Abstract
Abstract. Relative to extensive studies of interactions between the quasi 2-day wave and tides, nonlinear interaction of the 16-day wave with tides was reported less, in particular interaction with the diurnal tide. We present an observational study of a possible nonlinear interaction event between the 16-day wave and the diurnal tide based on meteor radar measurement at Maui. An obvious 16-day wave can be observed from raw wind data. Its maximum meridional wind amplitude can attain 18.0 m s−1 at a height of 92 km during the time of our attention, which is larger than that in previous reports. Sum and difference interactions between the 16-day wave and the diurnal tide are observed to have rather different intensities. Because sum nonlinear interaction is very intense, the secondary sum wave with a period of 22.59 h is stronger than the diurnal tide. However, weak spectrum of the secondary difference wave is hardly identified. The beat of the diurnal tide with the secondary sum wave leads to substantial modulation of the diurnal tide at a period of 16 days. Moreover, this strong secondary sum wave further interacts with the 16-day wave to generate a new secondary wave with a period of 21.33 h. Such an interaction may be also regarded as a third-order nonlinear interaction between the 16-day wave and the diurnal tide with two-step interaction. Hence, the third-order nonlinear interaction between planetary waves and tides may occur significantly in the MLT region.
Highlights
Atmospheric waves, including tidal, planetary and gravity waves, play important roles in determining large-scale circulation and thermal structure of the mesosphere and lower thermosphere (MLT) because these waves transport energy and momentum from one atmospheric layer to another, leading to global redistribution of atmospheric energy and momentum and coupling among various atmospheric layers
As the atmospheric density decreases, amplitudes of atmospheric waves propagating into the MLT increase, and many complex nonlinear processes of waves may occur in the MLT region, for example, nonlinear coupling between waves and background flow (Dickinson, 1969; Hartmann et al, 1984; Miyahara et al, 1993; Huang et al, 2010), nonlinear interactions among tidal, planetary and gravity waves (Fritts and Vincent, 1987; Stenflo, 1994; Nakamura et al, 1997; Jacobi et al, 1998, 2006; Liu and Hagan, 1998; Beard et al, 1999; Pancheva, 2000, 2006; Pancheva et al, 2000a, b; Liu et al, 2008; Stenflo and Shukla, 2009; Huang et al, 2009, 2013a), and breaking due to their amplitudes exceeding the instability thresholds (Lindzen, 1981; Leovy et al, 1985; Polvani and Saravanan, 2000; Xu et al, 2006)
Theoretical work indicates that planetary waves observed in the MLT region correspond to a series of classical westward propagating Rossby normal modes (s, n − s) (s is the zonal wavenumber and n is the meridional index) with periods of about 2, 5, 10
Summary
Atmospheric waves, including tidal, planetary and gravity waves, play important roles in determining large-scale circulation and thermal structure of the mesosphere and lower thermosphere (MLT) because these waves transport energy and momentum from one atmospheric layer to another, leading to global redistribution of atmospheric energy and momentum and coupling among various atmospheric layers. By using zonal and meridional winds observed by radar at the middle and high latitudes in two hemispheres, Pancheva (2000) and Pancheva et al (2002) demonstrated that there was considerable modulation of amplitudes of the semidiurnal tide at periods of planetary waves, evident at 10- and 16-day periods. Their analysis indicates that the nonlinear interaction between the semidiurnal tide and the 16-day wave was much stronger than those between the diurnal tide and the 16-day wave.
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