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F. L. Ludwig

Abstract

The dependence of plume growth rate on plume size is discussed. It is shown that the growth rates estimated by techniques used in some widely distributed puff models can be in error by as much as two orders of magnitude, although the errors are more commonly on the order of 10%. The concept of virtual travel distance is shown to provide a more realistic representation of the physical processes involved in plume growth after a change in atmospheric stability.

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F. L. Ludwig and Walter F. Dabberdt

Abstract

Two atmospheric stability classification schemes have been applied using National Weather Service data from Lambert Field in St. Louis. The results are compared with wind direction fluctuation and insolation measurements made in downtown St. Louis. One of the schemes considered was proposed by Turner in 1964. The other scheme is similar but uses opaque cloud, rather than cloud types and heights, for categorizing daytime solar radiation. The second scheme gives results that are in better agreement with stability classification based on fluctuations in the horizontal wind direction.

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F. L. Ludwig and Robert L. Street

Abstract

Multiresolution feature analysis (MFA), as originally proposed for estimating the fractal dimension of two-dimensional scalar fields, is described. MFA applies specified correlation filters to a data field (such as a gray-scale image) at different resolutions and examines the scaling of the intensities of the spatial peaks in the filter outputs at the different scales. These scaling properties can be related to different types of fractal dimension. One attractive aspect of MFA is that it gives the analyst flexibility to choose physically significant features for filtering. This paper describes the original MFA technique and then extends the technique from two-dimensional scalar fields to two-dimensional vector fields. In principle, the three-dimensional vector applications allow the estimation of the fractal dimension of the support for turbulent fluctuations but there are limitations on the applicability of the methodology, which are discussed. MFA requires definition of physically significant features, which take the form of small-scale patterns of motion when the technique is applied to three-dimensional flows. The authors describe statistical techniques (similar to principal component analysis) that can be applied to small subvolumes of data to identify those motion patterns that explain the most variance. These small-scale patterns of spatial variability then serve as the features in the version of MFA described here. Possible applications, modifications, and extensions of the methodology that has been developed are given.

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L. P. Reiche and F. L. Ludwig

An electronic instrumentation scheme for AC-type anemometers is described which permits the ten-minute-average wind vector to be determined with higher precision and reliability than by other means known to the authors. The average vector is conveniently given as two components in digital form.

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C. E. Junge, Elmer Robinson, and F. L. Ludwig

Abstract

This paper describes a program of aerosol measurements made in the fall of 1965 at Cape Blanco and Crater Lake, Ore. Aerosol size distributions were determined with a Royco light-scattering type particle counter and with condensation nuclei counters. Size distributions at Crater Lake (2200 m altitude) were found to be well approximated by power law distributions with exponents between 3 and 4. The Cape Blanco data agree well with earlier measurements of sea-spray, aerosol size distributions. A Goetz aerosol centrifuge was used, in conjunction with filter sampling, to determine the size distribution of the chloride- and sulfur-compound fractions of the aerosol.

At Crater Lake, sulfur-to-chloride ratios of about 8 were observed and considerable concentrations of other materials could be inferred. The data indicate that significant amounts of sea-spray aerosol did not penetrate upward to altitudes >2000 m. The aerosols identified with these 2000 m tropospheric levels are believed to be aged aerosols from continental sources. High-altitude tropospheric aerosols were also observed at Crater Lake during periods of subsidence; their concentrations were lower than those found under other synoptic weather conditions. The power law representations for subsidence aerosols had lower exponential values than those for the lower altitude tropospheric aerosols.

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F. L. Ludwig, John Horel, and C. David Whiteman

Abstract

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 along-valley 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.

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F. L. Ludwig, J. M. Livingston, and R. M. Endlich

Abstract

Observed winds and temperature profiles can be used to generate three-dimensional, mass-conserving wind fields that reflect topographical influences. The concept of critical dividing streamlines is used to define quasi-horizontal, flow-confining two-dimensional surfaces. Adjustment toward two-dimensional nondivergence on those surfaces forces flow around obstacles under stable conditions when some flow surfaces intersect higher terrain features. Unlike most mass-conserving wind models, the approach described here includes objective evaluation of the effects of atmospheric stability. Efficiency is achieved by casting the three-dimensional problem as several two-dimensional problems and by using an iterative scheme to adjust toward nondivergence. A 20 × 20 × 5 gridpoint analysis requires approximately 2 min on an IBM-AT personal computer.

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R. M. Endlich, F. L. Ludwig, C. M. Bhumralkar, and M. A. Estoque

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R. M. Endlich, F. L. Ludwig, C. M. Bhumralkar, and M. A. Estoque

Abstract

Terrain features and variations in the depth of the atmospheric boundary layer produce local variations in wind, and these variations are not depicted well by standard weather reports. We have developed a diagnostic model that computes local winds for use in estimating the wind energy available at any potential site for a wind turbine. The method uses the terrain heights of an area surrounding the site and a series of wind and pressure reports from the nearest four or five National Weather Service stations. An initial estimate of the winds in the atmospheric boundary layer is made and these winds are then adjusted to satisfy the continuity equation. In this manner the flow is made to reflect the influences of the terrain and the shape of the boundary-layer top.

We applied the method to seven sites in the United States for 1977. For four of the sites, we determined the sensitivity of the model to the values assigned to several parameters used in the computation. Suitable values of the parameters were selected by comparing the corresponding simulations to wind measurements that were made at the sites by the Department of Energy through its Pacific Northwest Laboratory. For the other three sites, simulations were made without adjusting the model. Seasonal and annual average wind speeds and frequency distributions were generated from the simulated 3 h u and v components. The simulated annual average speeds for the seven sites differ from measured values by amounts ranging from 0.1 to −1.8 m s−1, with an average absolute deviation of 0.7 m s−1. This level of accuracy is sufficient for determining whether particular sites merit the installation of wind-measuring equipment. Validation that wind energy is adequate to justify wind-turbine installation must be provided by site wind measurements for a period of one year or more.

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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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