Browsing by Subject "Boundary layer (Meteorology)"
Now showing 1 - 15 of 15
Results Per Page
Sort Options
Item A study of the radiative and satellite-observed properties of the dew point front(Texas Tech University, 1984-08) Mickelson, Mark AlanThe dryline is a narrow zone or discontinuity across which a sharp moisture gradient exists. The nighttime progression of this discontinuity is called the dew point front, and appears to reveal itself on satellite infrared photographs as a distinct boundary. The purpose of this study is to establish that the satellite-observed feature is indeed the dew point front. Two methods have been used in proving this assumption. The first was to utilize meteorological surface network data to analyze parameters which frequently indicate the presence of the dew point front. In this study, fields of temperature, dew point temperature, mixing ratio, adiabatic equivalent potential temperature, and streamlines were chosen. The position of the dew point front from each of these analyses was then compared to the satellite-inferred position of the dew point front. These comparisons showed that fields of mixing ratio, dew point temperature, and adiabatic equivalent potential temperature align to within 40-50 km of the satellite-inferred boundary, allowing a large measure of confidence to be placed in using the satellite photograph to interpret the position of the dew point front. The second method consisted of adapting a radiative model, based on standard assumptions and theories of radiative transfer, utilizing atmospheric sounding data to calculate the longwave radiative flux leaving the top of the atmosphere on either side of the boundary. If the difference in calculated fluxes across this boundary were to match the satellite-observed differences in fluxes, there would be no doubt that the satellite boundary was indeed the dew point front. Due to the simplicity of the model and its inability to treat cloud layers, the flux differences did not match. However, statistical analysis showed that the satellite imagery enhanced the ground temperature differences across the dew point front, amplifying the boundary on the infrared photographs. Thus, a second factor exists to confirm the fact that the visual satellite boundary is indeed the dew point front.Item Boundary layer characteristics just above sub-urban roughness(Texas Tech University, 2004-05) Dutta, Satya RanianWith regard to the reliable design of distribution lines (less than 60 ft) and other structures that stand just above the suburban roughness, it is very important to accurately study the complex turbulence structures in the region of suburban roughness sublayer. In this study, it has been found that the profiles of mean wind velocity, turbulence intensity and integral length scale parameters (longitudinal, lateral, and vertical) are not exactly the same as those used in ASCE 7, which considers the ideal case of turbulence above open terrain or well above the roughness. To accomplish this, wind tunnel experiments were carried out in Texas Tech University's wind tunnel facility and then the data was analyzed to determine the variation of turbulence and gust size just above the roughness. Very close simulations of open and suburban terrain were accomplished by respectively laying carpet and carpet plus 12 house models (in 4 rows) in the upstream section of the wind tunnel. To determine the integral length scales, cross correlation and auto correlation methods were employed with the help of Math CAD software. The profiles of different parameters were determined with respect to height and surface roughness. Also, to determine the effect of an abrupt change of terrain, a single row of 3 house models was placed in front of the line of measurement and the corresponding test data analyzed. It has been primarily concluded that the suburban roughness sublayer turbulence affects the gust effect factors which in turn affect the design wind load on distribution lines and the other structures that stand just above the suburban roughness.Item Bursting events in the stable atmospheric boundary layer(2008-05) Phillipson, Julie Ann; Basu, Sukanta; Gilliam, Xiaoning; Leary, ColleenThough recent experiments and field projects regarding the characteristics of the boundary layer have been conducted, there is still an overall lack of understanding about the mechanics of the stable boundary layer, whether it is transitionally stable, such as the nocturnal stable boundary layer in mid-latitudes, or persistently stable, such as the boundary layer in Polar Regions. ISCAT-00 is one such field project conducted in Antarctica, where an instrumented tower was used to record wind speed and temperature information, from which heat and momentum flux transfers can be discerned. Understanding of boundary layer processes is especially important for the stable layer since despite its stable nature, sporadic bursts of turbulence have been observed to occur, indicated by data collected from the ISCAT-00 campaign. These bursts, though short-lived, are responsible for most of the heat and momentum transfer that occurs within the otherwise stable layer. Since these bursts of turbulence disrupt the stable layer, they can not only pose problems for air quality forecasts, but they can also inhibit the performance of wind turbines. Transient loading from these bursting events as rotor blades pass through patches of organized, coherent turbulence can shorten the life span of a wind turbine by 5 to 10 years. However, despite the noted occurrence of turbulence bursting in the stable layer there is still very little known about its origins, causes, or basic properties. In order to explore the characteristics of turbulent bursting events in the stable layer, two methodologies are applied to analyze data obtained from the ISCAT00 campaign. The first methodology applied to discern the occurrence of turbulent bursting is one created by Nakamura & Mahrt (2005). The second methodology is a newly developed adaptive threshold methodology, which is much more robust, and removes much of the subjectivity of other turbulence bursting identification methods. The adaptive technique allows for the reduction of subjectivity in the data analysis phase, and is therefore felt to be more accurate. Also, spectral characteristics of polar turbulence are explored, and are found to have similar properties to spectra observed in mid-latitudes. This research focuses primarily on different methods, both old and newly developed that can help to further understanding about turbulence in the stable boundary layer and the corresponding heat and moisture flux properties.Item Characterization of hurricane gust factors using observed and analytical data(Texas Tech University, 2009-05) Edwards, Rebecca Paulsen; Schroeder, John L.; Gilliam, Kathleen; Smith, Douglas A.The nature of turbulence in the hurricane boundary layer has been the subject of much discussion. Two questions in particular continue to be the source for debate and ongoing research. The first question is whether or not hurricane GFs exhibit the same behavior as GFs from winds generated by extratropical systems (thunderstorms excluded). The second question is whether the structure of the wind, and the resulting gust factors, change at high wind speeds. This study seeks to address those two questions using a variety of data sources and analysis techniques. Observational data were collected from both landfalling tropical cyclones and synoptically generated extratropical wind. Analytical data at a variety of wind speeds were created using an inverse fast Fourier Transform of the universal spectrum for wind in the perturbed terrain. Gust factors and other parameters were computed for both types of data and the results assimilated in a data base. Analysis of these data yielded interesting results. A strong dependence on surface roughness was noted for gust factors from both observed and analytical data. However, once efforts were made to control for this dependency by stratifying the data into roughness regimes using the roughness length, slight differences between the tropical and extratropical gust factor data remained. Analysis of the artificial data, suggest spectral differences between the tropical and extratropical regimes due to the presence of additional low-frequency energy in the tropical regime. A slight decrease of the gust factor with increasing wind speed was noted in the high-speed analytical data. A similar decrease was suggested in the tropical data. It was concluded that the low-frequency spectral differences between the two regimes have less of an effect on the resulting gust factors as the wind speed increases, resulting in better agreement between the two distributions.Item Dynamic instability in the planetary boundary layer over the arid land(Texas Tech University, 1988-12) McCauley, Steven DClassic baroclinic theory is one of the most significant findings of modern meteorology and has often been used to investigate the nature of wave-like disturbances introduced onto a mean flow field. The theory provides an elegant explanation for the propagation speed, structure, growth and decay of the predominant synoptic-scale (approximately 4000 km in zonal wavelength) weather systems observed in the middle latitudes. However, not all major disturbances in the atmosphere possess a synoptic-scale wavelength. Subsynoptic-scale (1000 - 2000 km in zonal wavelength) disturbances are often found in the lower troposphere over arid land during the summer months. These disturbances appear to originate in a deep and neutrally stratified planetary boundary layer (PBL) of relatively mild horizontal thermal gradient. The deep neutral layer is caused and maintained by intense surface heating. When these shortwave disturbances encounter potentially unstable air, they can trigger intense convective events. Unfortunately, classic baroclinic theory cannot be used to describe the behavior of these systems. This study is a mathematical analysis of the governing equations of motion in order to account for the genesis and maintenance of these unique PBL disturbances. The goal of the analysis is to provide a better understanding of the dynamics of these shortwave disturbances in order that further improvement in the numerical weather prediction of these systems and their consequences can be achieved.Item Estimation of roughness lengths through gust factor analysis(Texas Tech University, 1999-08) Conder, Mark R.The aerodynamic roughness length is a parameter that quantifies the degree of friction that terrain elements have on the wind near the earth's surface. It is commonly used in the development of vertical profiles of the wind speed. However, uncertainty exists in the precision of current roughness length estimation schemes. A formula has been developed by Wieringa (1973) that calculates the roughness length from gust factors (ratio of the peak wind gust to the mean wind speed over a period). This method has an advantage in that it relies on readily available local wind data such as analog record charts found at the NWS. This study employed the Wieringa formula to evaluate its applicability and ease of use with a number of different types of wind records. First of all, the formula was applied to data collected at the Texas Tech Wind Engineering Research Field Laboratory (WERFL), where wind speed records are archived from five different vertical levels. Results indicated that both the gust factors and the roughness lengths obtained were higher than those from previous studies. Secondly, because both meteorologists and engineers are interested in vertical wind profiles in high winds, an attempt was made to obtain wind records from several hurricane passages. Four reliable hurricane wind records from anemometers located in the vicinity were acquired: Hurricane Bob (1991), Hurricane Iniki (1992), and two from Hurricane Bonnie (1998). The Wieringa formula requires specifications of the anemometry and since each record was produced by a different anemometer/recorder system, they provided a thorough assessment of the formula. Calculated roughness lengths were plotted and compared against the known topography of the site. The results showed a large variability according to the upstream fetch of the wind in the record. Another conclusion was that the gustiness of the wind during a hurricane is highly dependent upon the terrain and it is difficult to ascertain how turbulence caused by convection affects the gust factor and thus the roughness length.Item Fine-scale observations of surface boundaries utilizing mobile mesonets(Texas Tech University, 2001-05) Pietrycha, Albert EdwardOver the past decade, technological advances have permitted observations of the atmosphere in fresh and innovative ways. Observational platforms have been created and improved allowing atmospheric sampling, both remotely and via in situ data collection. Information gained from new platforms has aided the meteorological community to better understand an abundance of atmospheric thermodynamic, kinematic, and dynamic issues. Recently, a growing emphasis has been placed on investigation of the fine-scale structure of meteorological phenomena such as hurricanes, severe thunderstorms, drylines, and other boundaries. To provide the required resolution these investigations demand, a mobile observing platform, dubbed the mobile mesonet, was designed specifically for the sampling of small-scale features. The mobile platform was originally employed in the study of tornadoes and severe local storms. However, applications since have expanded to include a profusion of atmospheric surface features. This thesis presents a case study, exceptional in that it incorporates mobile mesonet observations. Included are the investigations of a low-level baroclinic zone interaction with Hurricane Floyd in 1999. The presentation of data and analyses also serve to illustrate the unparalleled capabilities and versatility of the mobile mesonet in resolving surface features in scales heretofore indistmguishable in atmospheric research.Item Hurricane Bonnie wind flow characteristics(Texas Tech University, 1999-12) Schroeder, John L.Whether convective influences result in localized areas of increased wind speeds and different turbulent structure than expected in a normal high wind situation has been debated for years. The question has been especially active concerning landfalling hurricanes. Given a lack of high-resolution wind speed data from within hurricanes necessary to resolve the debated questions, a field experiment was designed and conducted by Texas Tech University to acquire the necessary data. The Wind Engineering Mobile Instrumented Tower Experiment (WEMITE), the first successful field experiment to place a reinforced, self-powered, instrumented tower directly in the path of several hurricanes, successfully gathered high-resolution wind speed data from within Hurricane Bonnie as it made landfall near Cape Fear, North Carolina, on 26 August 1998, at 5:00 PM. These data are used to inspect the variations in turbulent characteristics of the wind during the passage of the storm. Specifically, turbulence intensities, integral scales, gust factors, and spectrograms are evaluated with respect to the surrounding meteorological events, such as the passage of rainbands. Comparisons are drawn between turbulence intensity and integral scale values present in Hurricane Bonnie, and those employed in the determination of the gust effect factor in ASCE 7-98. Wavelet analysis was also used to examine the incoherent model assumption employed in wind engineering.Item Saharan air layer interaction with Hurricane Claudette (2003)(Texas Tech University, 2004-08) Rothman, Gabriel SamuelIt is well understood that the Saharan air layer (SAL) influences the variability and the intensity of tropical cyclones and African easterly waves in the Atlantic Ocean Basin. The radiative and conductive properties of the mineral dust are thought to contribute to warming within the SAL, creating an anomalous baroclinic zone in the Tropical North Atlantic. Environmental baroclinic instability is a mechanism by which available potential energy is converted to eddy kinetic energy, leading to wave growth. However, this same baroclinic mechanism, along with the dry properties of the SAL could also lead to asymmetries in tropical cyclones, limiting tropical cyclone intensity. Claudette and its formative wave which occurred during the 2003 Atlantic Basin hurricane season propagated along the southern boundary of a broad but moderate SAL outbreak throughout its evolution. First, it was found that the presence of Saharan dust was well correlated to heating in the environment. Thus, the wave growth potential was diagnosed using the Chamey-Stem condition for combined barotropic-baroclinic instability. It was found that the Chamey-Stem condition was satisfied for the formative tropical wave throughout much of its evolution, corresponding to the mostly progressive wave growth which occurred over time. The lifting potential due to the SAL boundaries and the mid-level easterly jet were then investigated, and the current research suggests that there is a dual-celled set of circulations, forced by the SAL boundaries but modified by the mid-level easterly jet. The SAL continued to remain intact in the storm environment almost throughout Claudette's entire evolution, after tropical cyclogenesis and through its entire track into the Western Gulf of Mexico. Rapid intensification in the last 15 hours prior to landfall was concurrent with the breakdown of Saharan air intrusion that had been persistent in the storm environment. Although the presence of dry air may have limited the intensity of Claudette before the rapid intensification period, external factors from the SAL, such as oceanic hurricane heat potential, may have also played a role in Claudette's intensity fluctuations.Item Surface layer characteristics of thunderstorm outflow(Texas Tech University, 1997-12) Weinbeck, Stephen W.The objective of this study is to define the turbulent characteristics of thunderstorm outflow winds in the surface layer. Thunderstorms are one of the most common types of severe weather, and the climatological record for many stations shows many extreme wind events can be identified as having been generated by thunderstorms. However, until recently it has proved difficult to separate which high wind events are caused by thunderstorm outflows, and which are generated by typical extratropical low-pressure systems. The advent of the Doppler weather radar and the deployment of a national network of these radars across the continental United States provides a powerful new tool for the classification and analysis of thunderstorm outflow events. Studies have shown that in many locations the strongest winds on record are generated by thunderstorms (Twisdale and Vickery, 1992, 1995). In many cases, these straight-line winds can cause damage similar to that caused by tornadoes. Besides causing structural damage near the surface, the lifting of less dense ambient air by thunderstorm winds can be the focusing and/or riggering mechanism for further thunderstorm development to occur. It is the modification of the environment surrounding the storm, and the localized nature of the outflow and cloud system that has made the investigation and forecasting of thunderstorms difficult. It is possible to be very near a severe thunderstorm and yet not feel the effects of the rain, hail, or outflow winds. In many cases the thunderstorms are not captured by the surface observation network that the National Weather Service (NWS) has set up to observe weather systems that occur on much larger scale.Item Understanding the strengths and weaknesses of a new-generation numerical weather prediction model for application to short-term wind energy prediction(2012-12) Fowler, Padriac; Ancell, Brian A.; Schroeder, John L.; Swift, Andrew H. P.; McCaa, JimWind power is a growing economy and science. It has far reaching consequences in all aspects of society and if goals of energy sustainability and security are to be met, wind power will become crucial. Numerous studies, including the U.S. Department of Energy’s report “20% by 2030”, have already shown the incredible growth of wind power in the past 5 years. A compelling problem with this growth is that in spite of all the wind power currently online in the United States, according to the “20% by 2030” report, we have only utilized about 0.3% of the total available land based wind power, and are only 11% of the way to the 2030 goal of 300GW of installed wind. Wind power is entirely at the mercy of the wind, and in essence, the entire atmosphere. These are very dynamic systems and because of this, the energy distribution and market system of the United States are already feeling the strain of integrating wind as a significant resource. One of the main ways in which wind integration is achieved is by producing wind power forecasts. To date, several works have concluded that wind speed and wind power forecasting have economic benefits for wind farms and are necessary for the best integration into the electric grid. The work of this thesis is simulation conducted with the Weather Research and Forecasting model. The model is run as an ensemble of simulations varying each only by atmospheric boundary parameterizations schemes. Three case studies significant to wind power are assessed and discussed in terms of overall weather prediction, atmospheric boundary layer processes, and their significance to wind energy prediction.Item Vortex shedding from square plates near a ground plane: an experimental study(Texas Tech University, 1979-12) Matty, Rosemary RicohermosoVortex shedding frequencies were obtained experimentally for square plates near a ground plane in the Texas Tech wind tunnel. These frequencies, in the form of Strouhal numbers, were obtained as a function of Reynolds number, distance above a ground plane, and elevation and azimuth angle. Experiments involving various combinations of these parameters were conducted. From these experiments an equation describing the relationship between these parameters was obtained.Item Wind power systems in the stable nocturnal boundary layer(Texas Tech University, 2007-08) Walter, Kevin RobertHourly-averaged tall-tower data from a 200m tower located near Lubbock, Texas are used to examine static atmospheric stability as a governor of speed and direction shear in the atmospheric boundary layer. Meteorological forcing mechanisms for such shears include the thermal wind, inertial oscillation, Ekman spiral, and others. The inertial oscillation is highlighted as an atmospheric mechanism capable of generating discernable diurnal variations in average speed shear data in regions void of low-level jet association. Theoretical aerodynamic treatment shows the case of direction shear to differ from the case of turbine operation in yawed flow, and has therefore not been studied in wind power systems. Numerical simulations of power production in steady non-turbulent flow fields containing speed and direction shear show instantaneous power gain as great as 0.5% and depletion as great as 6% relative to a no shear baseline. Coupled with joint-probability distributions of speed and direction shear measured at Lubbock, instantaneous losses as great as 3% and gains as great as 0.5% are expected, while the average power change relative to the zero shear case is estimated to be -0.5%. Over the 20 year lifetime of a 100 MW wind power plant this finding translates to a $2.1 million loss in project revenue. Observational evidence shows the correlation coefficient between the average diurnal variation in static stability and power law shear exponent is 1.00. The correlation coefficient between the average diurnal variation in static stability and direction shear magnitude is found to be 0.93. The influence of static stability on speed and direction shear is hypothesized to be globally applicable. Observations from a second data platform in northwest Indiana support the magnitudes of direction shear found at Lubbock, and further suggest that the presence of direction shear is a more general result. Large magnitudes of wind direction shear are found to occur concurrently with large magnitudes of power law shear exponent at hub-height wind speeds greater than 8 m/s, making them present in a critical operating region where wind turbine control transitions from speed control to power regulation.Item Wind profile: estimation of displacement height and aerodynamic roughness(Texas Tech University, 1986-05) Abtew, WossenuNot availableItem Wind tunnel simulation of an atmospheric boundary layer(Texas Tech University, 2001-12) Burton, William VThis thesis examines the simulation of a rural and suburban area using the Texas Tech wind tunnel. The objective was to simulate a rural and suburban type terrain by matching key parameters to a reasonable unifying scale factor. Passive devices such as spires, fence barriers, and roughness elements were implemented in the tunnel for the simulations. ASCE 7-98 standards were used for similarity criteria between the full-scale ABL flow characteristics and the simulations. Comparisons of the velocity profiles, turbulence intensities, and power spectrum were performed. The rural exposure was simulated with the installation of triangular-shaped spires, two fence barriers, and carpet as the roughness fetch. For the suburban exposure simulation, the installation of the spires, fence barriers, and 50 mm square roughness blocks were used as the roughness fetch. Various scale factors were investigated until a reasonable match for all the key parameters were obtained. A reasonable compromise was found at a model scale of 1:350 for the rural simulation and a 1:300 for the suburban simulation. Adequate simulations were performed at these scale factors with the exception of insufficient turbulence intensity in the lower boundary layer.