Modeling and analysis of the structure and generation of the terdiurnal tide

The terdiurnal, or 8-h, tide has been seen in a number of midlatitude radar measurements and has recently been mapped globally at 95 km. A series of calculations and model simulations are used to examine the processes responsible for generating this tide and for its strong hemispheric asymmetry. The model tidal structure deviates significantly from the structure of a single Hough mode, in contrast to the diurnal tide. The dominant cause of the hemispheric asymmetry is the sensitivity of the propagation to the background state, particularly the static stability. Several conceptual views of the tide are explored to give a heuristic view of this asymmetry. This analysis predicts that particular modes are prohibited from propagating in large regions of the summer mesosphere. The tide occurs naturally in a three-dimensional nonlinear model extending from the tropopause to the lower thermosphere. The model tide shows a number of features in common with observations, including amplitude maxima in midlatitudes, a wintertime peak, and zonal winds larger than meridional winds. The tidal amplitude in the model is weaker than observed at 95 km. Several interactive model integrations are performed to determine the forcing mechanism for the tide. These calculations indicate that the direct solar forcing of the terdiurnal tide is the dominant mechanism occurring in the model at middle and high latitudes. Nonlinear interactions contribute to the low-latitude tide.