Browsing by Subject "Atmospheric turbulence"
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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 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 Real-time solar imaging with a 19-segment active mirror system: a study of the standard atmospheric turbulence model(Texas Tech University, 1990-08) Acton, Daniel ScottA 57-actuator segmented active mirror system designed for real-time correction of phase distortions due to atmospheric turbulence is described. Solar images and wavefront tilt data obtained with the active mirror system during a recent observing run are discussed. The standard model for imaging through atmospheric turbulence is described. Based on this model, mathematical expressions for the atmospheric coherence diameter, the tilt-corrected wavefront phase structure function, and the corrected and uncorrected modulation transfer functions are developed. Analytical expressions as well as functional relationships obtained through computer modeling are presented. The data are analyzed to check for agreement with the theoretical predictions from the standard model. The analyses indicate that while the standard model may be adequate to describe atmospheric turbulence in some instances, it is often quite inadequate. The analyses also show that the degree of correction performed by the active mirror system was less than ideal.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 Surface observations of landfalling hurricane rainbands: case studies of Hurricane Bonnie (1998) and Hurricane Dennis (1999)(Texas Tech University, 2003-05) Skwira, Gary D.This study examines the rainband-scale fluctuations of the meteorological parameters for Hurricane Bonnie (1998) and Dennis (1999). Since reliable surface observations near the locations of landfalling hurricanes are quite rare due to power and/or instrumentation failure. Wind Engineering Mobile Instrumented Tower Experiment (WEMITE) data are exploited to provide a unique look into the structure of the captured storms. The WEMITE data consists of high-resolution meteorological data—including wind speed and direction, temperature, relative humidity, and pressure— gathered from within the planetary boundary layer of landfalling hurricanes along the United States coastline. WEMITE data, along with supplemental data gathered by the National Weather Service, buoys, Coastal Marine Automated Network (CMAN) stations, dropsondes and hurricane hunter observations, are assembled and analyzed through the use of Geographic Information Systems (GIS). Furthermore, nearby Next Generation Weather Surveillance Radar (NEXRAD WSR-88D) data are used to identify and examine rainbands found within the hurricanes of interest. The observed meteorological profiles are compiled and the results are compared to previous rainband studies. The observed meteorological data suggest equivalent potential temperature minima, decreasing hurricane-relative inflow, and large-scale convergence to be commonly associated with intensifying or mature landfalling hurricane rainbands. Additionally, the results suggest larger rainbands (l00's of km long) promote updrafts and reflectivity redevelopment to the inside of their axis. The shorter bands (100 km or less in length), conversely, tend to form from one or a number of cells that are elongated into a band by strong hurricane winds with regeneration upband.Item Wind profile: estimation of displacement height and aerodynamic roughness(Texas Tech University, 1986-05) Abtew, WossenuNot available