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Steven R. Hanna

Abstract

It is suggested that helical roll vortices in the atmosphere are responsible for the formation of the longitudinal sand dunes that cover over half of the area of the large deserts of the world. The dunes are aligned in the direction of the prevailing wind and are spaced ∼2 km apart. Observations in the atmosphere and in the laboratory, and hydrodynamic stability theory, indicate that dominant forms of motion in the boundary layer of the atmosphere are counter-rotating helical roll vortices aligned along the wind and having diameters approximately equal to the thickness of the boundary layer. The necessary conditions for the formation of these roll vortices are fulfilled over large deserts and their spacings agree with the observed spacings of the dunes.

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Steven R. Hanna

Abstract

There is much evidence in the literature for the presence of mesoscale lateral meanders in the stable nighttime boundary layer. These meanders result in relatively high lateral turbulence intensities and diffusion rates when averaged over an hour. Anemometer data from 17 overnight experiments at Cinder Cone Butte in Idaho are analyzed to show that the dominant period of the mesoscale meanders is about two hours. Lidar cross-sections of tracer plumes from these same experiments show that the hourly average σ y is often dominated by meandering. Since meandering is not always observed for given meteorological conditions, it is suggested that nighttime diffusion cannot be accurately predicted without using onsite observations of wind fluctuations. In case no turbulence data are available, an empirical formula is suggested that predicts the hourly average lateral turbulence intensity as a function of wind speed and hour-to-hour variation in wind direction.

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Steven R. Hanna

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Lagrangian (neutral balloon) and Eulerian (tower and aircraft) turbulence observations were made in the daytime mixed layer near Boulder, Colorado. Average sampling time was ∼25 min. Average Lagrangian time scale is ∼70 s and average ratio of Lagrangian to Eulerian time scales (β = TL/TE) is about 1.7. The ratio β is inversely proportional to turbulence intensity i. These data support the formula β = 0.7/i. Lagrangian time scale for the vertical component of turbulence at heights above ∼100 m is given by the formula TL = 0.17zi/σ μ where zi is mixing depth. This formula is valid for the horizontal components of turbulence at all heights in the mixed layer. Lagrangian spectra in the inertial subrange are best represented by the formula Fr(n) = 0.2ε n −2.

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Steven R. Hanna

Abstract

Extensive meteorological and air chemistry measurements were obtained along the Ventura and Santa Barbara county coastal areas in California during four 2–3 day case studies conducted during the September–October 1985 South-Central Coast Cooperative Aerometric Monitoring Program (SCCCAMP 1985). An overview of the characteristics of ozone episodes during these four case studies is given, showing that the episodes are associated with warm, high pressure systems with light winds. In the absence of easterly winds, the observed ozone in the region is primarily due to local sources. At other times, easterly wind components transport ozone and its precursors from large source regions to the east (i.e., Los Angeles County). This transport sometimes occurs in inland valleys at elevations up to 600 m, and sometimes occurs over the ocean near the surface. Local sea breezes, mesoscale eddies, and terrain-generated winds often cause complex flow patterns and recirculation of pollutants.

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Steven R. Hanna and Ruixin Yang

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Mesoscale meteorological models are being used to provide inputs of winds, vertical temperature and stability structure, mixing depths, and other parameters to atmospheric transport and dispersion models. An evaluation methodology is suggested and tested with simulations available from four mesoscale meteorological models (Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, Regional Atmospheric Modeling System, Coupled Ocean–Atmosphere Mesoscale Prediction System, and Operational Multiscale Environmental Model with Grid Adaptivity). These models have been applied by others to time periods of several days in three areas of the United States (Northeast, Lake Michigan area, and central California) and in Iraq. The authors' analysis indicates that the typical root-mean-square error (rmse) of hourly averaged surface wind speed is found to be about 2–3 m s−1 for a wide range of wind speeds for the models and for the geographic regions studied. The rmse of surface wind direction is about 50° for wind speeds of about 3 or 4 m s−1. It is suggested that these uncertainties in wind speeds and directions are primarily due to random turbulent processes that cannot be simulated by the models and to subgrid variations in terrain and land use, and therefore it is unlikely that the errors can be reduced much further. Model simulations of daytime mixing depths are shown to be often within 20% of observations. However, the models tend to predict weaker inversions than are observed in interfacial layers capping the mixing depth. The models also underestimate the vertical temperature gradients in the lowest 100 m during the nighttime, which implies that the simulated boundary layer stability is not as great as that observed, suggesting that the rate of vertical dispersion may be overestimated. The models would be able to simulate better the structure of shallow inversions if their vertical grid sizes were smaller.

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Steven R. Hanna and Pasquale Franzese

Abstract

Observations of alongwind dispersion of clouds were collected from 11 field sites and from one wind tunnel and were used to test simple similarity relations. Because most of the observations consist of concentration time series from fixed monitors, the basic observed variable is σ t, the standard deviation of the concentration time series. The observed σ ts range from 0.3 to 9000 s. The concentration time series observations also allow the travel time t from source to receptor to be estimated, from which the cloud advective speed u e can be determined. Observed ts range from 2 to 40 000 s, and observed u es range from 0.5 to 16 m s−1. The alongwind dispersion coefficient σ x is then calculated from u e σ t. The resulting σ t and σ x observations support the similarity relations σ t = 0.1t and σ x = 2ut, where u∗ is friction velocity. About 50% of the observations are within a factor of 2 of these similarity relations.

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Steven R. Hanna and Joseph C. Chang

Abstract

The field program phase of the Lake Michigan Ozone Study (LMOS) took place during the summer of 1991. Observed ozone concentrations and weather variables have been analyzed for the Lake Michigan region and the eastern United States for four 1991 LMOS ozone episodes covering 21 days. It is found that all LMOS episodes are associated with large polluted regions with dimensions of 1000–2000 km, located on the western side of high pressure systems centered over the eastern United States. Consequently, the air coming into the LMOS region contains significant amounts of ozone, haze, and other pollutants advected along trajectories with anticyclonic curvature originating in upwind source regions from St. Louis, through the Ohio River valley, and into the northeast megalopolis. The local sources in the Gary-Chicago-Milwaukee region then add to this already polluted air mass, where concentrations are influenced by the strong stability of the boundary layer over Lake Michigan and the associated lake-land-breeze circulations. However, the magnitudes of ozone concentrations in the LMOS region are often quickly and significantly reduced by rain and by fronts, which are present in some portion of the domain during nearly all of the days that were studied. The portion of the urban ozone plume that is not influenced by rain and fronts is observed to be advected into rural areas at downwind distances of 100–200 km or more from Chicago, and often has concentrations higher than those in Chicago. Whether maximum concentrations are located on the western or the eastern shoreline of Lake Michigan depends on whether winds have an easterly or westerly component.

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Steven R. Hanna, Martin Pike, and Keith Seitter

Abstract

Time-lapse photography was used to estimate the speed of vortices in condensed plumes from a bank of mechanical draft cooling towers and a hyperbolic natural draft cooling tower. At a distance of about 30 m downwind from the towers, the median tangential velocity of the vortices at the edge of the plume is about 2 m s−1 in the downward direction, for ambient wind speeds of 7 to 13 m s−1. The standard deviation of turbulent fluctuations of tangential speeds of the vortices is about 1.8 m s−1 for both types of towers.

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Steven R. Hanna and Franklin A. Gifford

Large (10 000 to 50 000 MW) power parks are being studied as one means of satisfying the nation's demand for energy. The dissipation of waste energy from these installations may result in significant meteorological effects. It is shown that the rate of atmospheric dissipation of the waste energy from these power parks is approximately equal to the atmospheric dissipation of energy by geophysical phenomena such as thunderstorms, volcanoes, and large bushfires. Cumulus clouds and whirlwinds often result from these energy releases. There is a possibility that natural vorticity will be concentrated by large power parks. A theory of multiple plume rise is used to estimate the enhancement of plume rise from multiple cooling towers.

Calculations of plume rise, ground level fog intensity, and drift deposition due to emissions from cooling towers at a hypothetical 40 000 MW nuclear power park are made. The plume rise from 50 towers is estimated to be more than 110% of that from a single tower if the tower spacing is less than about 300 m. At locations within 100 km of the cooling towers, excess fog will occur about one or two percent of the time. The vapor plume will be appreciably longer than those from present installations; for instance it should be clearly visible from earth satellites most of the time. Since there are no power parks of this magnitude yet in existence, there are no measurements to test these calculations. The conclusions are highly tentative and indicate that much more research is required on this subject.

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Steven R. Hanna and Robert J. Paine

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The Hybrid Plume Dispersion Model (HPDM) was developed for application to tall stack plumes dispersing over nearly flat terrain. Emphasis is on convective and high-wind conditions. The meteorological component is based on observational and modeling studies of the planetary boundary layer. The dispersion estimates for the convective boundary layer (CBL) were developed from laboratory experiments and field studies and incorporate convective scaling, i.e., the convective velocity scale, w *, and the CBL height, h, which are the relevant velocity and length scales of the turbulence. The model has a separate component to handle the dispersion of highly buoyant plumes that remain near the top of the CBL and resist downward mixing. For convective conditions, the vertical concentration distribution is non-Gaussian, but for neutral and stable conditions it is assumed to be Gaussian. The HPDM performance is assessed with extensive ground-level concentration measurements around the Kincaid, Illinois, and Bull Run, Tennessee, power plants. It was also tested with limited data during high-wind conditions at five other power plants. The model is found to be an improvement over the standard regulatory model, MPTER, during light-wind convective conditions and high-wind neutral conditions.

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