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J. C. Doran

Abstract

A network of sodars was operated in the late summer and fall of 1993 to monitor the occurrence of nocturnal winds from a canyon in Colorado's Front Range near the Rocky Flats Plant and to determine the influence of those winds on the flow fields over the plant. The canyon flows could be broadly classified into two categories: well developed and irregular. The well-developed flows were generally stronger, deeper, and more continuous than the irregular ones, and the canyon's influence on the wind fields near the plant site was confined primarily to periods with these flows. These periods, in turn, usually followed days during which a deep mixed layer formed over the plains to the east of the mountains. Following days with shallower mixed layers, the canyon winds tended to be weaker and shallower.

Numerical simulations with a nested mesoscale numerical model were used to examine the mechanisms responsible for this behavior. The nighttime simulated temperature gradients between the air near the mountain slopes and the free air over the plains were found to be larger after days with deep mixed layers, resulting in stronger down-canyon flows at night. In addition, for the deeper mixed-layer cases, air parcels descending the slope do so in more neutrally stratified conditions so that their buoyant acceleration down the slope is maintained. A notable exception to this behavior occurred on one night when nonstationary waves appeared to disrupt the canyon flows. Marker particles released into the simulated flow fields were used to follow the motion of air parcels from the mountains out over the plains. They revealed a tendency for air parcels to remain elevated when they exit the valley on nights with lighter canyon winds and shallower afternoon mixed layers, thereby reducing the canyon's potential effect on the near-surface winds over the Rocky Flats Plant. Particle trajectories were also used to examine the concept of a well-defined airshed feeding a draining valley; the concept was found to be of limited validity for the topography in this area.

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S. Zhong and J. C. Doran

Abstract

Measurements from the Southern Great Plains Cloud and Radiation Testbed site, which is situated in Oklahoma and Kansas and extends over an area approximately 300 km × 350 km in extent, are combined with results from a three-dimensional mesoscale model to study the sensitivity of boundary layer properties to spatially varying surface sensible and latent heat fluxes. Four cloud parameterization schemes are used to estimate the fractional cloudiness expected over the site on three case study days with settled weather conditions during the summer of 1994. Comparisons between observations and model predictions show good qualitative agreement. Although local responses to varying surface fluxes can be found, the replacement of the spatially varying surface conditions with uniform ones makes little difference in the simulated cloud cover or the vertical profiles of potential temperature and water vapor mixing ratio when these are averaged over the full site. Spatial variations in the ambient meteorology were found to be more important than variations in surface fluxes in determining cloud amount and areas of preferred cloud formation. This conclusion is supported by additional simulations in which both the ambient meteorology and surface conditions are averaged over scales ranging from 6.25 km to 300 km. The results call into question the importance of mesoscale fluxes (i.e., fluxes arising from secondary circulations induced by heating contrasts over different surfaces) in coarse-resolution models such as general circulation models, at least for settled weather conditions similar to those considered in this study.

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Shiyuan Zhong and J. C. Doran

Abstract

A numerical modeling study was conducted to examine the response of the atmospheric boundary layer to inhomogeneous surface fluxes. The study was used to extend the results obtained from a field experiment carried out in spring 1992 in north-central Oregon over a region characterized by warm, dry sagebrush and grassland steppe and cooler, irrigated farmland. Characteristic scales of the two prominent land-use types were on the order of 10 km or more. A series of numerical experiments were carried out to analyze boundary-layer behavior on three days selected for detailed study, to perform a set of sensitivity tests to identify the principal mechanisms responsible for secondary circulations in the region, and, with selected two-dimensional simulations, to verify the role of advection in maintaining well-mixed layers over cool surfaces. Although contrasts in land use produce measurable secondary circulations over the study area, terrain effects and ambient winds can mask much of the response to the differential heating over the warm and cool regions. The depth of the mixed layer is poorly correlated with the local underlying surface heat fluxes but is governed instead by a combination of local fluxes, horizontal advection, convergence or divergence, and shear production of turbulence. An analysis of mesoscale heat fluxes, that is, the fluxes associated with secondary circulations that would not be resolved by a coarse resolution model, shows that the contribution, arising from land-use differences, to the domain-averaged atmospheric heating rate is small for this area. The authors suggest that modeling studies based on idealized terrain and land-use configurations may tend to overestimate the effect of mesoscale fluxes on the temperature structure predicted by coarse-resolution models applied to real world conditions. Even so, secondary circulations may be significant for other boundary-layer properties, such as mixed-layer depth and cloud formation.

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J. C. Doran and S. Zhong

Abstract

Current approaches to parameterizations of sub-grid-scale variability in surface sensible heat fluxes in general circulation models normally neglect the associated variability in mixed-layer depths. Observations and a numerical mesoscale model are used to show that the magnitude of such variability can be significant. Over a domain of (41 km)2, the standard deviation of simulated mixed-layer depths was found to be 21%–24% of the mean noontime values on three days, and the mean depths were not simply related to the mean sensible heat fluxes. Results obtained with two-dimensional simulations over idealized distributions of warm, dry and cool, or moist surfaces show that as the characteristic sizes of individual patches increase, the distributions of mixed-layer depths tend to assume a bimodal nature. under these conditions, the mean mixed-layer depth may have little physical relevance. Finally, the use of domain-averaged values of wind and temperature to compute surface fluxes is shown to be another potential source of error in flux parameterizations.

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J. C. Doran and S. Zhong

Abstract

An analysis of regional drainage flows in the Pacific Northwest is presented using results from a network of surface observations and a series of simulations carried out with a nested mesoscale model. The flows, which occur regularly in southeastern Washington during the late spring and summer months, are marked by an increase in wind speed and a shift to northwesterly wind directions early in the evening. The phenomenon occurs when a deep mixed layer forms cast of the Cascade Range, drawing cooler air from the west over the mountain crest. Anabatic and katabatic forcing, terrain channeling, and turning by the Coriolis force combine to produce the characteristic flow patterns.

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J. C. Doran and S. Zhong

Abstract

A southeasterly flow in the form of a low-level jet that enters the Mexico City basin through a mountain gap in the southeast corner of the basin developed consistently in the afternoons or early evenings during a four-week 1997 winter field campaign. Peak wind speeds often exceeded 10 m s−1. Although these winds have not been studied previously, the observations suggest that they are a regular feature of the basin wind system, at least during the winter months. The jets were found more frequently during the early part of the experiment, when conditions in the basin were generally warmer, drier, and less cloudy, than in the later part when conditions were cooler, more humid, and cloudier. The winds usually were stronger during the early period, also. Temperature measurements from radiosondes launched inside and outside the basin showed a dependence of the gap wind strength on the temperature differences between the two regions. A three-dimensional numerical model was used to simulate the characteristics of the gap flows to provide information on the mechanisms responsible for their development. The maximum speed of the jet usually is reached several hours after the occurrence of the maximum temperature gradient between the basin and the region to the south. An analysis of the momentum balance shows that the gap wind is initiated by a north–south pressure gradient across the gap in the lower boundary layer arising from temperature differences between the warmer basin and the cooler exterior air. Penetration of the gap wind into the basin is caused primarily by horizontal advection. The gap wind plays an important role in the formation of a convergence zone, which can have an important effect on surface air pollutant distributions in the basin.

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J. C. Doran and D. C. Powell

Abstract

Measured characteristics of gust amplitudes and times in the neutral surface boundary layer are presented. The probability of gust amplitudes exceeding a prescribed level is shown to decrease exponentially with amplitude, provided the amplitude is scaled with the root-mean-square turbulent speed. The 25 and 75 percentile conditional probabilities of gust duration obey power laws in the scaled amplitudes if the durations are normalized by N0, the frequency of occurrence of all gusts. These relationships are nearly independent of mean wind speed and measurement height. The effects of digital filtering of the data also are discussed.

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J. C. Doran and E. D. Skyllingstad

Abstract

Observations and numerical simulations with a hydrostatic model are used to examine effects of regional and local terrain, synoptic forcing, and stability on the wind fields of an intermountain basin. The study area is centered on the Hanford site in southeast Washington. Two wintertime case studies are presented, each characterized by a surface inversion but with different synoptic forcing. In both cases, the local topography produces a region of blocked flow and significant horizontal wind shear, but slight differences in the direction of the synoptic winds lead to marked differences in the subsequent development of the wind fields over the site. Numerical experiments show that these differences are related to flow through gaps in the Cascade Mountains over 100 km to the west. Additional experiments show that a modest increase in vertical wind shear can cause significant alterations in the local flow patterns, the elimination of the blocked-flow region, and less sensitivity to regional or local terrain effects. Increased solar insulation also weakens the blocking effect of local terrain, but the effects of the Cascade Mountains can still be discerned.

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J. C. Doran and T. W. Horst

Abstract

Measurements of simple nocturnal slope winds were taken on Rattlesnake Mountain, a nearly ideal two-dimensional ridge. Tower and tethered balloon instrumentation allowed the determination of the wind and temperature characteristics of the katabatic layer as well as the ambient conditions. Two cases were chosen for study; these were marked by well-defined surface-based temperature inversions and a low-level maximum in the downslope wind component. The downslope development of the slope flow could be determined from the tower measurements, and showed a progressive strengthening of the katabatic layer. Hydraulic models developed by Manins and Sawford (1979a) and Briggs (1981) gave useful estimates of drainage layer depths, but were not otherwise applicable. A simple numerical model that relates the eddy diffusivity to the local turbulent kinetic energy was found to give good agreement with the observed wind and temperature profiles of the slope flows.

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J. C. Doran and M. G. Verholek

Abstract

The adequacy of a vertical extrapolation scheme for Weibull wind speed distribution parameters proposed by Justus and Mikhail is examined. While useful for ensemble averages, the technique can lead to significant errors in individual cases.

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