Understanding Short Term Variations of the Migrating Diurnal Tide

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3389 Tides And Planetary Waves

Scientific paper

The upward propagating diurnal tide dominates the wind and temperature structure of the mesosphere and lower thermosphere. This phenomenon is forced in the lower atmosphere due to the absorption of infra-red radiation by water vapor in the troposphere and to a lesser extend the absorption of ultra violet radiation by ozone in the stratosphere. As the migrating diurnal tide propagate vertically, away from the source region, it can be impacted by a number of sources which can modify the amplitude and phase of the diurnal tide. These nonlinear interactions associated with the background zonal mean winds, dissipation and other large scale planetary waves can impact the amplitude and phase structure of the globally coherent migrating diurnal tide. Observations, primarily from ground-based radar and optical systems, have shown that the locally observed 24 hour oscillation, typically associated with the diurnal tide, can vary significantly over time scales from days to months. Satellite observations, which typically require integration times of months to extract the migrating tidal information have also shown the long period seasonal and interannual variations captured by the ground based observations. These long period variations have been replicated in both mechanistic and global circulation models, however the shorter period variations which occur on timescales of days have proven difficult to capture from observations and replicate using numerical models. One area that is problematic in understanding the day-to-day variability of the diurnal tide is the difficulty in separating potential sources of variability from the observations. From the ground-base perspective it is not clear if the observed variability is due to changes in the migrating diurnal tide or changes in other non- migrating components which cannot be separated from the migrating diurnal tide with measurements from multiple stations at similar latitudes. When utilizing measurements from a single satellite the difficulty arises from the fact that while the satellite provides nearly global coverage, all of the measurements on a give day are made at only one or two specific phases of the tide. To resolve this issue measurements are typically made over an extended time interval of months to build up measurements from multiple phases of the tide before inferring the tidal amplitudes. As a result of this process any short term variations in the tide are smoothed out. In this paper we will present an approach for utilizing observations from multiple satellite measurements to determine the global day-to-day variability of the migrating diurnal tide. Observations from the SABER instrument on the TIMED mission and the MLS instrument on the AURA mission will be combined to create estimates of the daily migrating diurnal tidal amplitude and phase. These results will be interpreted in an effort to understand the potential sources of variability associated with the migrating diurnal tide.

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