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The winter upper atmosphere is associated with semidiurnal tidal variants, referring collectively to enhancements of near-12h periodicities, including the lunar tide-like (M2) periodicity, solar semidiurnal (S2) spectral sidebands, and the quasi-semidiurnal westward propagating modes with zonal wavenumbers = 1 and 3 (qSW1 and qSW3). Here we formulate a multipoint technique and implement the technique for a configuration of two midlatitude meteor radars, from Germany and China, to investigate the tidal variants. Statistical results illustrate that the 12h periodicity is dominated consistently by the expected migrating mode ( = 2) between 2012 and 2016, consistent with the tidal climatology and in turn validating the technique. Our case study of 2013 sudden stratospheric warming reveals that the 11.6h periodicity is characterized by = 3, whereas the 12.4h periodicity is dominated by = 2 mode with a maximum amplitude 7.5m/s and also comprises an additional mode = 1 with a maximum amplitude 3.3m/s. These observational evidences demonstrate, explicitly and for the first time, that (1) two independently reported categories of the variants, namely, the sidebands and the qSW1/qSW3 enhancements, are two different perspectives of identical phenomena, namely, the secondary waves of nonlinear interactions between SW2 and planetary waves, and (2) while M2 and the qSW1-associated secondary wave are entangled in the 12.4h periodicity, M2 is superior to the sideband.

Plain Language Summary The oscillations induced by a propagating wave are coherent everywhere on the wave's path but typically in different phases. This phase difference provides a possibility to diagnose the wavenumber. The possibility was realized on the observations from the magnetospheric quadruple-spacecraft mission Cluster, through the so-called phase differencing technique. The current study consolidates the technique to a framework and further tailor the framework to a configuration of two ground-based radars to diagnose the semidiurnal tidal variants in statistical and case studies, identifying the dominant zonal wavenumber as a function of frequency. Results demonstrate that two independently reported categories of the semidiurnal variants are two faces of one coin, and among the two semidiurnal variants entangled in the 12.4h periodicity, the semidiurnal lunar tide is the dominant one.