Atmospheric Studies In Complex Terrain Research Articles

Inline Coupling of WRF–HYSPLIT: Model Development and Evaluation Using Tracer Experiments

AbstractThe Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT), a Lagrangian dispersion model, has been coupled (inline) to the the Weather Research and Forecasting (WRF) Model meteorological model in such a way that the HYSPLIT calculation is run as part of the WRF-ARW prediction calculation. This inline version of HYSPLIT takes advantage of the higher temporal frequency of WRF-ARW variables relative to what would be available for the offline approach. Furthermore, the dispersion calculation uses the same vertical coordinate system as WRF-ARW, resulting in a more consistent depiction of the state of the atmosphere and the dispersion simulation. Both inline and the offline HYSPLIT simulations were conducted for two tracer experiments at quite different model spatial resolutions: the Cross Appalachian Tracer Experiment (CAPTEX) in regional scale (at 9-km grid spacing) and the Atmospheric Studies in Complex Terrain (ASCOT) in finescale (at 333.3-m grid spacing). A comparison of the model with the measured values showed that the results of the two approaches were very similar for all six releases in CAPTEX. For the ASCOT experiments, the cumulative statistical score of the inline simulations was better than or equal to offline runs in four of five releases. Although the use of the inline approach did not provide any advantage over the offline method for the regional spatial scale and medium-range temporal scale represented by the CAPTEX experiment, the inline HYSPLIT was able to improve the simulation of the dispersion when compared with the offline version for the fine spatial and temporal resolutions over the complex-terrain area represented by ASCOT. The improvement of the inline over the offline calculation is attributed to the elimination of temporal and vertical interpolation of the meteorological data as compared with the offline version.

The Interaction of Katabatic Flow and Mountain Waves. Part II: Case Study Analysis and Conceptual Model

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.

Effects of stability on the profiles of vertical velocity and its variance in katabatic flow

The atmospheric katabatic flow in the foothills of the Front Range of the Rocky Mountains has been monitored by a network of towers and sodars for several years as part of the Atmospheric Studies in COmplex Terrain (ASCOT) program. We used three years of data from the network to explore the dependence on surface cooling and channeling by winds above the canyon of (1) profiles of the mean and variance of the vertical (perpendicular to the geopotential) component of motion and (2) the mean component of the wind perpendicular to the local terrain of Coal Creek Canyon. Previously we found that the magnitude of the near-surface temperature difference decreases with increasing surface cooling in light winds, apparently because of increasing turbulence caused when increasing drainage winds interact with surface topography. The variance of vertical velocity exhibits three types of vertical profiles, corresponding to different cooling rates and external wind speeds. The mean variance was found to depend strongly on a locally derived Richardson number.

Mesoscale Modeling and Four-Dimensional Data Assimilation in Areas of Highly Complex Terrain

Abstract A multiscale four-dimensional data assimilation (FDDA) technique, based on Newtonian relaxation, is incorporated into a mesoscale model and evaluated using meteorological and tracer data collected during the Atmospheric Studies in Complex Terrain (ASCOT) field experiment in the winter of 1991. The mesoscale model is used to predict the synoptically driven flows and small-scale circulations influenced by terrain along the Front Range in Colorado in the vicinity of Rocky Flats Plant for four nocturnal periods during the ASCOT field experiment. Data assimilation is used to create dynamically consistent analysis fields based on the mesoscale forecasts and the special asynoptic data taken during this experiment. Observations from towers, minisodars, airsondes, tethersondes, rawinsondes, and profilers near the Rocky Flats Plant, as well as observations from surface stations throughout Colorado, are incorporated into the high-resolution analysis fields. The wind and turbulence quantities produced by the...

Observations of a Terrain-Forced Mesoscale Vortex and Canyon Drainage Flows along the Front Range of Colorado

Abstract Observations taken during the February 1991 Atmospheric Studies in Complex Terrain (ASCOT) Winter Validation Study are used to describe the wind field associated with a terrain-forced mesoscale vortex and thermally forced canyon drainage flows along the Front Range of northeastern Colorado. A case study is presented of the night of 6/7 February 1991 when a weak vortex formed and propagated through the ASCOT domain. The NOAA/ERL Environmental Technology Laboratory Doppler lidar, one of an ensemble of instruments participating in the ASCOT field experiment, obtained high-resolution measurements of the structure of both the vortex and the canyon drainage flows. The lidar observations documented the kinematic and structural changes in the cyclone and their relationship to a drainage jet exiting a nearby canyon. Lidar analyses clearly show the layering and stratification present during this case, specifically the drainage jet flowing under the cyclone. A period of strong intensification of the drainag...

Transport and Dispersion in Complex Terrain

Atmospheric research related to the needs of emergency preparedness has been conducted in the US Department of Energy's (DOE's) Atmospheric Studies in Complex Terrain (ASCOT) programmes, most recently along the mountain-plains interface near DOE's Rocky Flats Plant in Colorado. ASCOT meteorological experiments in January and February 1991 were coordinated with Rocky Flats tracer releases for airflow and dispersion model evaluations. It was found that multiple scales of motion controlling dispersion varied significantly in three dimensions and on an hourly basis. Plume dispersion at night was held close to the surface within a stable drainage layer that followed regional flow features, intermittently interrupted by evolving mountain-canyon flows. Simulations of flow and dispersion used both wind interpolation techniques and primitive equation numerical models. Comparisons of simulations with plume observations showed relatively good agreement for general features but significant, evolving mesoscale variations were less reliably followed and require improved understanding and modelling.

Modeling Bulk Atmospheric Drainage Flow in a Valley

Abstract Most simulations of bulk valley-drainage flows depend heavily on parameterizations. The 1984 Atmospheric Studies in Complex Terrain (ASCOT) field experiment in Brush Creek Valley, Colorado, provided an unprecedented density of measurements in a natural valley of simple shape, allowing tests of assumptions and parameterizations developed from laboratory measurements and detailed numerical simulations. This paper uses the ASCOT data to test a model that computes total fluxes of mass (volume) and momentum—determining buoyant and pressure-gradient forces from measured temperature profiles, and parameterizing drag, entrainment, and sidewall and tributary drainage. Computed divergences of volume and momentum flux are within a factor of 2 of the Doppler lidar measurements in Brush Creek Valley. The relative importance of individual terms as parameterized in the model is discussed. A major problem for future work is the treatment of the interaction between valley drainage and ambient flow.

The Dispersion of Atmospheric Tracers in Nocturnal Drainage Flows

Abstract This paper summarizes the results of a series of perfluorocarbon tracer experiments that were carried out in the Brush Creek Valley in western Colorado under the auspices of the Atmospheric Studies in Complex Terrain (ASCOT) program. The results indicate that tracers entrained within the valley's nocturnal drainage flows displayed well defined plumes that were not influenced significantly by the larger scale flows above this deep and narrow valley. Thus, the spatial distributions of the tracers were primarily governed by the structure of the drainage flows. None of the tracers released within the valley were detected in significant quantities on the adjoining mesas or within the adjacent valleys prior to sunrise. The process of ventilating the tracers out of the valley was initiated shortly after sunrise by the upslope flows generated along the valley sidewall exposed to the morning sun. The rate of ventilation was influenced by the solar intensity, the ambient meteorology, and the location of th...

Transferability of a Three-Dimensional Air Quality Model between Two Different Sites in Complex Terrain

Abstract The three-dimensional, diagnostic, particle-in-cell transport and diffusion model MATHEW/ADPIC is used to test its transferability from one site in complex terrain to another with different characteristics, under stable nighttime drainage flow conditions. The two sites were subject to extensive drainage flow tracer experiments under the multilaboratory Atmospheric Studies in Complex Terrain (ASCOT) program: the first being a valley in the Geysers geothermal region of northern California, and the second a canyon in western Colorado. The domain in each case is approximately 10 × 10 km. The 1980 Geysers model evaluation is only quoted. The 1984 Brush Creek model evaluation is described in detail. Results from comparing computed with measured concentrations from a variety of tracer releases indicate that 52% of the 4531 samples from five experiments in Brush Creek and 50% of the 831 samples from four experiments in the Geysers agreed within a factor of 5. When an angular 10° uncertainty, consistent w...

Numerical Simulation of Drainage Flow in Brush Creek, Colorado

One of the objectives of the Atmospheric Studies in Complex Terrain (ASCOT) program is to develop numerical models that can be used to aid in the understanding and prediction of flow patterns observed over complex terrain. As part of this program, we have developed a three-dimensional dynamic model that uses a simplified finite element scheme combined with a variation of the forward Euler time integration scheme to solve the Boussinesq equations of motion over irregular terrain. We have used the model to simulate the development of nocturnal down-valley circulation in the area where the ASCOT field observations were made at Brush Creek, Colorado. The model and the simulation results are described in this paper.

Development of a Nested Grid, Second Moment Turbulence Closure Model and Application to the 1982 ASCOT Brush Creek Data Simulation

Abstract An improved, second-moment turbulence-closure model and a random particle kernel diffusion model are described and tested with the 1982 ASCOT (Atmospheric Studies in Complex Terrain) data collected in Brush Creek, Colorado. Three improvements of the models are the nested grid capability, inclusion of terrain shadows and application of a kernel method for concentration estimation. The present models are unique in a sense that wind variances needed for the computation of transport and diffusion of airborne materials are obtained directly from the second-moment turbulence-closure model.

Spatial Interpolation of Meteorological Data in Complex Terrain Using Temporal Statistics

Diagnostic wind field numerical models have significant difficulty developing representative wind velocities in complex terrain. A large of this difficulty begins with the initial wind field interpolation. If this interpolated wind field does not closely represent the true winder mass-consistent adjustments cannot retrieve the correct atmospheric flow patterns. Presently, the initial interpolation in diagnostic models is almost exclusively done using a simple one-over-separation-squared (1/r2) interpolation algorithm. This algorithm uses the closest or larger number of measurements of the wind velocity closest to the interpolation location. In this paper, we explore different interpolation algorithms using not only the measurement field at the interpolation time, but also the statistical relationships between stations. These algorithms were tested with data from the 1980 Atmospheric Studies in Complex Terrain (ASCOT) sponsored by the Department of Energy. The results show that, while consistent (though in most cases marginal) improvement in interpolated wind speeds was obtained, little improvement was derived for interpolated wind direction.

Observations of complex-terrain flows using acoustic sounders: Experiments, topography, and winds

Acoustic sounders have now been used extensively in a series of noctural drainage flow experiments carried out by the U.S. Department of Energy's Atmospheric Studies in Complex Terrain (ASCOT) program. Doppler acoustic sounders, located in three different valleys during the sequence of experiments, reveal drainage-wind profiles that depend strongly on the ambient meteorological conditions and the elevation of each observing site relative to surrounding terrain. In elevated sites that drain easily, Doppler-sounder derived wind profiles show a simply-structured flow; in lower lying areas, subject to topographic constriction and cold-air pooling, and where Archimedean forces are comparable to those due to synoptic and mesoscale pressure gradients, the wind profiles show considerable vertical and temporal variation. In particular, in the Geysers area of northern California, the seabreeze and the depth of the Pacific Coast marine inversion affect not only the initiation of drainage winds but also their subsequent evolution.

Doppler Lidar Measurements of Winds in a Narrow Mountain Valley

Abstract During September and October of 1984, the Wave Propagation Laboratory of NOAA used its pulsed infrared Doppler lidar in support of the U.S. Department of Energy's Atmospheric Studies in Complex Terrain (ASCOT) program. The lidar measured winds channeled within a narrow mountain valley on seven experiment nights, between 2300 and 1100 MST. We were able to quantitatively define the structure of the nocturnal drainage winds, monitor their decay in the morning, and sense the formation of a thermally driven up-valley flow later in the day.

Transport and Diffusion in the Transition Layer and Planetary Boundary Layer for Drainage Flows in Anderson and Putah Creeks, California, during ASCOT 1980

Abstract Approximately 100 constant-volume, superpressured balloons (tetroons) were tracked in the Anderson and Putah Creek drainages of the Geysers geothermal area of northern California as part of the ASCOT (Atmospheric Studies in Complex Terrain) program. These tetroons were tracked by radar. Each tetroon was equipped with a transponder which provided a 100 MHz shift in frequency, providing a means of discriminating the tetroon-transponder broadcast signal from radar signals reflected from the hard terrain. Tetroons were used to provide direct measurements of trajectories of individual air parcels, to provide transport speeds of individual air parcels and to provide direct measurements of the turbulence associated with an individual parcel of air. In addition, clusters of tetroons were released, providing direct measurement of the very low frequency turbulence associated with air flow meander within the two valleys. All tetroon flights were in the transition layer between the katabatic drainage flow an...

Experimental and model transport and diffusion studies in complex terrain with emphasis on tracer studies

The U.S. Department of Energy (DOE) Atmospheric Studies in Complex Terrain (ASCOT) program began in the fall of 1978 as a multiple DOE and other Federal Laboratory program devoted to developing a better physical understanding of atmospheric boundary layer flows in areas of complex terrain. The first technical challenge undertaken by the program was an investigation of atmospheric boundary layer phenomena associated with the development, continuation and breakup of nocturnal drainage wind flows. This paper discusses the general objectives the program has addressed during the past several years and focuses on results from a major field experiment conducted in 1980 in The Geysers area of northern California. Specifically, results from measurements of simultaneous tracer releases are compared to calculations from a mass-consistent wind field model coupled to a particle-in-cell transport and diffusion model. Results of these comparisons show that model calculations agree with measurements within a factor of 5 approximately 50 percent of the time. Part of the difficulty faced by the models in these comparison studies is associated with large variabilities between measurements made by samplers located one or two δx apart when compared to the resolution of the models. Space and time averaging improves the comparisons considerably, although the design of the field experiment did not allow the determination of optimum spacial and temporal averages.

Field studies of transport and dispersion of atmospheric tracers in nocturnal drainage flows

A series of tracer experiments were carried out as part of the Atmospheric Studies in Complex Terrain (ASCOT) program to evaluate pollutant transport and dispersion characteristics of nocturnal drainage flows within a valley in northern California. The results indicate that the degree of interaction of the drainage flows with the larger scale regional flows are strongly dependent on how well the shallow drainage flows are shielded by the surrounding topography from the external environment. For the valley under study, the drainage flows from about mid-slope elevations and below were generally decoupled from the externally generated flows; as evidenced by the similarity of the surface tracer distributions produced during widely varying regional flow conditions. However, tracers released immediately above the drainage flows near the ridge top did reveal considerable mixing between the transition layer flows and the underlying surface drainage flows. Likewise, the transport and dispersion of the tracers at elevated heights within the valley basin were extremely dependent on the influences of the regional scale flows on the valley circulations. The dispersion rates associated with the transition layer flows were dependent on topographic constraints but were appreciably higher than those reported for homogeneous flat terrain situations.

Cooling tower plume rise analyses by airborne lidar

As part of the Atmospheric Studies in Complex Terrain (ASCOT) program, five cooling-tower plume experiments were conducted at The Geysers geothermal area in California during August 1981. The experiments were designed to investigate plume behavior during conditions of valley nocturnal drainage flow and daytime vertical mixing on a mountain ridge. The Airborne Lidar Plume and Haze Analyzer (ALPHA-1) system was used during the experiments to observe plume geometry and aerosol structure of the boundary layer. Subvisible plume rise derived from the backscatter signatures is related to visible plume-rise results recently published. This analysis indicates that the lidar-observed plumes are about 10 times higher than visually observed plumes. The lidar-observed plume rise seemed to correspond with heights of elevated aerosol layers that typically indicate presence of temperature inversions. Pictorial displays are presented which provide information on atmospheric behavior over mountainous terrain.

Energy and air quality

Many coal, oil shale, and geothermal energy sources are located in areas where atmospheric transport and dispersion processes are dominated by the complexity of the terrain. The U.S. Department of Energy (DOE), responsible for developing new energy technologies that meet air‐quality regulations, developed a program aimed specifically at Atmospheric Studies in Complex Terrain (ASCOT) in 1978. The program uses theoretical atmospheric physics research, mathematical models, field experiments, and physical models. The goal is to develop a modeling and measurement methodology to (1) improve fundamental knowledge of transport and dispersion processes in complex terrain and (2) build on this improvement to provide a methodology for performing air quality assessments. The ASCOT team, managed by Marvin Dickerson and Paul Gudiksen of Lawrence Livermore Laboratory, Livermore, Calif., is composed of scientists from DOE supported research laboratories and university programs.