Abnormal Phase Structure Of Thermal Tides During Major Dust Storms On Mars: Implications For The Excitation Source Of High-Altitude Water Ice Clouds

Interactions among the thermal tide, airborne dust, and water ice clouds significantly influence the Martian atmosphere. The absence of a stratosphere on Mars allows easier identification of the thermal tides from observational data than that on Earth, making Mars a natural laboratory to study the tidal excitation and propagation processes. However, the global tidal phase structure and its response to the tidal excitation sources on Mars have not been studied from observations. Here, we present a comprehensive analysis of the phase structure of migrating diurnal tide (DW1) using multi-local-time observations of the Mars Climate Sounder. The DW1 phase at low-to-mid latitudes shows a downward phase progression, which indicates upward tidal energy and momentum propagation throughout the year but with an apparent hemispheric asymmetry. The entire DW1 phase structure rises and falls with the dust height at the southern mid-latitudes suggesting a modulation of vertical dust extent on the altitude of the excitation source. During major regional dust storms, the DW1 phase propagates anomalously upward from 10 to 1 Pa in the Southern hemisphere. The Hough analysis suggests that the DW1 in the dust layer can only propagate upward to 10 Pa while the downward propagating mode above 10 Pa may be excited from a high-altitude source. Evidence suggests that the enhanced high-altitude water ice clouds could be a potential excitation source for these downward propagating tidal modes. These findings demonstrate how thermal tides can locally behave like inertia-gravity waves and the importance of water ice clouds to Martian atmospheric dynamics.