Browsing by Author "Mthembu, Sibusiso Hector."

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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.
Studies on the short-term planetary wave activity in the MLT region over Southern hemisphere using super DARN HF radar.
(2016) Ngwane, Ntlakanipho.; Sivakumar, Venkataraman.; Mthembu, Sibusiso Hector.
The atmospheric vertical pressure profile, chemical composition and temperature distribution, together, define a set of conditions governing the manifestation of atmospheric dynamics. Observations and extensive research shows that lower atmospheric layers play host to the formation of forced atmospheric waves such as atmospheric planetary waves and solar tides. These waves serve as transportation modes for energy budgets and ascend to upper atmospheric layers where they induce significant meteorological processes. Planetary waves, tides and gravity waves often dissipate energy in the mesosphere lower-thermosphere (MLT) region. This dissertation presents a study of the planetary wave variability in the MLT region using the South African National Antarctic Expedition High Frequency (SANAE HF) radar data, a component of the Super Dual Auroral Radar Network (SuperDARN). The focus is on short term planetary waves with periods ranging from 2 to 6 days. This planetary wave variability in the MLT is investigated during the occurrence of minor sudden stratospheric warming (SSW) events. The study also investigates the assertion that there is a non-linear interaction between planetary waves and atmospheric tides. The mesospheric wind data considered stretched from year 1998 to year 2008. The criterion towards a conclusive investigation of short term planetary waves included determining years within the said interval (1998 to 2008) with minor SSW events. The 11 year long temperature data from the NCEP/NCAR reanalysis project was used for this. The study managed to show the previously stated finding that mesospheric wind reversal occurs a week prior the stratospheric wind reversal linked with the warming. Years 2002, 2003 and 2007 were shown to host minor SSW events. In year 2002, minor SSW events occurred in days 235, 243 and day 255. In year 2003, the minor SSW event occurred in day 280. The minor SSW for year 2007 occurred in day 263. This meant that the planetary wave variability and the non-linear interaction between planetary waves and tides is investigated in the said years only. The short time Fourier transform technique (STFT) was used to reveal the tidal wave behaviour in the MLT region. It was observed that the semi-diurnal tide (SDT) is the most active tide at high latitudes. The wavelet transform was used to show the planetary wave variability in the MLT region. Along the zonal component, the activity in year 2002 was shown to be the most robust compared to the activity in years 2003 and 2007. In the meridional component, the planetary wave behaviour in year 2007 was the most active compared to years 2002 and 2003.The wavelet transform was simultaneously used to implement the first phase towards asserting the non-linear interaction between planetary waves and atmospheric tides. This phase is termed SDT modulation. In year 2002, 2 day and 3 day p-waves possibly modulated an SDT along the zonal component while 3 day and 6 day p-waves possibly modulated an SDT in the meridional component. In 2003, both along the zonal and the meridionalcomponents, the 4 day p-wave possibly modulated an SDT. In year 2007, a 6 day and a 4 day p-wave possibly modulated an SDT along the zonal and meridional components respectively. The proposition that there is a possible non-linear interaction between p-waves and tides was further reinforced using the bispectrum analysis. This method revealed the interaction predicted by the modulation phase. Peaks signifying p-wave SDT wave interaction were observed. These peaks were all consistent with the modulation that was said to have occurred between p-waves and SDTs. The third step in validating the stated assertion involved looking for secondary waves that may have formed due to a possible primary wave (p-waves and SDTS) interaction. This step was conducted on periods suggested by the SDT modulation. In every p-wave SDT wave suspected interaction as per the SDT modulation, secondary waves were shown to exist.