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Montie M. Orgill

Abstract

An important component of a joint Environmental Protection Agency–Department of Energy field experiment in Brush Creek Valley, Colorado in July–August 1982, was an aircraft sampling task to help verify the early morning ventilation of a gaseous tracer from the valley. The aircraft sampling supplemented a surface sampling network in the 650 m deep northwest–southeast oriented valley. A 100 m elevated tracer release 11 km up Brush Creek Valley from its confluence with Roan Creek Valley was sampled using an airborne real-time tracer analyzer supplemented with periodic grab bag samples. Three experiments were conducted in the early morning hours (0400–1000 MST) of 31 July, 4 August, and 6 August. The tracer plume during the drainage wind period was observed to be confined below 400 m in the valley and was transported beyond 25 km down-valley from the source into Roan Creek Valley. Maximum tracer concentrations were observed on the lower east-sidewall during the early morning but moved to the lower west-sidewall following sunrise. Sixty to 90 minutes after sunrise, the tracer was detected by the aircraft's sampling instruments along the upper west-sidewall. Average concentrations were between 100 ad 200 ppt with peaks above 1000 ppt. Average scaled concentrations (χmacr;/Q̄) were between 0.4 and 0.7 × 10−6 s m−3. Concentrations over the shady east sidewall remained near background levels until about 2.5 hours after sunrise on the west-sidewall and 2 hours after the development of the up-valley winds. These observations indicate that in draining valleys such as Brush Creek Valley, tracer and pollutant material transported down the valley at night is ventilated from the valley following sunrise by upslope and up-valley winds which develop within convective boundary layers over the sunny sidewall and valley. The results from the aircraft sampling verified the existence of the ventilated tracer in higher regions of the valley sidewall, will aid in the study of the physical transport mechanisms operating during the morning transition period, and should assist in the evaluation of valley dispersion models.

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Sumner Barr
and
Montie M. Orgill

Abstract

Thermally driven local circulation in valleys has been studied for many years with the result that the underlying physics are reasonably well understood. ASCOT experiments of the early 1980s were formulated to help quantify predictive models and to apply the resulting methods to transport and dispersion of airborne materials. During the performance and analysis of experiments in two quite different valleys, it became clear that important aspects of the structure of the valley circulation depend on subtle differences in the ambient atmospheric conditions. In this paper we interpret nocturnal drainage structure in terms of ambient characteristics. We are able to describe changes in the depth of drainage and volume flux in terms of the influence of external wind and radiative effects on the collection of cool air in a valley airshed, and on erosion of an established drainage by turbulent entrainment. We describe evidence for internal buoyancy waves and rotors that can have a major effect on transport and dispersion in the nocturnal cool-air drainage regime.

Under ideal conditions of radiative cooling and light ambient winds the drainage depth fills the valley to the ridge level. The radiative driving factor is strongly suppressed by low cloud ceilings and this is reflected in drainage depths shallower than 25% of the valley depth. Further erosion of cold air drainage by turbulent entrainment of ambient air under conditions of moderate to strong ridge top winds results in a linear regression of the form:where v is the ridge top wind in m s−1. With these dependencies on clouds and wind it is not surprising that the “Climatology” of valley drainage winds favors the seasons of weak synoptic activity and low thunderstorm frequency.

The transition layer that bounds the top of the drainage is variable in time and space and depends on the thermal stability and wind speed and direction at ridge level. Standing internal waves, predicted a Froude number criterion, may govern the encroachment of ambient air into the local valley regime. Brush Crek Canyon is a narrow, steep-waled valley that may frequently exhibit a shear-induced helix imposed upon the down-valley drainage. This circulation is supported by meteorological and tracer data and can significantly influence pollution distributions.

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Montie M. Orgill
,
John D. Kincheloe
, and
Robert A. Sutherland

Abstract

The mesoalpha-scale upper-level sounding network data collected during the 1984 ASCOT meteorological and tracer experiments provided a unique opportunity to analyze the nocturnal drainage wind in four different valleys in western Colorado, and to examine the effects of the synoptic-and mesoscale ambient conditions on these valley drainage winds. The six experimental periods, although biased because of a “fair weather” selection process, provided an additional opportunity to examine “good” and “poor” drainage scenarios. The results show that drainage winds fill all four valleys up to 80%–l00% of their valley depths under favorable nocturnal radiative longwave cooling (1.0°–1.5°C h−1) and light (<5 m s−1) ambient winds. Valley drainage, once established, is rather resistant to erosion from above because of the large source regions of these valleys, their large volume fluxes and inertia, and the persistent stable conditions inside these valleys. Wind erosion was observed on three nights when the drainage depth was reduced to less than half of the valley depth. The principal contributing factors to wind erosion processes were above-valley along-valley wind component opposing the drainage, valley stability, height of the 5 m s−1 isotach above the valley, and total above-valley wind acceleration. Generally, wind erosion processes appear to be especially active when above-valley wind speeds exceed 5 m s−1 and accelerations exceed 0.00040 m s−1. Other contributing factors that cause variable or terminated drainage depths are precipitation-evaporation effects causing nonradiative drainage events, wind shear above the valley, cloudiness, frontal passages, and synoptic winds directed in the down-valley direction.

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Montie M. Orgill
,
John D. Kincheloe
, and
Robert A. Sutherland

Abstract

An experimental weather classification, analysis, and nowcasting system, based upon a combination of artificial intelligence techniques and conventional numerical modeling, and designed for use as a real-time range/field forecasting aid, is described. In particular, a computer-based prototype coupled knowledge-based system, called PROCANS (Prototype Coupled Analysis and Nowcasting System), tailored for applications at the U.S. Army field testing range at Fort Hunter Liggett, California, is used as an example to demonstrate and evaluate the overall concept. The components of the system are: 1) a rule-based meteorological scenario evaluator for analysis and classification of weather scenarios, 2) a nowcaster that uses four analogical and rule-based expert subsystems for nowcasting radiation fog, wind gustiness, thunderstorms, and precipitation, 3) a numerical transport and diffusion module based upon either Gaussian or Monte Carlo particle-trajectory models to simulate airflow and diffusion patterns, and 4) a master database for storing information for possible retrieval, comparing current weather scenarios with past scenarios for possible matching and for analogical and conceptual reasoning to aid future predictions. A preliminary evaluation of PROCANS shows that coupled knowledge-based systems have potential as an integrated local analysis and prediction tool or forecaster's aid for field operations such as smoke screening.

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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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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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