Studies on the short-term planetary wave activity in the MLT region over Southern hemisphere using super DARN HF radar.

Abstract

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.