• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1729
  • 162
  • 160
  • 160
  • 160
  • 160
  • 160
  • 157
  • 107
  • 61
  • 59
  • 52
  • 23
  • 12
  • 8
  • Tagged with
  • 3320
  • 1156
  • 971
  • 814
  • 522
  • 487
  • 229
  • 223
  • 202
  • 198
  • 191
  • 178
  • 176
  • 168
  • 162
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

An investigation of semi-diurnal fluctuations of winds at an altitude of ten kilometers.

Downey, Richard S. January 1961 (has links)
Approved for public release; distribution is unlimited / The purpose of this study is to make an analysis of a possible solar semi-diurnal variation in the winds at ten kilometers from the regular upper wind observations published in the Northern Hemisphere Data Tabulations. / http://www.archive.org/details/investigationofs00down / Commander, United States Navy
2

Dynamics of the Wind Field Expansion Associated with Extratropically Transitioning Tropical Cyclones

Unknown Date (has links)
Extratropical transition, or ET, can be characterized by the transformation of an initially symmetric, warm-core tropical cyclone into an initially cold-core, asymmetric extratropical cyclone. As a consequence of undergoing transition, changes in the synoptic and dynamic characteristics of the cyclone are realized. Of particular note is the wind field evolution, one of the aspects of ET that has seen little research into its causes. Previous informal theories toward understanding the wind field evolution based upon key meteorological conservation principles do not accurately account for its observed evolution, while formal studies into other aspects of the ET process (e.g. Ritchie et al. 2001, Jones et al. 2003) have only mentioned its existence or its resultant impacts. This study attempts to bridge this gap by analyzing the physical and dynamical mechanisms involved with both the expansion of the wind field and outward movement of the radial wind maximum during the transition process. One ET case, North Atlantic Tropical Cyclone Bonnie (1998), is modeled using the Pennsylvania State University/NCAR Mesoscale Model version 5 (MM5; Dudhia 1993) at 12km horizontal resolution. The evolution of the cyclone within the model output is found to be an accurate measure of reality when compared to the observed track and dynamical evolution of the cyclone. Analysis of the model output shows that the expansion of the wind field is brought about by the net import of absolute angular momentum from a midlatitude trough of low pressure along descending isentropic trajectories in the western semicircle of the cyclone. Export of absolute angular momentum in the outward branch of the secondary circulation in the eastern semicircle of the cyclone partially negates but does not balance the import to the west; thus a net import of momentum into the cyclone is seen. Redistribution of momentum within the cyclone is accomplished through vertical pressure torques. The overall evolution is found to be consistent with that for a developing extratropical cyclone as shown by Johnson and Downey (1976). Net cooling (warming) inside (outside) of the radial wind maximum is shown to lead to the outward movement of this feature via a hydrostatic response in the radial height gradient, a response opposite to that seen with eyewall contraction (Shapiro and Willoughby 1982) yet consistent with the transition into a cold-core vortex. The observed results are used to formulate a conceptual model for the evolution of the wind field during ET. Implications toward the wind field evolution with other post-ET structural evolutions, such as warm seclusion cyclones and those that remain cold core yet strengthen (e.g. Hart et al. 2006), are drawn in conjunction with the observed results. Related concepts of vertical wind shear and cyclone size are discussed as natural outgrowths of the wind field expansion process. / A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Master of Science. / Degree Awarded: Summer Semester, 2006. / Date of Defense: June 27, 2006. / Cyclones, Hurricanes, Tropical Storms, Extratropical Transition, Wind Field, Expansion / Includes bibliographical references. / Robert Hart, Professor Directing Thesis; Philip Cunningham, Committee Member; T. N. Krishnamurti, Committee Member.
3

The Thermodynamic Evolution of Recurving Tropical Cyclone Bonnie (1998)

Unknown Date (has links)
One of the defining characteristics of the extratropical transition of tropical cyclones is the transition of the warm core thermal structure associated with the tropical cyclone into an initially cold core thermal structure associated with the extratropical cyclone. Despite this being a defining characteristic of the extratropical transition process, the literature expresses no consensus agreement upon or a quantification and physical description of the factors that explicitly cause this transition to occur. Understanding this evolution is important in order to better forecast and describe the evolution of physical features within the cyclone such as its four-dimensional wind field structure and to begin to quantify the contributors to the poleward transport of heat energy associated with the transitioning cyclone and its impacts upon hemisphere weather patterns and model predictability. This work employs a suite of high resolution numerical simulations in order to quantify and physically describe the evolution of the thermodynamic structure associated with a typical extratropical transition case, North Atlantic Tropical Cyclone Bonnie of 1998. Thermodynamic budgets native to the numerical model's primitive equation set and physical parameterizations are computed during the transition phase of the cyclone within a four-dimensional analysis framework. The observed warm-to-cold thermal profile evolution is found to arise out of an imbalance between dynamical cooling and parameterized warming contributions. This dynamical cooling, as influenced by horizontal advection, vertical advection and adiabatic cooling, and total divergence, is of greater magnitude than warming associated with latent heat release due to condensation and deposition processes within the transitioning cyclone's delta rain region. While the net thermodynamic evolution is found to be relatively resolution-insensitive, specific details of the thermodynamic balance are found to vary depending upon the horizontal resolution of the given numerical simulation. The thermodynamic evolution is ultimately shown to be a natural outgrowth of the factors that influence extratropical transition as a whole and is found to closely resemble the mature and occluding stages of purely cold-core extratropical cyclone development. / A Dissertation submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Degree Awarded: Fall Semester, 2009. / Date of Defense: July 28, 2009. / Extratropical Transition, Extratropical Cyclones, Cold Fronts, Numerical Modeling, Thermodynamics, Tropical Storm, Hurricanes, Tropical Cyclone / Includes bibliographical references. / Robert E. Hart, Professor Directing Dissertation; James B. Elsner, Outside Committee Member; T. N. Krishnamurti, Committee Member; Paul Reasor, Committee Member; Paul Ruscher, Committee Member.
4

Microwave Radiative Transfer Modeling of Ice in the Atmosphere: A Critical Examination of Cloud Ice Utilizing Remote Sensing

Unknown Date (has links)
Tropospheric cloud ice has a significant impact on the earth's radiative balance and climate, and to help improve the ability to forecast short-term through climatological-scale weather, the importance of quantification of these ice particles is not to be underestimated. To that end, the study presented here describes an attempt to accomplish large spatial-scale integrated ice water quantity (known as ice water path) retrieval via remote sensing in the microwave band (80-300GHz), where effects from ice crystals become detectable via scattering of terrestrial radiation. At the heart of this study is the use of a radiative transfer model in conjunction with data from surface-based instrumentation to simulate atmospheric brightness temperatures at microwave frequencies, and to compare the simulated results to observational data from the Advanced Microwave Sounding Unit - B instrument on the NOAA-15 polar orbiting platform. However, this cannot be done without first discussing the nature of ice crystals in the atmosphere and the scattering modes that result from their interaction with energy, and the implementation of approximations thereof for model usage. Case studies are performed using the model to establish the sensitivity and behavior of the model under differing conditions, and these are compared to real-world data. Subsequently ice water path retrieval from satellite data utilizing Bayesian theory is attempted, with somewhat limited success, and the results hereof are discussed. Finally, known error sources are examined with possibilities for improvement, and ideas for future work in the field presented. / A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Master of Science. / Degree Awarded: Spring Semester, 2006. / Date of Defense: March 20, 2006. / Crystal, Ice, Atmosphere, Microwave, Radiative Transfer, IWP, Radar, Satellite / Includes bibliographical references. / Guosheng Liu, Professor Directing Thesis; Paul Ruscher, Committee Member; KwangYul Kim, Committee Member.
5

Modeling Streamflow Using Gauge-Only versus Multi-Sensor Rainfall

Unknown Date (has links)
This study evaluates the impacts of two types of rainfall input on simulated streamflow using a specialized, fully-distributed hydrologic model—the Watershed Assessment Model (WAM). We compare gauge-only Thiessen polygon input data with the gridded 4 × 4 km Florida State University (FSU) version of the National Weather Service (NWS) Multi-sensor Precipitation Estimator (MPE) scheme. Streamflow results are compared to observed amounts over a six year period (2000-2005) at two U.S. Geological Survey (USGS) stream gauge sites in the greater Florida Suwannee River basin. One catchment has an area of 1505 km2, while the smaller catchment is 500 km2. Previous comparisons have been made between the two different precipitation data types using mean areal precipitation calculations over several Florida basins. This study of streamflow expands on those findings. Results show significant differences in simulated streamflow when the higher-resolution FSU MPE rainfall data are input to WAM. However, the FSU MPE dataset does not always provide better results with this model configuration. The improvements in WAM simulated streamflow depend on a combination of factors, including the desired type of comparison with observed amounts (volume or correlation), rainfall pattern characteristics, and individual event scenarios. The accumulations of FSU MPE WAM streamflow generally are found to be more accurate than those from Thiessen polygons. During drought periods, MPE-derived streamflow provided more accurate accumulations, but coefficients of determination were not always improved. During years with more average rainfall events, FSU MPE produced greater underestimates of accumulation amounts, and thus a better approximation by the Thiessen polygon input. Seasonal results emphasized the weaknesses of each data source. Rain gauges usually are not able to capture the small scale spatial variability of summer rainfall events. And, radar-derived precipitation generally is underestimated during relatively low top stratiform winter events. When simulating streamflow with a hydrologic model using rain gauge input, it is apparent that gauge locations are very important. Generally speaking, increasing the spatial density of gauges will produce a better representation of rainfall. Our small basin was found to be prone to significant underestimates of accumulations and lower coefficients of determination regardless of the rainfall input. However, statistical differences between our larger and smaller basins are not as dramatic with the FSU MPE data. Current results are based on the WAM model as configured for this study. Results from other models and/or other configurations may be different. Although there appear to be errors in both WAM's ability to utilize the rainfall data properly and in the rainfall data measurements themselves, the results highlight areas where both can be improved. / A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Master of Science. / Degree Awarded: Spring Semester, 2008. / Date of Defense: March 18, 2008. / Multi-sensor, Gauge, Rainfall, Precipitation, Radar, Streamflow / Includes bibliographical references. / Henry E. Fuelberg, Professor Directing Thesis; Paul H. Ruscher, Committee Member; Guosheng Liu, Committee Member.
6

Climatological Characteristics of the Jet Streams over West Africa

Unknown Date (has links)
This paper examines the climatology of the major jet steams over West Africa. Three prominent jets occur at varying heights in the atmosphere, and while each jet is zonal in its flow, their sizes, magnitudes, and directions vary greatly. The Tropical Easterly Jet is shown to be the strongest and most consistent in its location at approximately 200 to 150 hPa. As its name implies, this jet stream consists of easterly flow and has been the topic of many studies over the Tibetan Plateau and Indian Ocean due to its relationship with the Indian Monsoon. On a smaller scale, the African Easterly Jet is prominent over West Africa at approximately 700 to 600 hPa. Although it is associated with the African Monsoon, its strength is related to the temperature contrast between the dry desert to its north, and the cool, moist south-westerlies to the south. The third jet stream is the only one that is westerly in direction. It is the least studied of the three jets. The Low Level Westerlies are located between 1000 and 850 hPa. Although they exhibit a smaller velocity, they are believed to exert a significant influence on the precipitation pattern over West Africa. The goal of this project is to determine the climatological characteristics of the three jet streams, especially during the summer months of June, July, August, and September. Along with building a database to analyze the climatological trends of the jets, their interrelationships are also studied. The speed of the Low Level Westerlies is shown to have a significant correlation with the speed of the Tropical Easterly Jet. And, although the African Easterly Jet occurs in the center of the atmospheric column between the other two jets, it does not have a significant relationship to either of the jets located above and below. The conclusions of this paper naturally lend themselves to further research to help explain not only the reason why the Tropical Easterly Jet and the Low Level Westerlies are related, but also the influence that these systems have on the local environment. In addition, future research should determine the larger scale implications of each jet's location in relation to the other jet streams / A Thesis submitted to the Department of Meteorology in partial fulfillment of the Requirements for the degree of Master of Science. / Degree Awarded: Spring Semester, 2007. / Date of Defense: December 15, 2006. / Jet Stream, Africa, Tropical Easterly Jet, African Easterly Jet / Includes bibliographical references. / Sharon E. Nicholson, Professor Directing Thesis; Henry E. Fuelberg, Committee Member; Ming Cai, Committee Member.
7

Development of the Finite-Volume Dynamical Core on the Cubed-Sphere

Unknown Date (has links)
The finite-volume dynamical core has been developed for quasi-uniform cubed-sphere grids within a flexible modeling framework for direct implementation as a modular component within the global modeling efforts at NASA, GFDL-NOAA, NCAR, DOE and other interested institutions. The shallow water equations serve as a dynamical framework for testing the implementation and the variety of quasi-orthogonal cubed-sphere grids ranging from conformal mappings to those numerically generated via elliptic solvers. The cubed-sphere finite-volume dynamical core has been parallelized with a 2-dimensional X-Y domain decomposition to achieve optimal scalability to 100,000s of processors on today's high-end computing platforms at horizontal resolutions of 0.25-degrees and finer. The cubed-sphere fvcore is designed to serve as a framework for hydrostatic and non-hydrostatic global simulations at climate (4- to 1-deg) and weather (25- to 5-km) resolutions, pushing the scale of global atmospheric modeling from the climate/synoptic scale to the meso- and cloud-resolving scale. / A Dissertation submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Degree Awarded: Summer Semester, 2007. / Date of Defense: May 17, 2007. / Cubed-Sphere, Shallow Water, Advection, Dynamical Core, Finite-Volume / Includes bibliographical references. / James J. O'Brien, Professor Directing Dissertation; Shian-Jiann Lin, Outside Committee Member; Richard Rood, Outside Committee Member; T. N. Krishnamurti, Committee Member; I. Michael Navon, Committee Member; Xiaolei Zou, Committee Member.
8

Examining Interannual Variability of the Short Rains for Different Categories of Wet and Dry Years

Unknown Date (has links)
East African "shorts rains" variability has long been a subject of interest both for its complexity and its sometimes dire human impacts. Four large-scale factors are thought to influence the shorts rains: Niño 3.4, the IOZM, and the surface and upper-level zonal winds over the central Indian Ocean. Recent research has shown that these established teleconnections are not as robust as previously thought. Particularly, the relationships between the short rains season and these factors have broken down since 1982, with several seasons having opposite conditions than what was expected. Furthermore, the debate over which large-scale factors have the most effect on the short rains season is on-going. In this study, the relationship between the interannual variability of the short rains and known large-scale forcing factors is examined with the goal of answering three questions: 1) Are the factors associated with dry and wet conditions different before versus after 1982? 2) Why did dry or wet conditions fail to occur when predicted in recent years? 3) Do dry seasons with different combinations of large-scale factors produce different local patterns in atmospheric circulation? To investigate these questions, different October-November seasons were designated into categories. Analysis was done on several diagnostic variables for each category of rainfall seasons. Contrary to what was expected, zonal wind field results showed few differences between pre- and post-1982, giving little answers as to why the relationships between the short rains and these factors has changed since 1982. Omega results highlight influence of a narrow area of upper-level sinking and rising motion over and just off the coast of East Africa. This study suggests that this feature, while indeed connected to the four established links, could shed new insight on the short rains interannual variability if evaluated as an individual factor. Results also reaffirmed prior research that shows wet and dry conditions have different sensitivities to large-scale factors. Additionally, it was found that different combinations of factors, which all produced dry conditions, had different local atmospheric patterns. Some of the greatest contrasts seen in local patterns were between factor combinations that include the IOZM compared to combinations without it. This suggests that different large-scale factors could counteract each other. / A Thesis submitted to the Department Of Earth, Ocean and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science. / Fall Semester 2015. / August 19, 2015. / east africa, interannual, rainfall, short rains, variability / Includes bibliographical references. / Sharon E. Nicholson, Professor Directing Thesis; Mark A. Bourassa, Committee Member; Vasubandhu Misra, Committee Member.
9

Algorithms for Advanced Tropical Cyclone Visualization

Unknown Date (has links)
With the events of recent years - especially in the Atlantic tropical basin, the need for additional forecast tools to aid in tropical cyclone (TC) prediction and understanding is apparent. A suite of algorithms to be used for 3-D TC visualization are presented. Utilizing the assets of the Amira visualization software, we present methods which constitute a real-time visualization routine of TC genesis, mesoscale, and TC centered features derived from the The Florida State University (FSU) adaptation of the Pennsylvania State University (PSU) and National Center for Atmospheric Research (NCAR) Fifth Generation Mesoscale Model (MM5). However, the algorithms we develop are generic in that they can be applied to any gridded output from a forecast model. A method for the removal TC vorticity fields allowing an approximation of the environmental steering levels is also developed. The development and use of an adaptive wavenumber refinement filter (AWRF) has proven to outperform various alternative methods for TC vortex removal and thus the preservation of the environmental flow. Case study inter-comparisons are performed and illustrate that the AWRF provides the smallest mean track errors in future forecast position compared to those determined by the model's use of minimum sea level pressure (SLP) and vorticity maxima algorithm for TC center location. Discussions are also provided suggesting areas of needed improvement within the algorithm as illustrated by the case studies presented. / A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Masters of Science. / Degree Awarded: Fall Semester, 2006. / Date of Defense: September 21, 2006. / Vortex Removal, Tropical Cyclone Visualization, Amira, Environmental Steering Flow / Includes bibliographical references. / Xiaolei Zou, Professor Directing Thesis; Gordon Erlebacher, Outside Committee Member; Mark A. Bourassa, Committee Member; Gousheng Liu, Committee Member.
10

The Development of Forecast Confidence Measures Using NCEP Ensembles

Unknown Date (has links)
Ensemble model data can provide a wealth of guidance to forecasters, especially in terms of forecast confidence. A model run where members diverge generally corresponds to a low confidence forecast, while a model run where members converge generally corresponds to a forecast of high confidence. The current NWS graphically based forecasts accessible to the public do not show this measure of uncertainty and thus communicate a false sense of confidence or precision. From August 2004 into 2006, approximately 2 years of individual GFS model ensemble data were analyzed. The result is a climatology of each ensemble member, which obviously does not match the observed climatology based on the NCEP reanalysis. The GFS model ensemble climatology was normalized so that there is a mapping between the current model ensemble value and the climatological value. Since there is only two years of data, the climatology is calculated on a 45 day temporal window. This method is similar, but more simplistic, to the method that is used in the FSU Superensemble (Krishnamurti 2000) of using temporal windows to increase climatology robustness in the training dataset. The variables analyzed here include 2-m temperature, 10-m wind speed and 10-m vorticity. Normalized climatology distributions have been calculated for each grid point within the ensemble member, with forecast confidence measures developed by comparing the normalized spread of the ensemble members to the model climatological spread, as described below. This normalized spread is compared to the typical spread for that time of year, location, and forecast length to arrive at a relative measure of forecast uncertainty. If the current model uncertainty is greater (less) than the uncertainty of the model climatology, then there is a lower (higher) than average confidence. Confidence graphics have been developed and analyses to see how confidence values behave with certain synoptic situations are ongoing. This overall behavior along with certain case studies will be featured. It has also been seen that there is a statistical significant difference in NWS forecast error between low confidence and high confidence regimes. Average NWS error for the below (above) normal GFS confidence forecasts was 5.20oF (3.08oF). A student t-test on these values revealed that there is a statistically significant difference to 95% confidence of the mean forecast error during low and high confidence GFS forecasts. That is, the mean WFO forecast error is significantly increased during times of low forecast confidence in the GFS ensemble. Therefore, forecasters have a-priori knowledge of the likely human forecast error when they see the GFS ensemble output-- before the NWS forecast even verifies. During cases of extreme low confidence where the current model standard deviation is greater than the 25-year observational standard deviation, a climatology forecast was found to be more accurate than the overall ensemble mean. Although the confidence graphics are only based on the GFS ensembles as of now, more models will be added in the future to see how they behave when compared to each other. The GFS ensembles and the corresponding confidence technique have been used in the FSU-MM5 to see how a mesoscale model affects the overall confidence for a specific case. Recent feedback from NWS employees suggests an additional development of confidence graphics based on the "poor man's ensemble", which is an ensemble of all the operational forecast models. Eventually these graphics of below and above average confidence may be implemented into the Graphical Forecast Editor (GFE) for use in the National Digital Forecast Database (NDFD). / A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Master of Science. / Degree Awarded: Summer Semester, 2006. / Date of Defense: June 29, 2006. / Hart, Weather, NWS / Includes bibliographical references. / Robert Hart, Professor Directing Thesis; Henry Fuelberg, Committee Member; T.N. Krishnamurti, Committee Member; Andrew I. Watson, Outside Committee Member.

Page generated in 0.0609 seconds