First Meteor Radar Observations of Gravity Waves in the Mesosphere and Lower Thermosphere Over Yinchuan (38.8°N, 106.8°E), China

By using the WHU-YC meteor radar observations (2019.11–2021.10) in Yinchuan (38.8°N, 106.8°E), we studied the winds, tides, and gravity waves (GWs) in the Mesosphere and Lower Thermosphere (MLT) over northwestern China for the first time. Distinct annual and semi-annual oscillations (AO and SAO) appeared in the background winds, showed maximum and minimum in summer (∼40 m/s) and spring (∼−14 m/s), respectively, and exhibited wind shear in summer and winter. Affected by the background winds, GW activity similarly exhibited AO, with maximum kinetic energy density (KED) in May (∼4.0 × 10⁻³ J/m³). KEDs consistently declined with altitude, indicating severe GW dissipation in the MLT. GW's momentum fluxes (MFs) are predominantly positive below ∼88 km and reverse negative above. MFs reached ∼10⁻⁴ J/m³ in summer and autumn, while weaker in winter and spring. The GW drag reached ∼443.8 m/s/d in zonal, closely correlated with the tendency, highlighting the substantial GW impact on the mean flow. The intense GW activities here approached some hot spots, possibly due to Yinchuan's characteristic topography, resulting in more terrain-excited GWs. Despite no discernible seasonal variation in GW's intermittency, the Gini coefficient steadily increased with altitude, implying GWs upward with smaller MF are easier to dissipate. The diurnal tide (DT) significantly modulated GWs at almost all heights. Despite comparable amplitudes of the semidiurnal tide (SDT) to DT, the weaker SDT modulation only concentrated in 84–86 km because of its longer vertical wavelength, emphasizing tides' vertical wavelengths are crucial parameters in GW's modulation.