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  • Author or Editor: James E. Bossert x
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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
,
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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