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James E. Bossert

Abstract

The Regional Atmospheric Modeling System (RAMS) is used to investigate the detailed mesoscale flow structure over the Mexico City region for a 3-day period in February 1991. The model simulation is compared with rawinsonde and tethersonde profile data and measurements from two surface stations in the southwestern part of Mexico City. The model results show that downward momentum transfer from aloft increases southerly winds near the surface on the first case day, effectively sweeping pollution from the basin surrounding Mexico City. Thermally driven circulations within the basin, in adjacent valleys, and over the slope of the Mexican Plateau strongly influence winds within the Mexico City basin on the second case day. These wind systems produce a complex interaction of flows, culminating in the propagation of a 1-km-deep density current circulation through Mexico City that displaces the polluted basin air mass aloft. Regional northeasterly flows develop early in the morning of the third case day and force the polluted basin air mass toward the southwestern portion of the basin where observed ozone concentrations are highest. The results show that both regional- and synoptic-scale flows influence the meteorology within the Mexico City basin over the 3-day period. The simulated circulations also provide a physical basis for understanding the high spatial and temporal variability of ozone concentrations observed over the city.

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Gregory S. Poulos and James E. Bossert

Abstract

The Atmospheric Studies in Complex Terrain Program conducted a field experiment at the interface of the Rocky Mountains and the Great Plains in the winter of 1991. Extensive meteorological observations were taken in northeastern Colorado near Rocky Flats to characterize overnight conditions in the region. Simultaneously, a tracer dispersion experiment using over 130 samplers to track plume development was conducted by Rocky Flats facility personnel. These two datasets provided an opportunity to investigate the accuracy and applicability of a fully prognostic, primitive equation, mesoscale model to the simulation of complex terrain dispersion.

Meteorological conditions in the Rocky Flats region are forecast for selected case nights using the Regional Atmospheric Modeling System initialized with sounding data taken during the experiment. The forecast winds and temperature are used in a Lagrangian particle dispersion model to predict tracer plume transport. The results of both models are compared to observations taken during the experimental period and qualitatively and quantitatively assessed. It is found that this modeling system is able to reproduce many features of the observed meteorology and dispersion for four overnight cases. Quantitatively, maximum ground concentrations are generally found to be within a factor of 2 of observations and located radially within approximately 50° of azimuth of the observed location. Additional model sensitivity simulations define the role of local terrain features on Rocky Flats area dispersion and indicate the need for improved model initialization techniques when multiple data sources are available. These experiments reveal a promising future for the application of prognostic mesoscale models to emergency response problems in regions of complex terrain.

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James E. Bossert and William R. Cotton

Abstract

A series of two- and three-dimensional idealized numerical experiments are conducted to examine the effects of different physical processes upon the development of the thermally driven regional-scale circulations over mountainous terrain simulated in Part I. The goal of this paper is to understand the conditions that enhance or suppress the formation of a westward-propagating density current within the mountain boundary layer. This current evolves from the Front Range mountain-plain circulation and was found in Part I to be responsible for unusual wind phenomena observed at mountaintop locations during the Rocky Mountain Peaks Experiment over western Colorado.

The idealized experiments show that the westward-propagating density current is a robust feature under summertime conditions of weak ambient flow and is initiated by differential heating across the Colorado Front Range between the plains and the intermountain region. In addition, the longevity of the thermally driven circulation system induces a steady southerly flow component, which persists over the intermountain region at night after the density current propagates away. The unique topography of the Colorado Rocky Mountain barrier—which features low plains on the east, a high dividing range, and a high plateau on the west—enhances the development of the current. The westward-propagating disturbance also develops over a range of low-level ambient wind speed, direction, and shear but is suppressed with low-level westerly flow, which also weakens the development of its progenitor, the Front Range mountain-plains solenoid.

Low-level stratification affects the depth and strength of the Front Range mountain-plains solenoid, which is most energetic in summertime conditions of near-neutral stability below 50 kPa. High stability in the lower troposphere suppresses the vertical development of the solenoid but increases the baroclinicity across the Front Range generated by surface heating, thereby still producing a significant density-current disturbance. Wet soil over the high terrain west of the Front Range also suppresses the formation and strength of the Front Range solenoid, while wet soil along the eastern slope of the Front Range and eastern plains with drier conditions over the high mountain terrain greatly enhances the baroclinicity within the solenoid and the subsequent density-current evolution. This couplet acts as an efficient conveyer of low-level moisture into the mountain region.

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James E. Bossert and William R. Cotton

Abstract

This study uses observed data and a numerical simulation to examine the generation of thermally driven flows across the Colorado mountain barrier on meso-β to meso-α scales. The observations were collected from remote surface observing systems at exposed mountaintop locations throughout the state of Colorado, over the summers of 1984–88, as part of the Rocky Mountain Peaks Experiment (ROMPEX). The data show the development of a recurrent circulation system across the Colorado mountain barrier, operating on a diurnal timescale. From the observations, the basic structure of the flow system appears as a daytime inflow toward the highest terrain, and a nocturnal outflow away from it. However, when examined in detail, the flow system exhibits more unusual behavior, especially west of the barrier crest. Here, winds in the early evening are occasionally observed to onset abruptly from an easterly direction, generally counter to the upper-level winds. Observations from ROMPEX for 26 August 1985 are used to provide comparison data for a numerical simulation with the Regional Atmospheric Modeling System (RAMS). This three-dimensional case study experiment is initialized with data from the National Meteorological Center and incorporates two-way interactive grid nesting.

From the observed data and case study simulation, four distinct phases of the regional-scale circulation system have been identified. In the development phase, a deep mountain-plains solenoid is generated through terrain heating along the Front Range. This circulation system transforms in the late afternoon transition phase into a westward-propagating density current (WPDC). The third phase, called the “density-current propagation phase,” occurs as the WPDC moves westward across the mountains, leaving in its wake strong southeasterly flow at the mountaintop level. This current appears to be the cause of the peculiar easterly component winds found in the ROMPEX mountaintop observations along the western slope. In the final late-night adjustment phase, the WPDC dissipates near the western edge of the Colorado mountains and a steady southerly flow evolves over the high mountain terrain. This southerly flow is the steady response to the differential heating that develops between the low-lying plains and the intermountain region.

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James E. Bossert, John D. Sheaffer, and Elmar R. Reiter

Abstract

Mountaintop data from remote stations in the central Rocky Mountains have been used to analyze terrain-induced regional (meso-β to meso-α) scale circulation patterns. The circulation consists of a diurnally oscillating wind regime, varying between daytime inflow toward, and nocturnal outflow from, the highest terrain. Both individual case days and longer term averages reveal these circulation characteristics. The persistence and broadscale organization of nocturnal outflow at mountaintop, well removed from valley drainage processes, demonstrates that this flow is part of a distinct regime within the hierarchy of terrain-induced wind systems.

The diurnal cycle of summertime convective storm development imparts a strong influence upon regional-scale circulation patterns. Subcloud cooling processes, associated with deep moist convection, alter the circulation by producing early and abrupt shifts in the regional winds from an inflow to outflow direction. These wind events occur frequently when moist conditions prevail over the central Rocky Mountains. Atmospheric soundings suggest that significant differences occur in the vertical profile of the topographically influenced layer, depending upon the dominant role of either latent or radiative forcing.

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Gregory S. Poulos, James E. Bossert, Thomas B. McKee, and Roger A. Pielke Sr.

Abstract

Via numerical analysis of detailed simulations of an early September 1993 case night, the authors develop a conceptual model of the interaction of katabatic flow in the nocturnal boundary layer with mountain waves (MKI). A companion paper (Part I) describes the synoptic and mesoscale observations of the case night from the Atmospheric Studies in Complex Terrain (ASCOT) experiment and idealized numerical simulations that manifest components of the conceptual model of MKI presented herein. The reader is also referred to Part I for detailed scientific background and motivation.

The interaction of these phenomena is complicated and nonlinear since the amplitude, wavelength, and vertical structure of the mountain-wave system developed by flow over the barrier owes some portion of its morphology to the evolving atmospheric stability in which the drainage flows develop. Simultaneously, katabatic flows are impacted by the topographically induced gravity wave evolution, which may include significantly changing wavelength, amplitude, flow magnitude, and wave breaking behavior. In addition to effects caused by turbulence (including scouring), perturbations to the leeside gravity wave structure at altitudes physically distant from the surface-based katabatic flow layer can be reflected in the katabatic flow by transmission through the atmospheric column. The simulations show that the evolution of atmospheric structure aloft can create local variability in the surface pressure gradient force governing katabatic flow. Variability is found to occur on two scales, on the meso-β due to evolution of the mountain-wave system on the order of one hour, and on the microscale due to rapid wave evolution (short wavelength) and wave breaking–induced fluctuations. It is proposed that the MKI mechanism explains a portion of the variability in observational records of katabatic flow.

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Gregory S. Poulos, James E. Bossert, Thomas B. McKee, and Roger A. Pielke

Abstract

The mutual interaction of katabatic flow in the nocturnal boundary layer (NBL) and topographically forced gravity waves is investigated. Due to the nonlinear nature of these phenomena, analysis focuses on information obtained from the 1993 Atmospheric Studies in Complex Terrain field program held at the mountain–canyon–plains interface near Eldorado Canyon, Colorado, and idealized simulations. Perturbations to katabatic flow by mountain waves, relative to their more steady form in quiescent conditions, are found to be caused by dynamic pressure effects. Based on a local Froude number climatology, case study analysis, and the simulations, the dynamic pressure effect is theorized to occur as gravity wave pressure perturbations are transmitted through the atmospheric column to the surface and, through altered horizontal pressure gradient forcing, to the surface-based katabatic flows. It is proposed that these perturbations are a routine feature in the atmospheric record and represent a significant portion of the variability in complex terrain katabatic flows.

The amplitude, wavelength, and vertical structure of mountain waves caused by flow over a barrier are themselves partly determined by the evolving structure of the NBL in which the drainage flows develop. For Froude number Fr > ∼0.5 the mountain wave flow is found to separate from the surface at higher altitudes with NBL evolution (increasing time exposed to radiational cooling), as is expected from Fr considerations. However, flow with Fr < ∼0.5 behaves unexpectedly. In this regime, the separation point descends downslope with NBL evolution. Overall, a highly complicated, mutually evolving, system of mountain wave–katabatic flow interaction is found, such that the two flow phenomena are, at times, indistinguishable. The mechanisms described here are expanded upon in a companion paper through realistic numerical simulations and analysis of a nocturnal case study (3–4 September 1993).

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Elmar R. Reiter, John D. Sheaffer, James E. Bossert, Richard C. Fleming, William E. Clements, J. T. Lee, Sumner Barr, John A. Archuleta, and Donald E. Hoard

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, sufficient data were obtained to verify the regional-scale organization of strong convective outflow at mountaintop elevations. In addition, the usefulness and feasibility of a mountain-peak weather-data network for routine synoptic analysis is demonstrated.

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