Dissertations / Theses on the topic 'VLF atmosphere'

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1991.
Includes bibliographical references (p. 179-184).
by Keith Michael Groves.
Ph.D.

Thesis (M.S. in Telecommunication Systems Management)--Naval Postgraduate School, March 1990.
Thesis Advisor(s): Davidson, Kenneth L. Second Reader: Tulloch, A. W. "March 1990." Description based on signature page as viewed on August 26, 2009. DTIC Descriptor(s): Atmospheric Refraction, Very High Frequency, Ultrahigh Frequency, Communication And Radio Systems, Tools, Parameters, Wind, Forecasting, Accuracy, Theses, Electromagnetic Radiation, Pressure, Radiosondes, Refraction, Pacific Ocean, Climatology, Troposphere, Humidity, Gradients, Shores, East(Direction), Computer Printouts, Television Display Systems, Weather, Guided Missiles, Synoptic Meteorology, Test And Evaluation, Data Bases. DTIC Identifier(s): IREPS (Integrated Refractive Effect Prediction System). Author(s) subject terms: Communications, IREPS, refraction. Includes bibliographical references (p. 70-71). Also available online.

In addition to a proper radar calibration, quantitative estimation of precipitation from VHF radars requires the extraction of the precipitation signal out of the Doppler spectra. It also requires the proper conversion of this precipitation signal into a reflectivity factor.
This research develops a multi-faceted approach for the calibration of VHF vertically-pointing radars, by combining a first calibration method that compares the recorded VHF signal to power coming from a noise generator and a second calibration method that compares recorded VHF signal to cosmic radiation. This approach allows the retrieval of antenna and receiver parameters (such as noise levels, efficiency, and gain), and four other equations for the corresponding errors. In addition, we develop an equation for calibrating Doppler spectra.
The analysis is focused on rain observations with VHF radar. We verify the hypothesis that |K|2 = 0.93 for most of the rain observations at VHF band. A signal-processing algorithm for extracting the rain signal out of the VHF power spectra is then presented. This work also derives a general version of the radar equation valid for vertically pointing radars, as well as a particular version of this equation valid for the McGill VHF radar. The study then makes numerical simulations of several profiles of precipitation signal at VHF band, by combining high-resolution profiles of precipitation signal (from a calibrated X-band radar) and the VHF antenna pattern in our general version of the radar equation. The analyses indicate that VHF reflectivity at gates above the melting layer is artificially enhanced by the precipitation signal collected from the side lobes.
This work also studies the effect of precipitation in the scattering properties of clear air. We analyze several cases of stratiform and convective rain, occurring in a continental mid-latitude environment (Montreal, Lat.45.41°N, Long.73.94°W). For these cases, Doppler spectra taken by a VHF vertically-pointing radar were used to retrieve simultaneous co-located values of precipitation intensity (rainrates) and degrees of refractive index fluctuation (structure-function parameter for refractivity turbulence, Cn2). We validated these retrievals using co-located, calibrated measurements of precipitation signal at X-band. The comparison between equivalent reflectivity factors at X and VHF bands agrees within 1 dB. The study includes rainrates between 0.3 and 78 mm/h, and Cn2 values between 10-16 and 10-12 m-2/3 , retrieved from the VHF spectra at 2.5 km height. The study finds that the occurrence of rain is associated with distinctive changes in the structure of air refractive index fluctuations, and that these changes are of a turbulent nature for the most intense rainrates.

Depuis 2003 l’imagerie directe d’exoplanètes a permis d’obtenir des informations spectroscopiques et photométriques pour caractériser l’atmosphère de ces planètes géantes. Notre but a été tout d’abord de développer un outil simple pour interpréter les observations venant de SPHERE (Spectro-Polarimetric-High-contrast Exoplanet REsearch) au VLT et GPI (Gemini Planet Imager) au Gemini Sud pour déterminer les paramètres physiques des planètes. Nous avons développé un modèle d’exoplanètes géantes jeunes (nommés Exo-REM pour Exoplanet Radiative-convective Equilibriul Model). Les paramètres d’entrée du modèle sont la gravité de surface g, la température effective (Teff) et la composition élémentaire. Les sources d’opacité comprennent l’absorption par le continuum induit par collisions H2 –He et les raies atomiques et moléculaires de 8 corps différents (incluant la liste de raies du méthane d’Exomol). Les absorptions par des nuages de fer et de silicates sont aussi prises en compte sans diffusion. J’ai utilisé Exo-REM avec des observations photométriques et spectroscopiques des planètes naines et brunes HD 95086 b, beta Pictoris b, kappa Andromedae B, HR 8799 b, c, d, e, GJ 758 B. J’ai enfin étudié avec quelle précision pouvait-on dériver les paramètres physiques des planètes à partir de différents jeux d’observations, photométriques et spectroscopiques dans le cadre de SPHERE
Since 2003 direct imaging of exoplanets allows us to obtain spectroscopic and photometric data to characterize their atmospheres. First, our goal was to develop a simple tool to interpret data from SPHERE and GPI to derive planetary physical parameters. We developed a model of young giant exoplanet (called Exo-REM for Exoplanet Radiative-convective Equilibrium Model). Input parameters are surface gravity g, effective temperature (Teff) and an elemental composition. Opacity sources include the H2 –He collision-induced absorption and atomic and molecular lines from eight compounds (including CH4 updated with the Exomol linelist). We consider clouds of iron and silicates without scattering. I have used Exo-REM to analyse photometric and spectral observations of HD 95086 b, beta Pictoris b, kappa Andromedae B, HR 8799 b, c, d, e, GJ 758 B. I finally investigated the presicion to which the above parameters can be constrained from SPHERE measurement

Les processus dynamiques présents en zone de swash ont un impact significatif sur l'évolution des zones côtières. Une part importante du transport sédimentaire cross-shore se produit dans cette zone, plus particulièrement dans cette zone où se produisent le run-up et le run-down. La zone située au-delà de la ligne de rivage au repos est le plus souvent décrite par des modèles intégrés sur la verticale. La décroissance des vagues est bien reproduite, cependant l'étude du transport sédimentaire impose une paramétrisation du frottement sur le fond. Nous présentons les résultats de simulations RANS de la propagation d'un mascaret (obtenu par un "lâcher de barrage") sur une plage en pente et le run-up et le run-down ainsi générés. Les résultats numériques sont comparés aux résultats expérimentaux de Yeh et al. (1989). Les simulations ont été réalisées avec le code Navier-Stokes diphasique AQUILON. Deux méthodes de suivi d'interface VOF (VOF TVD ET VOF PLIC) sont implémentées. La viscosité turbulente est calculée par un modèle V2-F (Durbin, 1991). Une estimation des grandeurs turbulentes k et epsilon basée sur la théorie des ondes longues pour la propagation d'un ressaut hydraulique est présentée. Une modélisation VOF-PLIC & V2-F est appliquée pour reproduire les caractéristiques macroscopiques du lâcher de barrage, qui comme on pouvait s'y attendre dépendent peu de la turbulence. Nous étudions aussi l'impact des conditions initiales sur k et epsilon sur l'établissement de l'écoulement turbulent. Après ces validations vis-à-vis de la turbulence, des simulations du cas décrit par Yeh et al. (1989) sont menées pour optimiser le choix des paramètres de calcul. La théorie de Whitham (1958), prédit un effondrement du mascaret au niveau de la ligne de rivage au repos. La théorie de Shen and Meyer (1963) est toujours à l'heure actuelle le modèle de référence. Les résultats expérimentaux de Yeh et al. (1989) montrent clairement un phénomène différent. L'utilisation conjointe de la technique VOF-TVD et du modèle de turbulence V2-F semble apporter les meilleurs résultats par rapport aux expériences de Yeh et al. (1989). Une étude de la transition mascaret/lame de swash est proposée. Nos résultats montrent que la théorie de Whitham décrit de façon assez précise le mécanisme de d'effondrement du mascaret. Les résultats de nos simulations sont utilisés pour décrire la transition entre l'effondrement du mascaret et l'écoulement du run-up. L'analyse des processus de frottement dans le jet de rive met en évidence une forte dissymétrie entre le run-up et le run-down avec cisaillement plus faible lors du run-down

Doppler radar wind profilers are already widely used to measure atmospheric winds throughout the free troposphere and stratosphere. Several methods have been developed to quantify atmospheric turbulence with such radars, but to date they have remained largely un-tested; this thesis presents the first comprehensive validation of one such method. Conventional in-situ measurements of turbulence have been concentrated in the surface layer, with some aircraft and balloon platforms measuring at higher altitudes on a case study basis. Radars offer the opportunity to measure turbulence near continuously, and at a range of altitudes, to provide the first long term observations of atmospheric turbulence above the surface layer. Two radars were used in this study, a Mesosphere-Stratosphere-Troposphere (MST) radar, at Capel Dewi, West Wales, and the Facility for Ground Based Atmospheric Measurements (FGAM) mobile boundary layer profiler. In-situ measurements were made using aircraft and tethered-balloon borne turbulence probes. The spectral width method was chosen for detailed testing, which uses the width of a radar's Doppler spectrum as a measure of atmospheric velocity variance. Broader Doppler spectra indicate stronger turbulence. To obtain Gaussian Doppler spectra (a requirement of the spectral width method), combination of between five and seven consecutive spectra was required. Individual MST spectra were particularly non-Gaussian, because of the sparse nature of turbulence at its observation altitudes. The width of Gaussian fits to the Doppler spectrum were compared to those from the `raw' spectrum, to ensure that non-atmospheric signals were not measured. Corrections for non-turbulent broadening, such as beam broadening, and signal processing, were investigated. Shear broadening was found to be small, and the errors in its calculation large, so no corrections for wind shear were applied. Beam broadening was found to be the dominant broadening contribution, and also contributed the largest uncertainty to spectral widths. Corrected spectral widths were found to correlate with aircraft measurements for both radars. Observing spectral widths over time periods of 40 and 60 minutes for the boundary layer profiler and MST radar respectively, gave the best measure of turbulence intensity and variability. Median spectral widths gave the best average over that period, with two-sigma limits (where sigma is the standard deviation of spectral widths) giving the best representation of the variability in turbulence. Turbulent kinetic energies were derived from spectral widths; typical boundary layer values were 0.13 m 2.s (-2) with a two-sigma range of 0.04-0.25 m 2.s (-2), and peaked at 0.21 m 2.s (-2) with a two-sigma range of 0.08-0.61 m 2.s (-2). Turbulent kinetic energy dissipation rates were also calculated from spectral widths, requiring radiosonde measurements of atmospheric stability. Dissipation rates compared well width aircraft measurements, reaching peaks of 1x10 (-3) m 2.s (-3) within 200 m of the ground, and decreasing to 1-2x10 (-5) m 2.s (-3) near the boundary layer capping inversion. Typical boundary layer values were between 1-3x10 (-4) m 2.s (-3). Those values are in close agreement with dissipation rates from previous studies.

En zone littorale, la houle subit de fortes transformations par effets bathymétriques. Une meilleure compréhension de ses modifications et des transferts d'énergie associés permet de mieux appréhender les problèmes de dimensionnement de structures côtières et d'aménagement du littoral (protection du littoral, influence des ouvrages sur la côte).
L'objectif de ce travail est d'étudier expérimentalement et numériquement la propagation et le déferlement
d'ondes de gravité.
La première partie, expérimentale, propose des méthodes de calcul, basées sur les houles de Stokes, pour la mesure d'ondes partiellement stationnaires à partir d'instruments de type électromagnétique (S4) ou
acoustique (ADV) donnant des mesures synchrones de vitesses et/ou de pression. Les influences du courant,
de la direction de propagation, de la profondeur d'immersion des appareils ainsi que des effets non
linéaires sont alors étudiés à partir de données en bassin et in situ.
La deuxième partie, numérique, consiste en la validation d'une méthode de suivi de surface libre de type
SL-VOF (Semi-Lagrangian Volume Of Fluid), insérée dans un code de calcul industriel (code EOLE de la
société Principia R&D). L'onde de gravité est modélisée par un soliton. L'étude de la propagation et du
déferlement du soliton est effectuée pour deux applications : sur une marche (discontinuité du fond) puis sur un fond de pente constante 1/15. L'évolution de la surface libre, son élévation et le champ de vitesses
sont alors comparés aux résultats expérimentaux.

Le vif interet suscite au sein de la communaute scientifique par l'expansion de la radarmareteorologie a conduit le laboratoire de sondages electromagnetiques de l'environnement terrestre a developper, a partir de 1982, un radar de teledetection stratosphere-troposphere (radar provence). Cependant, la volonte d'explorer les couches atmospheriques inferieures a, par la suite, necessite la conception d'un mini-radar specifique, dont l'exploitation se heurte a un probleme majeur, inherent aux caracteristiques du systeme retenu: l'emploi conjoint d'une antenne de dimensions reduites, imposee par le respect du critere basse troposphere, et de la gamme vhf (very high frequencies), classiquement la plus favorable aux observations en air clair, entraine un elargissement consequent du faisceau radar qui, combine a un angle de tir nettement plus rasant, resulte en un fort recouvrement des domaines d'altitude sondes par les cellules radiales successives. L'obtention des profils atmospheriques de reflectivite et de vitesse ne peut donc plus s'effectuer de facon immediate (comme c'est le cas pour le radar provence) et il doit etre fait appel pour cela a une methode de restitution de parametres par deconvolution ou methode inverse. Dans ce contexte, l'objet du travail presente ici consiste en l'etude et la mise en application d'une technique d'inversion appropriee ; deux methodes differentes sont abordees, la methode des moindres carres et celle du maximum d'entropie, ces deux methodes se presentant a priori comme particulierement adaptees aux deux principaux types de profils atmospheriques pouvant etre distingues (respectivement les profils reguliers et les profils a forts gradients). Une etude theorique est tout d'abord menee, visant a modeliser les principes physiques regissant l'obtention des donnees radar exploitees (moments d'ordres 0 et 1 des spectres doppler). Le modele ainsi etabli, combine a un jeu de profils atmospheriques theoriques dont les proprietes sont ajustees a volonte, permet de pratiquer diverses simulations systematiques, contribuant a evaluer la faisabilite des processus d'inversion envisages et autorisant une adaptation optimale des methodes employees au cas particulier du mini-radar. La mise au point theorique des methodes en question etant assuree, leurs performances effectives sont alors testees a l'aide de donnees reelles acquises sur le mini-radar, les resultats obtenus etant compares a ceux fournis en alternance sur le meme site par le radar provence, servant ici de reference. Les modalites concernant l'implantation pratique des procedes mis en uvre et les perspectives ainsi ouvertes sont enfin discutees, tandis que certains axes de recherche meritant de plus amples developpements, sous la forme de travaux specifiques, sont suggeres

Cette étude a été motivée par la nécessité de pouvoir décrire la variabilité biogéochimique avec la même résolution que les paramètres physiques. Des profileurs optiques ont permis de mesurer à haute fréquence les propriétés optiques inhérentes dans la colonne d'eau. Par comparaison avec des données discrètes acquises simultanément à nos mesures, les coefficients optiques à des longueurs d'onde précises ont été convertis en grandeurs biogéochimiques quantitatives. Nous avons estimé la concentration en chlorophylle a et en carbone particulaire total ainsi que la répartition verticale de la matière organique dissoute colorée (CDOM). La nature qualitative du matériel particulaire et dissous a été abordée en combinant différents coefficients optiques ainsi qu'en analysant leur dépendance spectrale. En préalable aux campagnes dans l'Atlantique nord (campagnes POMME), une expérience en milieu contrôlé a été menée sur une population d'organismes phytoplanctoniques calcifiants (Emiliania huxleyi) afin de caractériser ses propriétés biooptiques ainsi que leurs modifications consécutives à une augmentation de la pCO2 atmosphérique du niveau actuel (360 ppm) au niveau prévu pour la fin du siècle (700 ppm). Lors des campagnes entre le Portugal continental et les Açores nous avons effectué le suivi biooptique de la variabilité à méso-échelle à trois périodes clés de l'année : fin de l'hiver, printemps et fin de l'été. L'observation à haute résolution de la colonne d'eau a permis de mettre en évidence certaines relations entre les caractéristiques hydrologiques et biogéochimiques. Le déploiement de nos instruments le long de radiales avec un système tracté autorisant une résolution de 2 milles (Tow-Yo) a conduit à préciser l'influence des structures tourbillonnaires de moyenne échelle.

Bibliography: p. 249-267.
xxiv, 267 p., [1] leaf of plates : ill. ; 30 cm.
Title page, contents and abstract only. The complete thesis in print form is available from the University Library.
Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1996

碩士
國立中央大學
太空科學研究所
84
Using the datasets of frontal precipitation passed on Dec. 12 1995 by Chung-Li VHF radar, proceed the correlation analysis of atmospheric and prcipitation parameters. We can distinguish two distinct precipitation events from the observational time according to the variation of precipitation echo varied with height and time, and recognize the bright band by the data of airsonde in time and the vertical motion of backound atmosphere. Correlation analysis applied on series of precipitation echo varied with height can present the delay phenomena of raindrop''s falling process, which can be regarded as a symbol of tracking raindrops. Perfect correlations exist between raindrops'' falling velocities and precipitation echo, which can be described as a exponential form V=a*P^b. There is a obvious turning at the melting layer respond to the height variation of coefficient "b". Additionally , it is weak that the vertical motion of background atmosphere reacted on the hydrometers for the datasets obtained from 1995''s observations. The study of interactions between precipitation particles and background atmosphere is concerned about that whether the hydro- meter will be frozen-in the background atmosphere or not. At the time, we define the checking parameters "df" and "rf" to be the quantized index of frozen-in level. In addition, the parameter settings with transmitting and receiving in time for vertical and off-vertical beams was applied on the observation of frontal precipitation on Apr. 13. 1996. In such way, we do not have to consider if the time resolution as the setting of 1995 could describe the variation of the targets. After these experiments, that hydrometers were not frozen-in background atmosphere is approved,and the correlations such as checking parameters versus falling velocities and radial wind velocities are also not quite obvious. Finally, the effect of anti-frozen-in for ice crystals are more severe than raindrops.

This thesis describes the development, modification and refinement of a high-powered hybrid Stratospheric Tropospheric (ST)/meteor radar at the University of Adelaide’s Buckland Park (BP) field station. This thesis also describes the process of statistically comparing results obtained from multiple co-located independent measurement sources. Also included are statistical comparisons made between meteor radars at BP,Darwin, Northern Territory, and Davis Station, Antarctica, with other independent sources of measurement. Previous meteor radar systems have generally been low powered (∼8 kW peak) and as such could only afford low count rates at frequencies of the order of 50 MHz. While it has been shown that the echo detection rate is inversely proportional to frequency to the power of 1.5, the use of lower VHF frequencies within Australia is restricted by government regulations. As such, this has lead to the development of a high powered meteor radar system at 55 MHz which has served to facilitate higher echo rates at this frequency. The aim of improving the echo rate is to improve the statistical accuracy of results generated by the meteor technique. Also presented are descriptions of the meteor radar systems used to provide the data for this study and the basic principles of the meteor technique. Basic descriptions of the other systems and the techniques used to provide data for comparison are also presented. Two key components in the development of the high-powered meteor system are the high-powered all-sky crossed-dipole transmit antenna and the high-powered 1:2 splitter-combiner required to drive the antenna. The antenna was designed using standard equations for Yagi-Uda antenna design found in literature and modeled using the EZNEC modeling programe. After successful modeling, the antenna was prototyped and refined into a low powered version to investigate the antenna’s performance characteristics. Once the performance of the antenna was verified, the process of upgrading the antenna to handle the full output power from a VTX transmitter was performed. This upgrade also spawned the design and development of the highpowered 1:2 splitter-combiner which would be used to feed the high-powered version of the antenna. The successful operation of the high-powered system over several periods of observation has allowed for a more in-depth investigation into the statistical reliability of the meteor technique. Along with the comparison of standard atmospheric parameters, i.e. temperatures and wind velocity, the high-powered system has allowed for the verification of the relationship between echo rate and radar parameters found by McKinley, which is frequently referred to in many papers dealing with meteor observations. Along with the comparisons made with the results from the high-powered meteor radar system at BP, comparisons of atmospheric parameters derived from meteor observations and other techniques were made at Davis Station and Darwin. Of particular interest is the unique comparison of atmospheric winds made at Davis between two independent meteor radar systems and a Medium Frequency (MF) radar. Previous comparison studies have only enjoyed the benefit of having two independent sources of measurement to compare and as such have not allowed for a unique solution to be obtained for the uncertainties of the techniques using the method of Hocking et al. [2001]. Davis Station is unique in that it has two independent meteor radars in addition to a MF radar. This has enabled for the reduction in the number of degrees of freedom in the statistical comparison process, and as such has allowed for unique solutions to be determined for the uncertainties when comparing two independent techniques; i.e. meteor and MF wind comparisons. Atmospheric temperatures in the Mesospheric and Lower Thermospheric (MLT) region were determined through the use of meteor diffusion coefficients and derived atmospheric pressure models at Davis Station, BP and Darwin. Comparisons are made between the meteor technique and other co-located independent measurements. These include; airglow, satellite and falling sphere measurements at Davis Station, airglow and two independent satellite measurements at BP and two independent satellite observations at Darwin. This thesis as a whole demonstrates the successful operation of the highpowered ST/meteor hybrid radar at BP. It also demonstrates the successful comparisons of MLT winds and temperatures made between meteor radar and other independent sources of MLT measurements. The validation of using the high-powered meteor radar at BP coupled with the successful comparison of atmospheric parameters derived using the meteor technique and other forms of MLT observations serves to re-affirm the statistical accuracy and benefit of the meteor technique in observations of the MLT region.
http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1474902
Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 2010

This thesis describes the development, modification and refinement of a high-powered hybrid Stratospheric Tropospheric (ST)/meteor radar at the University of Adelaide’s Buckland Park (BP) field station. This thesis also describes the process of statistically comparing results obtained from multiple co-located independent measurement sources. Also included are statistical comparisons made between meteor radars at BP,Darwin, Northern Territory, and Davis Station, Antarctica, with other independent sources of measurement. Previous meteor radar systems have generally been low powered (∼8 kW peak) and as such could only afford low count rates at frequencies of the order of 50 MHz. While it has been shown that the echo detection rate is inversely proportional to frequency to the power of 1.5, the use of lower VHF frequencies within Australia is restricted by government regulations. As such, this has lead to the development of a high powered meteor radar system at 55 MHz which has served to facilitate higher echo rates at this frequency. The aim of improving the echo rate is to improve the statistical accuracy of results generated by the meteor technique. Also presented are descriptions of the meteor radar systems used to provide the data for this study and the basic principles of the meteor technique. Basic descriptions of the other systems and the techniques used to provide data for comparison are also presented. Two key components in the development of the high-powered meteor system are the high-powered all-sky crossed-dipole transmit antenna and the high-powered 1:2 splitter-combiner required to drive the antenna. The antenna was designed using standard equations for Yagi-Uda antenna design found in literature and modeled using the EZNEC modeling programe. After successful modeling, the antenna was prototyped and refined into a low powered version to investigate the antenna’s performance characteristics. Once the performance of the antenna was verified, the process of upgrading the antenna to handle the full output power from a VTX transmitter was performed. This upgrade also spawned the design and development of the highpowered 1:2 splitter-combiner which would be used to feed the high-powered version of the antenna. The successful operation of the high-powered system over several periods of observation has allowed for a more in-depth investigation into the statistical reliability of the meteor technique. Along with the comparison of standard atmospheric parameters, i.e. temperatures and wind velocity, the high-powered system has allowed for the verification of the relationship between echo rate and radar parameters found by McKinley, which is frequently referred to in many papers dealing with meteor observations. Along with the comparisons made with the results from the high-powered meteor radar system at BP, comparisons of atmospheric parameters derived from meteor observations and other techniques were made at Davis Station and Darwin. Of particular interest is the unique comparison of atmospheric winds made at Davis between two independent meteor radar systems and a Medium Frequency (MF) radar. Previous comparison studies have only enjoyed the benefit of having two independent sources of measurement to compare and as such have not allowed for a unique solution to be obtained for the uncertainties of the techniques using the method of Hocking et al. [2001]. Davis Station is unique in that it has two independent meteor radars in addition to a MF radar. This has enabled for the reduction in the number of degrees of freedom in the statistical comparison process, and as such has allowed for unique solutions to be determined for the uncertainties when comparing two independent techniques; i.e. meteor and MF wind comparisons. Atmospheric temperatures in the Mesospheric and Lower Thermospheric (MLT) region were determined through the use of meteor diffusion coefficients and derived atmospheric pressure models at Davis Station, BP and Darwin. Comparisons are made between the meteor technique and other co-located independent measurements. These include; airglow, satellite and falling sphere measurements at Davis Station, airglow and two independent satellite measurements at BP and two independent satellite observations at Darwin. This thesis as a whole demonstrates the successful operation of the highpowered ST/meteor hybrid radar at BP. It also demonstrates the successful comparisons of MLT winds and temperatures made between meteor radar and other independent sources of MLT measurements. The validation of using the high-powered meteor radar at BP coupled with the successful comparison of atmospheric parameters derived using the meteor technique and other forms of MLT observations serves to re-affirm the statistical accuracy and benefit of the meteor technique in observations of the MLT region.
Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 2010

碩士
國立中央大學
太空科學研究所
95
One of the most important topics in researching atmospheric gravity waves is to discussing the energy source of the waves and the mechanicism of excitation。 Owing better resolution of time-space and the capability to detect the echoes from precipitation and clear air, the VHF radars became good researching tool for study atmospheric waves in precipitating environment. This research using Chung-Li VHF radar as a researching tool has found that when Tai-kei typhoon moving northward, she has tilted up a 15°-27° angle from the surface of the earth, and she disturbed the change of wind direction. It is also found that the depletion of the air echoes, because the increasing of turbulence mixing effect. Finally,it is found that the precipitation particles do excite gravity waves propagating upward.

The high power, broadband very low frequency (VLF, 3--30 kHz) and extremely low frequency (ELF, 3--3000 Hz) electromagnetic waves generated by lightning discharges and propagating in the Earth-ionosphere waveguide can be used to measure the average electron density profile of the lower ionosphere (D region) across the wave propagation path due to several reflections by the upper boundary (lower ionosphere) of the waveguide. This capability makes it possible to frequently and even continuously monitor the D region electron density profile variations over geographically large regions, which are measurements that are essentially impossible by other means. These guided waves, usually called atmospherics (or sferics for short), are recorded by our sensors located near Duke University. The purpose of this work is to develop and implement algorithms to derive the variations of D region electron density profile which is modeled by two parameters (one is height and another is sharpness), by comparing the recorded sferic spectra to a series of model simulated sferic spectra from using a finite difference time domain (FDTD) code.

In order to understand the time scales, magnitudes and sources for the midlatitude nighttime D region variations, we analyzed the sferic data of July and August 2005, and extracted both the height and sharpness of the D region electron density profile. The heights show large temporal variations of several kilometers on some nights and the relatively stable behavior on others. Statistical calculations indicate that the hourly average heights during the two months range between 82.0 km and 87.2 km with a mean value of 84.9 km and a standard deviation of 1.1 km. We also observed spatial variations of height as large as 2.0 km over 5 degrees latitudes on some nights, and no spatial variation on others. In addition, the measured height variations exhibited close correlations with local lightning occurrence rate on some nights but no correlation with local lightning or displaced lightning on others. The nighttime profile sharpness during 2.5 hours in two different nights was calculated, and the results were compared to the equivalent sharpness derived from International Reference Ionosphere (IRI) models. Both the absolute values and variation trends in IRI models are different from those in broadband measurements.

Based on sferic data similar to those for nighttime, we also measured the daytime D region electron density profile variations in July and August 2005 near Duke University. As expected, the solar radiation is the dominant but not the only determinant source for the daytime D region profile height temporal variations. The observed quiet time heights showed close correlations with solar zenith angle changes but unexpected spatial variations not linked to the solar zenith angle were also observed on some days, with 15% of days exhibiting regional differences larger than 0.5 km. During the solar flare, the induced height change was approximately proportional to the logarithm of the X-ray fluxes. During the rising and decaying phases of the solar flare, the height changes correlated more consistently with the short (wavelength 0.5-4 Å), rather than the long (wavelength 1-8 Å) X-ray flux changes. The daytime profile sharpness during morning, noontime and afternoon periods in three different days and for the solar zenith angle range 20 to 75 degrees was calculated. These broadband measured results were compared to narrowband VLF measurements, IRI models and Faraday rotation base IRI models (called FIRI). The estimated sharpness from all these sources was more consistent when the solar zenith angle was small than when it was large.

By applying the nighttime and daytime measurement techniques, we also derived the D region variations during sunrise and sunset periods. The measurements showed that both the electron density profile height and sharpness decrease during the sunrise period while increase during the sunset period.

Dissertation

碩士
國立中央大學
太空科學研究所
84
VHF雷達對低層大氣降水現象的觀測，經由前人的努力，對降水環境下的 大氣結構變化，已有相當程度的了解，如鋒面下的大氣和降水回波功率、 降水型態、降水粒子大小和終端速度、融解層及以上的冰晶粒子回波功率 、粒徑大小等，已做相當程度地分析研究。至於整體大氣的研究，以往皆 將低層大氣降水觀測，及較上層的對流層頂觀測分開，此處用4μs波寬觀 測，同時看到降水及對流層頂，空間解析度比以往8μs脈波寬觀測到的高 ，比較傳統探空氣球測得的對流層頂位置，VHF雷達所測得高度是較高， 對於以後欲觀測低層大氣的整體結構提供了很好的選擇。文章中將針對降 水環境下，討論大氣回波功率與大氣參數之間的關係，及大氣回波功率與 大氣結構變化的情形；發現大氣亂流回波功率和折射指數梯度平方、比濕 、比濕梯度相關性較高。由於觀測參數設定是以2μs和 4μs交互觀測， 於比較2μs和4μs降水及大氣回波功率、頻寬、垂直速度變化，發現降水 粒子分布的不均勻性及降水粒子受波束加寬效應的影響不大。 Using the VHF radar, precipitation is observed in the low atmosphere. It is understood that the structure of atmosphere varies in the precipitation environment. For example, during the front,the atmophere and precipitation echo power, the form of precipitation, the size distribution and terminal velocity of precipitation particle, the melting layer, the echo power and size of ice particle are studied suitably. However, in the past, the observation of precipitation and tropopause for studying entire atmosphere were separate. Here, the pulse width of observation is 4μs that will see both precipitation and tropopause. The height measured by the VHF radar is higher than rawinaode data. In the future, using pulse width of 4μs to make observation will provide a good opportation for studying the entire structure of low atmosphere. This dissertation will discuss the relation between the atmospheric echo power and atmospheric parameters, and the relation between the echo power and atmospheric structure. Furthermore, it will also make comparison among power, spectral width and vertical velocity of the atmosphere and precipitation echo by using the alternative observations of the pulse width of 2μs and 4μs. It is found that the distribution of precipitation particle is nonuniform in the space and that the affection of beam broading effect is small.

碩士
國立中央大學
太空科學研究所
104
The antenna array which consists of five Yagi antenna was used in this research.Five Yagi antenna was placed in a cross shape and the distances of each Yagi were different. The position of the target in three dimensional space can be derived by different phases.The targets may fail to locate because of different phases during operating in radar system and phase confusion. By using IGRF model, different phases during operating in radar system can be solved and geometric logic can deal with the phase confusion. Positions of the meteor tails can be located successfully and the three dimensional wind field of the mesosphere can be estimated with sliding window method.After surveying the HWM wind field observed, they are consist with my results and the Diurnal Tide features are quite obvious.