Phase relationships between tidal horizontal wind components observed in the mid-latitude winter upper mesosphere

Abstract

Wind measurements obtained from the Wuhan/China MF radar during winter 2001 are used to study the phase relationships between tidal horizontal winds in the upper mesosphere. The tidal initial phases in any time window are uniformly defined in radians. The meridional and zonal initial phases of each tide exhibit similar time variation trends at each height, which is associated with near tidal horizontal phase differences at three continuous heights. However, different tides have different phase variation trends and phase differences. Tidal meridional components generally lead the zonal. The domains of tidal phase ‘quasi-quadrature’ and ‘quasi-in-phase’ are strictly specified. In most of ∼14 days, the diurnal ( Δ ϕ 24 ) and semidiurnal ( Δ ϕ 12 ) phase differences are ‘quasi-quadrature’ while the terdiurnal phase difference ( Δ ϕ 8 ) is surprisingly ‘quasi-in-phase’. The appropriate tidal meridional and zonal quadratic phase coupling (QPC) equations are estimated, respectively. The calculated terdiurnal phases and those observed agree well. The tidal phases calculated by discrete Fourier transform (DFT) and least-squares fitting (LSF) are equivalent as are the tidal QPC revealed, respectively, by bicoherence spectrum and LSF analyses. By subtracting the estimated tidal zonal QPC equation from the meridional, a tidal phase difference equation is obtained. The calculated terdiurnal phase differences and those observed again agree with each other. Within the 14th time window, all three tides are generally elliptically rather than circularly or linearly polarized. However, both Δ ϕ 24 and Δ ϕ 12 in this window are ‘quasi-quadrature’ at all heights while Δ ϕ 8 are ‘quasi-in-phase’ at 92.0 and 94.0 km. So the estimated tidal QPC equations, the deduced tidal phase difference equation, the observed terdiurnal ‘quasi-in-phase’ phase differences and the typical tidal polarization plots are all indications of real QPC between the observed 24-, 12- and 8-h tides.