A Comprehensive Study of the Short-term Variability of the Migrating Diurnal Tide in the Mesosphere and Lower Thermosphere

Speaker: Ashan Vitharana, University of Louisville (PhD Thesis Defense)

Abstract: Our ability to understand and predict space weather has become vital due to its significant societal impacts, for example, communication, transportation, and national defense. One of the most exciting discoveries in the last decade has been the realization that tropospheric weather can strongly influence the space weather. It is now recognized that the atmospheric waves (gravity waves, atmospheric tides, and planetary waves) play a key role in coupling the lower and upper atmosphere. In this dissertation, we focused our study on atmospheric tides. While climatology of tides has been extensively studied, little is known about the tidal weather (tidal variability < 30- days). This dissertation constitutes a study to make the step from “seasonal/climatological tides” to “tidal weather/short-term variability” using the data from the extended Canadian Middle Atmosphere Model (eCMAM), and temperature observations from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the TIMED satellite. Particular attention is given to the short-term variability of the migrating diurnal tide (DW1). A hierarchy of statistical models, e.g. autoregressive (AR), vector AR (VAR), and parsimonious AR (PVAR), are developed to predict tidal weather based on the statistical properties such as the quasi 23‐day oscillation and the spatial correlations found in the tidal weather. We can predict the next day’s tidal weather at near R2 = 90% accuracy (correlation coefficient of 0.95). The total tidal variability is modulated on various temporal scales, hence a multi-linear regression model is fitted to the DW1 temperature amplitude using deterministic variables (solar cycle, ENSO, QBO, and the seasonal harmonics) and the fitted coefficients/amplitudes are examined. The absolute and relative variability of the short-term tidal time series shows significant 6-month variation. Short-term tidal variability contributes ~50-75% of the total tidal variability between 7 to 30-day window. Physical mechanisms for the short-term DW1 variability are also investigated using the eCMAM thermodynamic budget. Wave-mean flow interaction is mostly responsible for the tidal weather in DW1 in the mesosphere. Above 140 km, short-wave solar heating becomes the governing force for the short-term DW1 variability.