This project is to characterize the role of nonlinear interactions, including instability, between tides and traveling and stationary planetary waves in generating variability in the upper mesosphere and lower thermosphere (MLT). The work will focus particularly on the two day wave (TDW) in the summertime southern hemisphere, including studies on its amplification due to resonance with a subharmonic of the diurnal tide and consequent generation of a parametric subharmonic instability (PSI) as well as on the apparent association between phase-locked TDWs and pronounced enhancements of airglow emission brightness. In addition, this project will investigate nonlinear forcing mechanisms which modulate tidal amplitude and/or produce non-migrating tides, particularly the westward semidiurnal tide which is dominant in the high-latitude upper mesosphere. This effort will utilize a systems science methodology which combines ground-based wind and airglow intensity measurements, temperature measurements from the NASA TIMED satellite, the TIME-GCM general circulation model, and a global data assimilation model. This work will be conducted as a collaboration between international and U.S. educational, research, and small business institutions.
The overarching objective of this project is to characterize and explain the variability in tides and planetary waves induced by interaction between them. Rapid amplification of planetary waves at two-day period has been observed in the Southern Hemisphere during austral summer. This rapid amplification of the two-day wave (TDW) occurs when the period is close to 48 hours (typically the period ~ exceeds 52 hours) and the TDW is thus resonant at a sub-harmonic frequency of the tides. Walterscheid and Vincent [1996] have proposed that the phase-locked TDW is a manifestation of a parametric sub-harmonic instability (PSI) that proceeds when the free period is very close to a tidal sub-harmonic. There is observational and modeling support for this suggestion [Walterscheid and Vincent, 1996; Palo et al., 1999; Hecht et al., 2010]. The PSI may be the single most important amplification process in the MLT. The same mechanism can modulate lower frequency planetary waves (those with periods near 5, 8 and 15 days) when background conditions cause their frequencies to be sufficiently close to a tidal sub-harmonic frequency. Four different topics were studied; 1) Two-day wave in the southern hemisphere summer high latitude upper mesosphere Characterized the relative efficacy of different interaction schemes for exciting the TDW including zonally symmetric and migrating tides Identified the source region where interactions are most effective in driving the phase-locked two day wave. Determined the latitudinal extent of the January events and whether they extend into the northern hemisphere to an appreciable extent. Modeled the wave induced transports induced by the TDW Understood the airglow response using a global circulation model 2) Subharmonic generation of longer period traveling planetary waves Characterized the effectiveness of interactions in producing an exchange of energy between tides and longer period planetary waves (e.g., 5, 8, 15 days). 3) Modulation of tidal amplitudes Determined the effectiveness of sideband generation in explaining the modulations of tidal amplitudes at planetary wave frequency and the extent to which tidal modulation observed locally is affected by phase speed dispersion. 4) Nonmigrating westward s=1 semidiurnal tide Characterized local forcing versus interhemispheric exchange of energy in generating the high latitude SDW1 tide in the upper mesosphere during austral spring and summer. Determined the amplitude and seasonality of the stratospheric SDW1 tide. The work resulted in: a) 2 papers in preparation for publication, describing results of research with new scientific findings b) successful collaborations between ASTRA, Aerospace Corporation, Dixie College, CIRES/CUBoulder, HAO-NCAR c) ASTRA mentored two REU students as part of the University of Colorado/LASP REU program. One of the students worked on analysis of planetary waves in the middle atmosphere and ionosphere. She received training in scientific computing and developed computer programs to analyze NCEP re-analysis data products to quantify planetary waves. d) This work identified an area of improvement in the NCAR- TIME-GCM model. We worked with collaborators at NCAR to develop a more robust lower boundary forcing scheme for planetary waves. The TIME-GCM is a community model and is widely used by the upper atmospheric community. Any improvement made to the lower boundary condition in the model will benefit the community at large.
