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Francis L. Ludwig and Douglas Sinton

Abstract

An automated system has collected meteorological data hourly in the San Francisco Bay Area since October 1995. Data from sites operated by the National Weather Service, Federal Aviation Administration, U.S. Navy, U.S. Air Force, the San Francisco Bay Area Air Quality Management District, and others have been integrated into a common database and archived by the U.S. Geological Survey. Data for 0300 and 1500 UTC during 1996 were taken from that archive and used to evaluate the performance of the Winds on Critical Streamline Surfaces objective analysis code in this area of complex terrain, where altitudes range from 0 to over 1000 m above mean sea level. The comparisons of observed and modeled winds are for a longer time period and a wider variety of meteorological conditions than are available from limited-duration field studies. The authors chose 4-, 7-, and 11-station “networks” for analysis; there were 461 h when data were available from all the sites in the 4-station network, 439 h for the 7 stations, and 343 for the 11 stations. The essential features of the model are described, and examples are given to show how changing some of the features affects the comparisons between modeled and observed winds. The effects of increasing the number of stations used for the analyses also are discussed. The model evaluation techniques described here can be applied to wind analyses from any source, but application certainly is more feasible with simple diagnostic models that can be run repeatedly so that sensitivities and performance changes can be examined. The performances of more complex models also can be compared with these results to determine if their greater complexity produces significant improvements.

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Ying Chen, Francis L. Ludwig, and Robert L. Street

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This paper describes observed and simulated interactions among atmospheric forcing, cold-pool development, and complex mountain terrain at the south end of the Salt Lake valley, near the Jordan Narrows and the Traverse Range. The Advanced Regional Prediction System (ARPS), a three-dimensional, nonhydrostatic compressible new-generation large-eddy simulation code in generalized terrain-following coordinates with advanced model parameterizations, was used. Past studies showed that a finer resolution produces more accurate simulations, and so this study used six one-way nested grids to resolve the complex topography. Horizontal grid spacing ranged from 20 km (initialized by Eta 40-km operational analyses) to 250 m; the finest grid had 200 vertically stretched levels between 5 m and 20 km above the surface. Two intense operating periods with weak synoptic forcing, stable stratification, and pronounced nighttime drainage were selected for simulation from the October 2000 Vertical Transport and Mixing (VTMX) experiment. Qualitative agreement between simulations and observations at four stations was good. Usually, the quantitative agreement was also good. Finer horizontal and vertical resolution improved agreement, capturing daytime and nighttime temperature structures, including inversion-layer strength. The simulations showed a complex flow near the Jordan Narrows, with hydraulic jumps and internal waves initiated by the Traverse Range to either side.

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Cristina L. Archer, Mark Z. Jacobson, and Francis L. Ludwig

Abstract

A shallow cyclonic circulation that occurs in the summertime over the Monterey Bay (California) is investigated. Since it is often centered offshore from the city of Santa Cruz and has never been studied in detail before, it is named the Santa Cruz eddy (SCE) in this study. Its horizontal size is 10–40 km, and its vertical extent is 100–500 m. The SCE is important for local weather because it causes surface winds along the Santa Cruz coast to blow from the east instead of from the northwest, the latter being the climatological summer pattern for this area. As a consequence of the eddy, cool and moist air is advected from the south and southeast into the Santa Cruz area, bringing both relief from the heat and fog to the city.

The SCE is unique in its high frequency of occurrence. Most vortices along the western American coast form only during unusual weather events, whereas the SCE forms 78%–79% of the days during the summer. The SCE frequency was determined after analyzing two years of data with empirical orthogonal functions (EOFs) from a limited observational network and satellite imagery. An explanation of the formation mechanism of the SCE will be provided in Part II of this study.

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Francis L. Ludwig, Fotini Katopodes Chow, and Robert L. Street

Abstract

This paper demonstrates the importance of high-quality subfilter-scale turbulence models in large-eddy simulations by evaluating the resolved-scale flow features that result from various closure models. The Advanced Regional Prediction System (ARPS) model was used to simulate neutral flow over a 1.2-km square, flat, rough surface with seven subfilter turbulence models [Smagorinsky, turbulent kinetic energy (TKE)-1.5, and five dynamic reconstruction combinations]. These turbulence models were previously compared with similarity theory. Here, the differences are evaluated using mean velocity statistics and the spatial structure of the flow field. Streamwise velocity averages generally differ among models by less than 0.5 m s−1, but those differences are often significant at a 95% confidence level. Flow features vary considerably among models. As measured by spatial correlation, resolved flow features grow larger and less elongated with height for a given model and resolution. The largest differences are between dynamic models that allow energy backscatter from small to large scales and the simple eddy-viscosity closures. At low altitudes, the linear extent of Smagorinsky and TKE-1.5 structures exceeds those of dynamic models, but the relationship reverses at higher altitudes. Ejection, sweep, and upward momentum flux features differ among models and from observed neutral atmospheric flows, especially for Smagorinsky and TKE-1.5 coarse-grid simulations. Near-surface isopleths separating upward fluxes from downward are shortest for the Smagorinsky and TKE-1.5 coarse-grid simulations, indicating less convoluted turbulent interfaces; at higher altitudes they are longest. Large-eddy simulation (LES) is a powerful simulation tool, but choices of grid resolution and subfilter model can affect results significantly. Physically realistic dynamic mixed models, such as those presented here, are essential when using LES to study atmospheric processes such as transport and dispersion—in particular at coarse resolutions.

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Francis L. Ludwig, Douglas K. Miller, and Shawn G. Gallaher

Abstract

The results from a hybrid approach that combines the forecasts of a mesoscale model with a diagnostic wind model to produce high-resolution wind forecasts in complex coastal orography are evaluated. The simple diagnostic wind model [Winds on Critical Streamline Surfaces (WOCSS)] was driven with forecasts (on a 9-km grid) from the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) to obtain detailed near-surface wind forecasts with 3-km horizontal spacing. Forecasts were produced by this hybrid model for four cold-season cases—two frontal and two nonfrontal—over the central California coastal region. They were compared with 3-km forecasts from the innermost COAMPS full physics model nest and with winds observed at 35 surface sites scattered throughout the study domain. The evaluation sought to determine the conditions for which the hybrid approach performs well and those for which it does not. The performance (in terms of bias and root-mean-square error) was evaluated 1) when there were and were not fronts and 2) for the early (6–18 h) and late (21–36 h) periods of the mesoscale model forecasts. The geographic distribution of performance was also examined to see if forecasts were affected by mountains and oceans. The hybrid approach performed best during stable, nonfrontal conditions. There were no clearly defined geographic effects on hybrid performance. The computation requirements of the full physics mesoscale model nested down to 3 km are substantially greater than those of the hybrid approach. Suggestions are given for further improvements.

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Nelson L. Seaman, Francis L. Ludwig, Evelyn G. Donall, Thomas T. Warner, and Chandrakant M. Bhumralkar

Abstract

The Pennsylvania State University (PSU)/National Center for Atmospheric Research (NCAR) mesoscale model was modified and used to simulate the evolution of meteorological conditions in the vicinity of St. Louis, Missouri, from near sunrise to noon on 25 July 1975. Observations obtained during the METROMEX (Metropolitan Meteorological Experiment) and RAPS (Regional Air Pollution Study) field programs were available for comparison with modeled conditions. The PSU/NCAR model used a nested grid with two-way interaction between the coarse mesh (7.5 km) and the fine mesh (2.5 km), where the fine domain covered the city and its immediate suburban and rural surroundings. Realistic three-dimensionally variable initial and lateral boundary conditions were obtained from the observations so that the numerical experiments could be used for quantitative evaluation of certain urban effects. After simulation of the actual conditions (control experiment), the importance of a number of processes on the urban planetary boundary layer (PBL) structure were investigated. The PBL effects were isolated by using realistic surface parameters as well as those based on the preurban conditions and an expanded urban environment. Sensitivities to surface evaporative fluxes, radiative processes, and different surface roughness associated with urbanization were examined. The control simulations of the temperature, boundary layer depth, specific humidity and wind fields exhibited essentially the same behavior as observed in the urban PBL throughout the morning forecast period. Unlike many other documented cases that displayed strong urban-induced low-level convergence, the confluence on this morning was relatively weak, with the center of the heat island displaced (in both the simulation and the observations) downwind (south) of the city. A relative minimum in windspeed was associated with the center of the displaced heat island. The sensitivity experiments clearly demonstrated and maintenance of the urban PBL perturbation.

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Plamen B. Savov, Toni S. Skakalova, Ivan N. Kolev, and Francis L. Ludwig

Abstract

Lidar experiments were conducted in the mountainous region of Bulgaria to determine the spatial and temporal distribution of major aerosol sources and the zones of aerosol accumulation. When these lidar data are combined with conventional meteorological observations of temperature and wind profiles they provide a clear picture of the physical processes that lead to the accumulation and subsequent dispersion of aerosols and other pollutants in the valleys. The observations showed that the valley gradually fills with cool air after sunset, producing an inversion that traps aerosols and other pollutants emitted at night. After sunrise a convective boundary layer develops in the valley; its evolution is delayed by the confining valley walls. Insolation causes airflow up the slope, producing divergence near the surface and subsidence of the inversion core. The one winter experiment conducted suggests that weaker winter insolation delays the process until much later than in the summer, sometimes to the extent that the inversion persists throughout the day, or even for several days. The findings described here are in good agreement, qualitatively and quantitatively, with the model described by Whiteman and McKee. The results also demonstrate the power of combining conventional meteorological observations with lidar techniques for determining the nature of boundary layer processes in a valley.

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Alison F. C. Bridger, Allen J. Becker, Francis L. Ludwig, and Roy M. Endlich

Abstract

Applications of the Winds on Critical Streamline Surfaces (WOCSS) model in the San Francisco Bay Area are described. Three case studies, chosen to represent important classes of airflow in the region, were conducted. Two cases involved a prevailing northeasterly flow with or without an inversion, and the third case involved northeasterly flow at the time of the Oakland hills firestorm of 20 October 1991. The dependence of model results (surface winds) on input winds and on the specification of inversion topography is discussed. Dependable results are produced with relatively few well-placed surface observations and with a single sounding. The results suggest that the model is quite suitable for routine, real-time analyses and other practical applications.

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