Long‐Term Variation of Lunar Semidiurnal Tides in the MLT Region Revealed by a Meteor Radar Chain

Abstract

Atmospheric tides are one of the most important dynamic processes in coupling the lower atmosphere and the upper atmosphere. They are usually observed by radars and satellites in the mesosphere and lower thermosphere (MLT) region. The solar thermal and lunar gravitational tides are the predominant periodic oscillations in the atmosphere. The solar tides, modulated by the variations of water vapor, ozone, and molecular oxygen in the atmosphere, are mainly generated by solar heating (Forbes, 1995). In contrast, the lunar tides are primarily generated by the gravitational forcing of the Moon on the lower atmosphere (Chapman & Lindzen, 1970). Since the forcing is virtually constant, the variations of lunar tides in the MLT region are related to the changes in the background atmosphere (Forbes & Zhang, 2012;Sandford et al., 2007;Yamazaki et al., 2012). Hence, the determination of the lunar tides in the MLT region is an excellent tool to understand the coupling of the atmosphere (Forbes et al., 2013;Stening et al., 1997).Using numerical simulations, Chapman and Lindzen (1970) predicted that the lunar tides potentially consist of three main modes: Long-term semimonthly lunar tide, lunar diurnal tide, and lunar semidiurnal (LSD) tide. The observed LSD with a period of 12.42 hr consists of a migrating tide with zonal wavenumber 2 (M 2 ) and a nonmigrating component, which has the largest amplitude among the three modes (Pedatella et al., 2012;Vial & Forbes, 1994). Observations of the LSD in the atmosphere are difficult because the frequency of LSD is very close to the frequency of solar semidiurnal tide. Thus, a high spectral resolution in the data sets is required to distinguish the solar and lunar components.