Challenges in Observing Short-term Tidal Variability

Modeling the upper levels of the Earth’s atmosphere, is one of the strengths of the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) supported as a community model by scientists at the High Altitude Observatory.

TIME-GCM modeling image
The Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) is a strong method for modeling the upper levels of the Earth’s atmosphere.

In order to model the complicated interactions occurring with in this region it is necessary to include forcing from above and below. The forcing from above includes changes in solar activity and dynamics within the magnetosphere. The forcing from below includes impacts from lower atmosphere a major portion of which are communicated via tides. Using the data from Modern-Era Retrospective Analysis for Research and Application (MERRA) HAO scientist Maura Hagan and colleagues including postdoctoral scientist Kathrin Hausler a new method for including these tides was developed for the TIME-GCM. The 3-hour MERRA data inherently includes realistic daily variations both the diurnal and semi-diurnal tidal fields. As the top panel of Figure 1 shows this method allows the model to capture eastward propagating diurnal tide as seen in the peaks of neutral temperature. These features are not as clearly evident in version of the model that utilze climatological tidal data as the lower boundary condition. 

Simulations such as these provide a unique opportunity to assess the ability of observing platforms such as the CHAMP and GRACE missions to measure these tidal variations. Since satellite mission like these can only monitor the region of space they are current flying through in order to produce a global picture like that shown in the top panel of Figure 1 they need to produce a climatology by average data over many orbits together. The second panel shows the result of doing this averaging over 10-days. In this panel only a weak trace of the diurnal temperature variation is present. Unfortunately, the produce a global picture the CHAMP and GRACE mission need to average the data over a 72-day interval and the result of doing that averaging on the TIME-GCM simulation results is shown in the bottom panel of Figure 3. As is clearly evident the figure this level of average completely removes the signal of the tidal variations.