteorological parameters. Our purpose here is both to introduce some of the fundamental characteristics of the tropical western Pacific region and to show some interesting properties of the data that have emerged from our analyses thus far. The column concentration of water vapor is very high in this region and dominates the downward flux of infrared at the surface. For shortwave radiation, clouds dominate the surface budget. We have compared the surface shortwave cloud forcing from the ARM site with previous observations from this region. Finally, we have examined the vertical distribution of clouds in periods of active and suppressed convection and find interesting contrasts between these regimes.

Nighttime Valley Waves

Abstract This paper describes a regular oscillation observed in nighttime drainage airflow in a valley under relatively light upper-level wind conditions. The period of these oscillations is about 20 minutes with at least one harmonic at about 10 minutes. A strong coherence between tributary flow and main valley fluctuations was observed, with the phase of the tributary flow leading the valley oscillation; this indicates the importance of tributaries as major contributors to the dynamics of cold air flow in valleys.

A video image analysis system for concentration measurements and flow visualization in building wakes

-A video image analysis technique for concentration measurements and flow visualization was developed for the study of diffusion in building wakes and other wind tunnel flows. Smoke injected into the flow was photographed from above with a video camera, and the video signal was digitized in real time during the experiment and during playback with a video image analysis system. The relation between the digitized smoke intensity and the vertically-integrated concentration of smoke particles was obtained from calibration experiments in which the smoke was replaced by a mixture of ethane and air. The time-averaged vertically-integrated concentration of the ethane tracer was measured at discrete points throughout the flow field and correlated with the time-averaged smoke intensity data. After the system was calibrated, the instantaneous and time-averaged fields of vertically-integrated concentration were obtained by simply photographing the smoke plume. Color-contoured images of the plume were created and displayed in real time thus creating a pseudo-color movie that was very useful for flow visualization. This technique was used to study the effects of building geometry and source location on the concentration and flow patterns in building wakes. Key word index: Building wakes, dispersion in building wakes, flow visualization, video image analysis systems, wind tunnel modeling. I. I N T R O D U C T I O N The short-range dispersion of material released into the atmosphere from vents on buildings is dominated by the complex, unsteady flow in the building wake. There are no general theoretical models for diffusion in such flows, and most of our knowledge comes from wind tunnel experiments in which the building is immersed in a simulated, turbulent atmospheric boundary layer. The most important parameters in these simulations are the geometry of the building, the location of the vent, and the direction and vertical variation of the ambient wind. The emphasis in most wind tunnel experiments has been on defining the structure of the flow field by measuring local velocities and turbulence, and on defining concentration patterns by releasing a tracer and measuring point concentrations on the building surface and on the ground downstream of the building. Comprehensive reviews of wind tunnel studies and semi-empirical models * Earth and Environmental Sciences Division, Geoanalysis Group (MS-D466), Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. t Space Science and Technology Division, Data Analysis Group (MS-D440), Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. ~/Fluid Modeling Facility (MD-81), U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, U.S.A. (On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.) based upon this database are presented by Hosker (1984) and Meroney (1982). Image analysis techniques have been used in various fluid dynamics experiments for flow visualization, panicle tracking and concentration measurements; Yang (1985) and Vrret (1987). These techniques use lasers or other high intensity light sources to illuminate a thin sheet of a flow that has been seeded with smoke, small tracer particles or a substance which flouresces in the laser light. The major advantage of these techniques over point measurements is their definition of the spatial structure of the flow field. In some diffusion problems the quantity of primary interest is the concentration integrated along a path through a plume. For example, visibility depends upon the concentration integrated along the line-ofsight of the observer, and many remote sensing instruments measure concentration integrated along a propagation path in the atmosphere. Wind tunnel simulations in which integrated concentrations are measured directly can be useful in these areas. Also, as will be shown in this paper, a surprising amount of information on the horizontal structure of the flow field is contained in the vertically-integrated concentration field. The objectives of this study are to develop a video image analysis technique for measuring the vertically integrated concentration in wind tunnel experiments and to apply this technique to the study of diffusion in building wakes. Similar studies with less emphasis on the quantitative measurement of concentrations were

Plume dispersion in a nocturnal drainage wind

Abstract A series of tracer experiments were conducted under nocturnal drainage wind conditions in a complex terrain setting in the Piceance Basin of western Colorado. Concurrent meteorological information including profiles of wind and temperature as well as gross turbulence fluctuations from fixed 2-m stations provided the basis to test plume growth and dilution prescriptions for this moderately complex site. Plume parameters exhibited slightly greater diffusion than would be indicated by simple stability-based prediction methods or the gross turbulence indicators. Two terrain-related mechanisms appear to contribute to the development of the plume. A meandering component immediately downwind of the confluence of two valleys gives the appearance of an abnormally wide time-integrated plume. Further downstream the mean wind direction stabilizes and the plume dimension reflects diffusive spread due to small-scale turbulence.

Fluorescent particle lidar

A lidar system designed to detect the return from small fluroescent particles has been built. (AIP)

ROMPEX—The Rocky Mountain Peaks Experiment of 1985: Preliminary Assessment

Abstract During the late summer of 1985 a field experiment was conducted to investigate mountaintop winds over a broad area of the Rocky Mountains extending from south central Wyoming through northern New Mexico. The principal motivation for this experiment was to further investigate an unexpectedly strong and potentially important wind cycle observed at mountaintop in north central Colorado during August 1984. These winds frequently exhibited nocturnal maxima of 20 to 30 m·s−1 from southeasterly directions and often persisted for eight to ten hours. It appears that these winds originate as outflow from intense mesoscale convective systems that form daily over highland areas along the Continental Divide. However, details of the spatial extent and variability of these winds could not be determined from “routine” regional weather data that are mostly collected in valleys. Although synoptic conditions during much of the 1985 experiment period did not favor diurnally recurring convection over the study area, ...

Some Physical Concepts in Complex Terrain Meteorology Derived from the U. S. Department of Energy S Ascot Program

In 1978 the United States Department of Energy (DOE) initiated a program directed specifically at atmospheric studies in complex terrain (ASCOT). The two broad objectives of the ASCOT program are: To improve fundamental knowledge of atmospheric transport and dispersion processes in complex terrain. Building on this improvement in the understanding of the physics to provide a methodology for performing air quality assessments.

Demonstration of a High Pulse Rate Lidar for Studying Airflow

Abstract In response to a recognized need for an inexpensive, low power, portable lidar for meteorological applications in remote areas, a system has been designed and constructed. The lidar, termed MELS (Mini-Environmental Lidar System), operates on 20 watts of power from a generator or battery, and can be easily deployed by two people on foot. After a series of calibration runs the MELS was demonstrated in two field experiments in Colorado valleys. One experiment was concerned with the measurement of natural aerosols and another with smoke released from a point source. Meteorological interpretation of the data shows consistency with expectations; the data provide a valuable adjunct to the conventional observations for deducing boundary layer dynamics.