Tesis
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Esta comunidad contiene documentos de investigación de pre-grado y postgrado que han sido financiados por nuestra institución.
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Browsing Tesis by Subject "Atmosphere"
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Item Open Access A study of radar aspect sensitivity in the lower atmosphere(Cornell University, 2001) Chen, CharlesThe goal of this thesis is related to atmospheric temperature measurements using in situ techniques in tandem with a direct numerical simulation to better understand the zenith angle dependence of VHF (30-300 MHz) radar backscatter from the atmosphere. We begin our study with a high-resolution balloon-borne in situ temperature measurement made over Wichita, KS, in 1995. Very steep vertical temperature gradients were found at the edges of vertical potential steps, regions of near zero vertical potential temperature gradient. We use wavelet analysis to isolate the organized components of the signal and, after subtraction from the original signal, the residual signal is found to have the characteristics of isotropic turbulence. This confirms our hypothesis that the measured temperature profile is a superposition of coherent structures and a background isotropic turbulence. From a radar perspective, we show that this wavelet analysis allows us to predict the radar backscatter as a function of zenith angle from a high- resolution one-dimensional temperature measurement. Unfortunately, radar measurements were not available at this point. We next explore the cause of aspect sensitivity directly via a multi-instrument investigation of the lower atmosphere over the Jicamarca Radio Observatory (JRO) near Lima, Peru. The joint analysis of radar backscatter and in situ measurements of the temperature structure shows that a combination of Fresnel scattering and turbulence is the most likely explanation for aspect sensitive echoes. Furthermore, the strong backscatter seems to originate from vertical potential temperature steps; such as those observed over Wichita, KS. Finally, we show that the measured potential temperature steps and the structures seen in a direct numerical simulation (DNS) of a Kelvin-Helmholtz instability (KHI) are remarkably similar. Not only do we find good agreement between the observation and the simulation; the similarity is also seen in the wavelet spectrum, which is the behavior of the wavelet coefficient as a function of scale size. We extend the results from experimental observations and numerical simulation by predicting the characteristic radar backscatter and show that it is consistent with observations.Item Open Access Characterization of the convective boundary layer through a combination of large-eddy simulations and a radar simulator(University of Oklahoma, 2011) Scipión, Danny; Palmer, Robert; Chilson, PhillipThe boundary layer (BL) is the lowest part of the atmosphere where the flow field is directly influenced by interactions among air, heat, and the Earths surface. The structure of flow in the BL is dynamic due to the fact that the atmospheric flow is turbulent. Turbulence in the daytime convective boundary layer (CBL) is primarily caused by buoyancy forced from the heated underlying surface. Wind profilers are one of the many instruments used to study and characterize the atmosphere. In addition to in-situ observations, numerical large-eddy simulations (LES) have been probed to adequately reproduce the CBL under different conditions. The focus of this work is to bring the advantages of LES techniques to assist in the interpretation of data from wind profilers. The present study focuses on an example of flow structure of the CBL as observed in the U.S. Southern Great Plains Atmospheric Radiation Measurement Climate Research Facility in Lamont, Oklahoma on June 8, 2007. The considered CBL flow has been reproduced using LES, sampled with a LES-based virtual boundary layer radar (BLR), and probed with an actual operational wind profiler. The LES-generated CBL flow data are then ingested by the virtual BLR and treated as a proxy for prevailing atmospheric conditions. The virtual BLR has been used to simulate radar signals obtained from wind profilers through the synthesis of Doppler beam swinging (DBS) and spaced antenna (SA) techniques and to retrieve the three-dimensional wind fields. Comparisons of the estimates of the structure parameter of refractive index [special characters omitted], wind fields, vertical velocity variance, and vertical velocity skewness have been presented for the LES, virtual BLR, and actual radar. It has been observed that during the presence of strong horizontal shear of vertical velocity, the estimates of the horizontal wind fields are biased. This study will quantify the effect of this shear for both wind estimation techniques under different conditions. Additionally, it has been noticed that this shear also biases the estimates of turbulence kinetic energy (TKE) calculated from the variances of the wind fields. Finally, the TKE (eddy) dissipation rate ε can be obtained from radar estimates of Doppler spectral width. Values of ε are obtained from the different oblique and vertical beams and contrasted with the LES estimates obtained through a parameterized expression that relates the dissipation rate to sub-grid TKE and turbulence length scale.Item Open Access Entwicklung eines multistatischen kodierten CW-Radars mittels USRP zur Analyse der mittleren Atmosphäre(University of Applied Sciences, 2016-05-03) Geese, SvenMeteor radars are used for continuous atmospheric observations for many years. The collected data can be used to create wind fields in heights between 70 and 120 km, which can further be used in comparison with other types of measurments to conclude interactions between atmospheric layers like vertical waves. The goal of this thesis is therefore the development of a multistatic continuous-wave radar with pseudo random coding. The idea behind this is a radar network with multiple transmit and receive stations, which creates multiple baselines for meteor detection. With that the observable volume of the atmosphere is enlarged and additionally the system is modularly expandable by adding transmit or receive systems, which allows to increase the performance of the system by means of meteor rates easily in the long term.Item Open Access Examination of various techniques for measuring wind velocities using clear-air radars, with emphasis on vertical wind measurements(University of Colorado, 1998) Chau Chong Shing, Jorge Luis; Balsley, Ben B.In this work, I examine various techniques for measuring wind velocities using clear-air radars. Special emphasis has been given to the vertical wind measurements. All the observations were obtained in the lower atmosphere (below 20 km) using the Jicamarca 50 MHz radar system near Lima, Peru. I have concentrated my efforts on: (a) examining: different radar techniques for horizontal wind estimation, (b) improving the understanding of angle-of-arrival radar measurements, and (c) comparing a number of techniques for the measurement of the vertical wind velocities. I have measured horizontal wind velocities by a variety of both time- and frequency-domain spaced antenna (SA) techniques. Comparisons of these techniques for both zonal and meridional components have been carried out in a statistical sense. Two sets of data were analyzed under both "quiet" (low wind, low variability) and "active" (high wind, high variability) conditions. I have found that the simpler techniques that assume horizontally isotropic scattering compare well with the more complicated full correlation analysis techniques. Briefly, while all SA techniques give essentially the same horizontal wind direction, considerable discrepancies are apparent in the wind speed, particularly above 15 km. With regard to technique comparisons for vertical velocity, two types of measurement techniques using a concurrent data set are compared. Both Zenith (i.e., using a vertically-directed single beam) techniques and spaced antenna (SA) techniques are compared using a concurrent 5-day data set obtained with a special configuration of the Jicamarca 50 MHz radar antenna. Two separate Zenith techniques are implemented using different antenna beam-widths (a 0.85º and a 3º beam-width). Different SA approaches are also implemented using a: "quasi" Doppler beam swinging (DBS) approach and four "classical" SA approaches. "Classical" SA approaches make use of horizontal wind and AOA measurements to get 'corrected' vertical velocities. Statistical results are presented for both quiet and active conditions.Item Open Access Study of waves observed in the equatorial ionospheric valley region using Jicamarca ISR and VIPIR ionosonde(University of Illinois at Urbana-Champaign, 2017) Reyes, Pablo Martín; Kudeki, ErhanIncoherent scatter (IS) radar and ionosonde (VIPIR, vertical incidence pulsed ionospheric radar) data were taken concurrently at Jicamarca during campaigns of January, April, June, and July 2015, January 2016, and most recently April 2017 to bring more insight into the state and dynamics of the ionospheric E-F valley region and the 150-km radar echoes detected from this region. To better understand the rich and dynamic vertical structure of 150-km echoes observed at the Jicamarca Radio Observatory (JRO) and other equatorial stations and to contribute to the understanding of the physics of this region, we used JRO ISR and VIPIR ionosonde techniques to perform high spatial and temporal resolution measurements. We found correlations between VHF backscatter radar measurements and fluctuations detected with the VIPIR ionosonde, which is an indication of gravity waves playing a role in modulating the space-time structure of the 150-km echoes. Fluctuations with periods from 5 to 15 minutes are observed in VIPIR ionograms as well as in the layers found in the 50 MHz radar range-time-intensity (RTI) plots. The quiet-time stratified electron density contours are being rippled by waves propagating through the ionosphere. Evidence for this is the fluctuation of virtual reflection heights and angle of arrival (AOA) of the ionosonde echoes. The AOA is provided by interferometry, which indicates that the echo is not always coming from overhead. Scatter plots of the AOA in the receiving antenna’s orthogonal baselines give us the propagation direction. Plots of virtual height and AOA obtained using VIPIR data show phase fronts propagating downwards, which is characteristic of internal gravity waves (IGW). Other characteristics of IGW are present in the oscillations of virtual height: their frequencies are just below the Brunt-Väisälä frequency, their amplitudes increase with altitude, and shorter vertical wavelengths seen in lower altitudes are heavily damped in higher altitudes. The observed IGW exhibit fluctuations similar to those seen in the thin “forbidden” or “quite” zone of the 150-km echo undulations, which indicates some IGW-driven modulations of the 150-km echo as has been suggested previously [e.g. Kudeki and Fawcett, 1993; Chau and Kudeki, 2013]. Phase profiles of cross- correlation pair of antennas in the IS Faraday rotation experiment exhibit a smooth progression with altitude. That means that there are no sharp density gradients that could be a source of plasma instabilities. Still, density variations across the magnetized plasma in the region can be key to explaining the enhanced echoes observed via the electrodynamics that they can drive. We also found that there exist sub-minute quasi-periodic (SMQP) fluctuations when zooming into high time resolution RTI plots. This is a new observation that has not been reported in the literature to date. A method was designed in order to validate the existence of SMQP fluctuations. The method consisted of identifying episodes of sub-minute fluctuations in a non-exhaustive search of high resolution RTI plots using a web-based interactive tool designed for zooming in and marking the episodes where the sub-minute period fluctuations were found. We found a wide range of sub-minute periods, with a predominance between 15 and 20 seconds. This was a first step towards reporting SMQP; a more exhaustive method to search for these fluctuations is being produced. This multi-instrument approach helps us to characterize the daytime electron density fluctuations in the equatorial valley region, and aims to contribute to the goal of understanding better the fundamental physics of the region.Item Open Access Three-Dimensional Wind Measurements and Modeling Using a Bi-Static Fabry-Perot Interferometer System in Brazil(University of Illinois at Urbana-Champaign, 2013) Fisher, Daniel J.; Makela, Jonathan.The earth’s upper atmosphere has been studied for over a century now, and while a multitude of data has been collected studying the plasma in the ionosphere, there are not equivalent amounts of data on neutrals in the thermosphere to pair with these plasma observations. The Remote Equatorial Nighttime Observatory for Ionospheric Regions (RENOIR) project consists of two observing systems stationed in northeastern Brazil in Cajazeiras (6.87◦S, 38.56◦W) and Cariri (7.38◦S, 36.52◦W) since 2009. They are separated by 232.28 km and each is equipped with a Fabry-Perot interferometer (FPI) to measure neutral wind velocities and neutral temperatures using observations of the 630.0-nm emission caused by the dissociative recombination of O +2. The FPI systems can operate in two different modes: cardinal and common volume (CV). In cardinal mode, each FPI looks at a 45◦ elevation angle towards geographic north, east, south, and west followed by a zenith and laser image. In common volume mode, the two FPIs are synchronized to observe three common locations followed by both a zenith and laser observation. Two common volume points, one to the north and one to the south, are where the two FPIs have the same elevation angle but are looking orthogonal to one another in the horizontal plane. The third point is an inline measurement to the midpoint of the two sites. Vector neutral winds in the horizontal plane can be computed at the two common volume points, and a vertical neutral wind can be found at the inline point. FPIs are phase-based instruments, meaning a zero-reference is needed in order to get an absolute wind measurement. Since there is no practical 630.0-nm lab source, the zero-Doppler source is typically obtained from the zenith airglow measurements because the vertical winds are assumed to be zero across the night. However, the inline zenith measurements give non-zero winds under this assumption, consequently contradicting it. Therefore, the observations of the frequency-stabilized HeNe laser are used as a zero-Doppler reference to improve the derived vertical and horizontal neutral winds. This thesis validates this new technique by both developing an analytical model for the CV winds and by confirming it with results from actual observations made in Brazil.