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Jerome D. Fast

Abstract

The nonhydrostatic version of the NCEP Meso-Eta Model is used to perform simulations that differ by only the vertical coordinate to determine the differences in forecasted valley circulations associated with the step-mountain and terrain-following vertical coordinates and whether one coordinate produces consistently superior forecasts at meso-γ and micro-α scales. A horizontal grid spacing of 850 m is used. The model forecasts are evaluated using data from the October 2000 Vertical Transport and Mixing (VTMX) field campaign in the Salt Lake valley. The forecasts of the diurnal evolution of the dominant circulations in the Salt Lake valley, including valley, slope, and canyon flows, and their modification by synoptic forcing during five intensive observation periods, were qualitatively similar to the measurements. Forecasts produced by the step-mountain and terrain-following vertical coordinates each have their own advantages and disadvantages and neither vertical coordinate outperformed the other overall. In general, the terrain-following coordinate simulations reproduced the observed surface wind directions over the valley sidewalls better, while the step-mountain coordinate simulations of nighttime near-surface temperatures and wind speeds were closer to the observations. Significant differences in wind speed and direction between the simulations were also produced in the middle valley atmosphere at night, with the terrain-following coordinate simulations somewhat better than the step-mountain coordinate simulations. Similar forecast errors produced by both simulations probably resulted from the physical parameterizations, rather than the choice of vertical coordinate.

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Shiyuan Zhong and Jerome Fast

Abstract

This study presents what is, to the authors' knowledge, the first intercomparison and evaluation of three state-of-the-art mesoscale numerical models, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MMS), the Regional Atmospheric Modeling System (RAMS), and the NCEP Meso-Eta, at horizontal resolution finer than 1 km. Simulations were carried out for both weak and strong synoptic forcing cases during the Vertical Transport and Mixing (VTMX) field campaign conducted in the Salt Lake valley in October of 2000. Both upper-air and surface observations at high spatial and temporal resolution were used to evaluate the simulations with a focus on boundary layer structures and thermally driven circulations that developed in the valley. Despite differences in the coordinate systems, numerical algorithms, and physical parameterizations used by the three models, the types of forecast errors were surprisingly similar. The common errors in predicted valley temperature structure include a cold bias extending from the surface to the top of the valley atmosphere, lower than observed mixed-layer depths when the observed mixed layers were relatively high, and much weaker nocturnal inversion strengths over the valley floor. Relatively large wind forecast errors existed at times in the midvalley atmosphere even in the case of strong synoptic winds. The development of valley, slope, and canyon flows and their diurnal reversals under weak synoptic forcing were captured better by RAMS and MM5 than by Meso-Eta. Meso-Eta consistently underpredicted the strengths of these terrain-induced circulations and the associated convergence and divergence over the valley floor. As operational mesoscale modeling moves toward subkilometer resolution in the near future, more detailed forecasts of the circulation patterns and boundary layer structure can be produced for local-scale applications. However, this study shows that relatively large forecast errors can still exist even with sufficiently fine spatial resolution, indicating that the future for accurate local forecasting still lies in improved model parameterization of longwave radiation and turbulent mixing.

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Jerome D. Fast and Michael D. McCorcle

Abstract

Observations have shown the existence of a diurnal oscillation of the wind profile in the springtime boundary layer of the central United States called the Great Plains low-level jet. This low-level jet is a mesoscale phenomenon that is not easily predicted by larger-scale numerical forecast models, which lack adequate resolution of the boundary layer. It has been postulated that this nocturnal jet can act as a triggering mechanism in the development of localized convective precipitation. If the mechanisms that cause this low-level jet are better understood and modeled, more accurate short-range local forecasts of these convective events could be made.

The dominant forcing mechanisms in the development of the Great Plains low-level jet are the coupled frictional and thermal oscillations in the planetary boundary layer. In this study, a two-dimensional planetary boundary-layer model linked with a soil hydrology system is used to determine the sensitivity of the low-level jet to perturbations in the thermal and frictional forces caused by various surface characteristics. This research provides a theoretical analysis of how the thermal and frictional forces are modified by secondary effects such as variation of slope, latitude, soil type, and soil moisture content and distribution.

Typical springtime synoptic conditions are used to initialize the forecast model. The magnitude and structure of the simulated low-level jet is very sensitive to small variations in surface slope and soil moisture content and distribution. The jet is affected by variations in latitude and soil type, but relatively large changes in these parameters are necessary to significantly alter the wind profile.

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Jerome D. Fast and Michael D. McCorcle

Abstract

Thermally induced circulations, similar to sea breezes, may be established in the presence of horizontal gradients in soil moisture, soil type, vegetation, or snow cover. The expense of extensive observational networks and the relatively small-scale circulations involved has made examining these circulations very difficult. Recent numerical studies have indicated that sharp gradients in soil or vegetation properties may induce mesoscale circulations in the absence of synoptic forcing.

The current study employed a three-dimensional, hydrostatic mesoscale model to evaluate the effects of horizontally heterogeneous soil moisture and soil type on the passage of a summer cold front in the central United States. Grid-scale condensation, precipitation, latent heat release, and cumulus conviction are not accounted for in this model; moisture was affected only by advection, diffusion, and evaporation. Numerical simulations demonstrated that evaporation of soil moisture significantly affected the boundary layer structure embedded in the baroclinic circulation. Although the position of the front was not altered, the thermal and momentum fields were effected enough to weaken the front near the surface. Evaporated soil moisture was advected ahead of the cold front, far from its source region. Moisture convergence was significantly enhanced in several locations, indicating that soil moisture may play an important role in modifying the spatial distribution and intensity of precipitation.

The impact of surface inhomogeneities in soil moisture and soil type on the atmosphere is expected to be highly dependent on the particular synoptic conditions.

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Jerome D. Fast and Lisa S. Darby

Abstract

A mesoscale model, a Lagrangian particle dispersion model, and extensive Doppler lidar wind measurements during the Vertical Transport and Mixing (VTMX) 2000 field campaign were used to examine converging flows over the Salt Lake valley in Utah and their effect on vertical mixing at night and during the morning transition period. The simulated wind components were transformed into radial velocities to make a direct comparison with about 1.3 million Doppler lidar data points and to evaluate critically the spatial variations in the simulated wind fields aloft. The mesoscale model captured reasonably well the general features of the observed circulations, including the daytime up-valley flow; the nighttime slope, canyon, and down-valley flows; and the convergence of the flows over the valley. When there were errors in the simulated wind fields, they were usually associated with the timing, structure, or strength of specific flows. The simulated flow reversal during the evening transition period produced ascending motions over much of the valley atmosphere in the absence of significant ambient winds. Valley-mean vertical velocities became nearly zero as down-valley flow developed, but vertical velocities between 5 and 15 cm s−1 occurred where downslope, canyon, and down-valley flows converged, and vertical velocities greater than 50 cm s−1 were produced by hydraulic jumps. A fraction of tracer released at the surface was transported up to the height of the surrounding mountains; however, higher concentrations were produced aloft for evenings characterized by well-developed drainage circulations. Simulations with and without vertical motions in the particle model produced large differences in the tracer concentrations at specific locations and times, but the amount of tracer moving out of the valley atmosphere differed by only 5% or less. Despite the stability, turbulence produced by vertical wind shears mixed particles several hundred meters above the surface stable layer for the particle model simulation without vertical motions.

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Jerome D. Fast and Warren E. Heilman

Abstract

A meteorological–chemical model with a 12-km horizontal grid spacing was used to simulate the evolution of ozone over the western Great Lakes region during a 30-day period in the summer of 1999. Lake temperatures in the model were based on analyses derived from daily satellite measurements. The model performance was evaluated using operational surface and upper-air meteorological measurements and surface chemical measurements. Reasonable agreement between the simulations and observations was obtained. The bias (predicted − observed) over the simulation period was only −1.3 ppb for the peak ozone mixing ratio during the day and 5.5 ppb for the minimum ozone mixing ratio at night. High ozone production rates were produced over the surface of the lakes as a result of stable atmospheric conditions that trapped ozone precursors within a shallow layer during the day. In one location, an increase of 200 ppb of ozone over a 9-h period was produced by chemical production that was offset by losses of 110 ppb through vertical mixing, horizontal transport, and deposition. The predicted ozone was also sensitive to lake temperatures. A simulation with climatological lake temperatures produced ozone mixing ratios over the lakes and around the lake shores that differed from the simulation with observed lake temperatures by as much as 50 ppb, while the differences over land were usually 10 ppb or less. Through a series of sensitivity studies that varied ozone precursor emissions, it was shown that a reduction of 50% in NOx or volatile organic compounds would lower the 60-ppb ozone exposure by up to 50 h month−1 in the remote forest regions over the northern Great Lakes. The implications of these results on future climate change and air quality in the region are discussed.

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Shiyuan Zhong, Jerome D. Fast, and Xindi Bian

Abstract

A detailed case study of one complete episode of a typical summertime Great Plains low-level jet (LLJ) using data collected by the NOAA wind profiler demonstration network is presented. The high temporal and spatial resolution of the data from the profiler network permits a much more detailed picture of the Great Plains LLJ than is possible from previous studies of this phenomenon. A three-dimensional mesoscale numerical model is also used to simulate the episode and to provide information on the physical mechanisms responsible for the initiation, evolution, maintenance, and decay of the LLJ. The position and width of the jet core, as well as the diurnal variation of wind speed and direction inside the jet core are well predicted by the model. The analysis and modeling suggest that the diurnal oscillation of horizontal pressure gradient over sloping terrain is secondary to the inertial oscillation mechanism resulting from the release of frictional constraint in the evening and throughout the night in driving this example of the summertime Great Plains LLJ. The meridional variation of the Coriolis parameter as air moves northward appears to enhance the strength of the jet. A larger amplitude of the diurnal oscillation of the jet speed is found to be associated with drier soil, while rising motion downstream of the jet core is stronger for wetter soil. This enhanced vertical motion appears to be associated with latent heat release due to precipitation. A horizontal variation of soil moisture content also appears to be important in reproducing the observed convergence and precipitation patterns in this case.

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Jerome D. Fast, Shiyuan Zhong, and C. David Whiteman

Abstract

A mesoscale model is used to simulate the nocturnal evolution of the wind and temperature fields within a small, elliptical basin located in western Colorado that has a drainage area of about 84 km2. The numerical results are compared to observed profiles of wind and potential temperature. The thermal forcing of the basin wind system and the sources of air that support the local circulations are determined. Individual terms of the basin atmospheric heat budget are also calculated from the model results.

The model is able to reproduce key features of the observed potential temperature profiles over the basin floor and winds exiting the basin through the narrow canyon that drains the basin. Complex circulations are produced within the basin atmosphere as a result of the convergence of drainage flows from the basin sidewalls. The strength of the sidewall drainage flow varies around the basin and is a function of the source area above the basin, the local topography, and the ambient winds. Flows on the basin floor are affected primarly by the drainage winds from the northern part of the basin. The near-surface sidewall drainage flows converge within the southern portion of the basin, producing a counterclockwise eddy during most of the evening. Evaluation of the individual terms of the atmospheric heat budget show that the forcing due to advection and turbulent diffusion is significantly larger above the sidewalls than over the basin floor; therefore, measurements made over the basin floor would not be representative of the basin as a whole. The cooling in the center of the basin results from the local radiative flux divergence and the advection of cold air from the sidewalls, and the cooling above the basin sidewalls is due primarily to turbulent sensible heat flux divergence. A high rate of atmospheric cooling occurs within the basin throughout the evening, although the strongest cooling occurs in the early evening hours. Sensitivity tests show that the thermal structure, circulations, and rate of cooling can be significantly affected by ambient wind direction and, to a lesser extent, vegetation coverage.

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Jerome D. Fast, B. Lance O'steen, and Robert P. Addis

Abstract

Atmospheric transport and diffusion models are an important part of emergency response systems for industrial facilities that have the potential to release significant quantities of toxic or radioactive material into the atmosphere. An advanced atmospheric transport and diffusion modeling system for emergency response and environmental applications, based upon a three-dimensional mesoscale model, has been developed for the U.S. Department of Energy's Savannah River Site so that complex, time-dependent flow fields not explicitly measured can be routinely simulated. To overcome some of the current computational demands of mesoscale models, two operational procedures for the advanced atmospheric transport and diffusion modeling system are described including 1) a semiprognostic calculation to produce high-resolution wind fields for local pollutant transport in the vicinity of the Savannah River Site and 2) a fully prognostic calculation to produce a regional wind field encompassing the southeastern United States for larger-scale pollutant problems. Local and regional observations and large-scale model output are used by the mesoscale model for the initial conditions, lateral boundary conditions, and four-dimensional data assimilation procedure. This paper describes the current status of the modeling system and presents two case studies demonstrating the capabilities of both modes of operation. While the results from the case studies shown in this paper are preliminary and certainly not definitive, they do suggest that the mesoscale model has the potential for improving the prognostic capabilities of atmospheric modeling for emergency response at the Savannah River Site. Long-term model evaluation will be required to determine under what conditions significant forecast errors exist.

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Jerome D. Fast, Joel C. Torcolini, and Randy Redman

Abstract

As part of an air-quality field campaign conducted in Phoenix, Arizona, during the summer of 2001, a network of temperature dataloggers and surface meteorological stations was deployed across the metropolitan area for a 61-day period. The majority of the dataloggers were deployed along two intersecting lines across the city to quantify characteristics of the urban heat island (UHI). To obtain pseudovertical temperature profiles, some of the instrumentation was also deployed along a mountain slope that rose to 480 m above the valley floor. The instrumentation along the mountain slope provided a reasonable approximation of the vertical temperature profile of the free atmosphere over the valley center during the night and a few hours after sunrise. Mean differences of 0.63 and 0.92 K and standard deviations of 1.33 and 1.45 K were obtained when compared with the in situ radiosonde and remote radio acoustic sounding system measurements, respectively. The vertical temperature gradients associated with temperature inversions within 200 m of the surface during the morning were also close to those obtained from the radiosondes. The average UHI during the measurement period was between 2.5° and 3.5°C; however, there was significant day-to-day variability, and it was as large as 10°C during one evening. The peak UHI usually occurred around midnight; however, a strong UHI was frequently observed 2–3 h after sunrise that coincided with the persistence of strong temperature inversions obtained from the radiosonde and the pseudovertical temperature profiles. The nocturnal horizontal temperature gradient was somewhat different than that reported for other large cities, and the UHI did not decrease with increasing wind speeds until the wind speeds exceeded 7 m s−1.

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