| dc.description.abstract | Atmospheric tides are global-scale waves whose periods are an integer fraction of a solar or lunar day. While the tides are primarily excited in the lower atmosphere, their amplitudes can become very large at high altitudes. As a result, the tides can strongly impact the chemistry and dynamics of the upper atmosphere and ionosphere. In this thesis, the drivers of the seasonal and short-term variations of the atmospheric tides are investigated by means of meteor radar wind measurements and mechanistic tidal model simulations.
Meteor wind measurements made by a global-scale array of SuperDARN radars are utilized to measure the sun-synchronous, or migrating, components of the tides in the mid- to high-latitude mesosphere and lower thermosphere (MLT). The SuperDARN radars span over 180◦ of longitude on a latitude band centered on 60◦N, and make hourly wind measurements based on the backscatter signal of meteor ablation trails. Leveraging the geographical extent and time of operation of the SuperDARN radars, unambiguous observations of the migrating tides are presented for a time period spanning 16 years.
The semidiurnal tide (SDT) is a major source of variability in the mid- and highlatitudes. To investigate the driving mechanisms of the seasonal variations of the SDT, simulations made using a mechanistic tidal model are validated against SuperDARN observations of the migrating SDT for the year 2015. Numerical experiments identify the impact of tidal dissipation, the background atmosphere, and surface reflections on establishing the simulated SW2 tide. The background atmosphere and eddy diffusion are found to strongly impact the seasonal behavior of the SDT, while the interference between the upward propagating tide and its surface reflection plays an important role during the summer months.
To investigate the drivers of short-term tidal variability, the SDT response to the 2013 major sudden stratospheric warming (SSW) event is simulated. The simulation results are validated against meteor wind observations made by the CMOR (43.3◦N, 80.8◦W), Collm (51.3◦N, 13.0◦E), and Kiruna (67.5◦N, 20.1◦E) radars, and against the migrating SDT measured by the SuperDARN radars. Numerical experiments identify the relative importance of the background atmosphere, non-linear interactions between the migrating SDT and quasi-stationary planetary waves, and variations in the tidal forcing. In addition, special attention is paid to the individual role of the solar and lunar SDT components. Results find that the solar SDT accounts for the majority of the net SDT variability, while being strongly impacted by the background atmosphere and by non-linear wave-wave interactions.
The SuperDARN model sampling technique, represented by a Gaussian vertical averaging kernel following the SuperDARN meteor echo distribution, is used to compare the mean zonal winds measured by SuperDARN radars against those simulated by the WACCMX-DART model for the 2009 SSW event. The temporal and spatial evolution of the measured winds compare favourably to the simulated winds, giving confidence in the representation of the polar vortex in the WACCMX-DART model. Further model analysis, in conjunction with temperature and nitric oxide (NOX) volume mixing ratio observations, finds that the downward transport of NOX during the SSW event was a factor of five greater in the trough than in the ridge of the polar vortex. | en_US |
