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Doctoral Degrees (Physics)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6603

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Browsing Doctoral Degrees (Physics) by Subject "Atmospheric waves."

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    Applications of light scattering and refraction by atmospheric gases.
    (2002) Moorgawa, Ashokabose.; Michaelis, Max M.
    LIDAR, an acronym for LIght Detection And Ranging, is a system used for studying the scattering of laser light incident on a parcel of air. This thesis investigates the atmosphere above the Durban region using two atmospheric LIDARs, referred to, in this study, as the "old LIDAR" and the "new LIDAR". The old LIDAR was used in a campaign of observation from July to October 1997 in a study of aerosol concentrations over Durban. This thesis will focus on, among other things, the local aerosol profiles for low altitude (0 to 10 km) and high altitude (10 to 35 km). In particular, the focus will shift on any long persistence in this region (it was found that the aerosol layer observed by M. Kuppen (1996) on June 1994 at 25 km may have moved to the higher altitude of 28 km in October 1997. This may be explained by stratospheric upwelling, carrying the layer to higher altitude. These aerosols are known to influence the local climate). This investigation will give some useful insight into the local atmospheric dynamics. The new LIDAR system (Rayleigh-Mie LIDAR) has been used to measure atmospheric temperatures from 20 to 60 km as well as aerosol extinction coefficients from 15 to 40 km. Height profiles of temperature have been measured by assuming that the LIDAR returns are solely due to Rayleigh scattering by molecular species and that the atmosphere obeys the perfect gas law and is in hydrostatic equilibrium (Hauchecorne and Chanin 1980). Since its installation in April 1999, the new LIDAR has been used to monitor stratospheric temperatures and aerosol concentrations from 10 to 40 km. In this study, we discuss in chapter 7 the results of a validation campaign conducted during the period of April 1999 to December 2000. Average monthly LIDAR temperatures are computed from April 1999 to December 1999 and compared with radiosonde temperatures obtained from the South African Weather Service (SAWS) at Durban. The monthly LIDAR temperature profiles over two years (1999 and 2000) were also computed and compared with the climatological model Cospar International Reference Atmosphere (CIRA)-1986 and with the average monthly European Centre for Medium Range Weather Forecast (ECMWF) temperatures . The results show that there is good agreement between LIDAR and SAWS radiosonde temperatures in the 20 and 30 km altitude range. Between 20 and 40 km, the monthly LIDAR temperatures agree closely with the CIRA-86 and ECMWF profiles. However, during winter, in the altitude range 40 to 60 km, LIDAR temperatures are warmer than CIRA-1986 and ECMWF temperatures, and they show large variability. These variations could be due to relatively fast transient phenomena like gravity waves or planetary waves propagating vertically in the stratosphere. As part of the validation process, the aerosol extinction coefficients retrieved from the LIDAR data have also been compared with the extinction coefficients measured by Stratospheric Aerosol and Gas Experiment (SAGE) II close to the LIDAR location and on coincident days. Appendix E of this thesis also investigates the concept of refraction by atmospheric gases as applied to gas lenses. A simple spinning pipe gas lens (SPGL) has been used as the objective lens of a camera to take pictures of the moon and sun spots. The SPGL is a varifocal length lens which depends on the temperature of the pipe and the angular velocity at which it spins. For our purpose a focal length of 8 m has been used. The moon pictures are compared with a lunar map so as to identify the maria.
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    Studies on atmospheric tides and planetary waves in the mesosphere-lower thermosphere (MLT) region using SuperDARN HF radars and meteor radar.
    (2013) Mthembu, Sibusiso Hector.; Venkataraman, Sivakumar.; Malinga, Sandile B.; Pillay, Sadhasivan Rangan.
    In this work, observational results of atmospheric dynamics caused by upward propagating atmospheric waves (tides, planetary waves and their interactions) in mesosphere-lower thermosphere (MLT) region are presented. This study is imperative as it contributes toward an understanding of various physical and dynamical processes that take place in this region. The seasonal and inter-annual variations of tides are investigated using MLT winds recorded simultaneously by SuperDARN HF radars situated at Halley (75°S, 26°W), SANAE (72°S, 3°W) and Syowa (69°S, 36°E) from 1998 to 2007. The seasonal variation of tides was found to be characterized by maximum amplitudes in summer and minimum amplitudes in winter. The semidiurnal tides showed additional enhancement of amplitude in autumn. The seasonal behavior of the diurnal tide (semidiurnal tide) was found to be similar to that of tropospheric specific humidity (stratospheric ozone mixing ratio) which suggests a forcing mechanism as a possible source of tidal variation. Long-term variation of semidiurnal tide was found to be correlated to F10.7 solar flux, which suggests solar activity as a possible driver of the semidiurnal tide variation. The variability of tides prior and post 2002 sudden stratospheric warming (SSW) event was studied using MLT winds derived from SuperDARN HF radars at Halley, SANAE and Syowa. Forcing mechanism using the ozone mixing ratio was found to be a possible source of semidiurnal tide (SDT) variability before the SSW event (160-250). Nonlinear interaction between planetary waves and tides on the other hand, was found to be a possible source responsible for the SDT variation just before, during and after the SSW event (250-300). Nonlinear interaction between planetary waves and tides in the MLT region was studied using wind velocity data collected from meteor radar located at Rothera (68°S, 68°W) Antarctica during the year 2005. Wavelet analysis conducted on the wind data showed that the MLT region is dominated by SDT’s and planetary waves with period ~ 5, 10, 16 and 23 days. Further analysis showed that SDT’s are modulated at the periods of ~5, ~16 and ~23 days. However, non-linear interaction between the SDT and 16-day planetary wave was found to be mostly responsible for the variability of the SDT than the interaction between the SDT and 5- as well as 23-day planetary. Study on the coupling between neutral atmosphere and ionosphere was conducted using SuperDARN HF radar and magnetic field data, both data sets were recorded from SANAE. The results showed that the quasi-16-day periodicity observed in the ionosphere most probably originated from the neutral atmosphere. This was established on the basis of the travel time of oscillation from the neutral atmosphere to the ionosphere. Modulation of semidiurnal tide at quasi-16-day periodicity was found to be the mechanism responsible for the neutral atmosphere/ionosphere coupling through ionospheric electrodynamo effect. Magnetosphere/ionosphere coupling was also observed at quasi-20- and -23-day periodicity using Dst index as the magnetospheric parameter. Solar-ionosphere coupling on the other hand was not observed.