Francis L. Ludwig

Numerical Studies of Urban Planetary Boundary-Layer Structure under Realistic Synoptic Conditions

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 planet...

Using EOF Analysis to Identify Important Surface Wind Patterns in Mountain Valleys

Empirical orthogonal functions (EOFs) have been determined for three wind datasets from stations in valleys south of the Great Salt Lake in Utah. Two of the datasets were for summer months, with individual days selected from the MesoWest archive to represent conditions conducive to well-developed thermally driven flows. The remaining dataset was for the month of October 2000 and was derived from a combination of MesoWest data and data collected during intensive observation periods of the Vertical Transport and Mixing Experiment (VTMX) conducted in the Great Salt Lake area. This experiment investigated stable atmospheric conditions in the complex urban terrain around Salt Lake City, Utah. In all three datasets, the primary EOFs represented flows that were directed predominantly along valley axes and were caused by channeled or thermally driven flow. Diurnal variations in EOF intensity showed that thermal effects were the most common causal mechanism. The alongvalley EOFs accounted for 43%‐58% of the variance in the wind component datasets (8 or 10 stations each). The second EOFs accounted for 13%‐18% of the variance. In the summer datasets, the second EOF appeared to represent day‐night transition periods; there was evidence of both side canyon flows and day‐night transitional effects in the October dataset. The EOF approach has promise for classifying wind patterns and selecting representative cases for simulation or for further detailed analysis.

Stably Stratified Flows near a Notched Transverse Ridge across the Salt Lake Valley

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.

Lidar Investigation of the Temporal and Spatial Distribution of Atmospheric Aerosols in Mountain Valleys

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.

Evaluating an Objective Wind Analysis Technique with a Long Record of Routinely Collected Data

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 s...

Effect of Turbulence Models and Spatial Resolution on Resolved Velocity Structure and Momentum Fluxes in Large-Eddy Simulations of Neutral Boundary Layer Flow

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 difference...

Evaluation of the WOCSS Wind Analysis Scheme for the San Francisco Bay Area

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.

Evaluating a hybrid prognostic-diagnostic model that improves wind forecast resolution in complex coastal topography

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...

Analysis of small-scale patterns of atmospheric motion in a sheared, convective boundary layer

Multiresolution feature analysis has been applied to data from NOAAs Phoenix II dual Doppler radar observations of a convective boundary layer near Boulder, Colorado, during June 1984 and to large eddy simulation (LES) results (using the regional atmospheric modeling system (RAMS) developed at Colorado State University) corresponding to a subset of that data. The data and LES results provide winds on a three-dimensional grid with spacing of ∼200 m to a depth of about 2 km over a 9 × 9 km square for the observations. LES results were for an elongated (in the east-west direction) volume. The prevailing circulation maintained a strong shear (synoptic westerlies aloft above upslope easterlies at the surface) despite strong afternoon heating. The analysis defined preferred motion patterns (for both the observed and the simulated data) over 3 × 3 × 3 grid points using empirical orthogonal functions. In the case of the observations the patterns strengthened or weakened the shear locally for the most stable cases, but other patterns became relatively more important with increasing convection. Among these were a vortical pattern tilted in the shear direction. Differences in the peak intensity statistics for two different smoothings were used to estimate their support dimension from the observed data during moderately unstable conditions; most fell between 2.2 and 2.6. When the LES results corresponding to a moderately unstable observed atmosphere were analyzed, they showed that the preferred motion patterns were more like the observed stable case than the unstable cases, and the support dimension estimates were all greater than 2.6, indicating less intermittency than was observed.