The Variations of Gravity Wave Activities During the Formation of the Large Horizontal Wind Shears in the Static State Lower Thermosphere.

Large horizontal winds and shears across the mesopause region have been observed by sounding rockets and satellites but with limited simultaneous temperature profiles to date. This feature is critical to the diagnosis of the insitu dynamic conditions. Theories suggest this feature may be related to variations of the atmosphere static stability, Brunt‐Väisälä frequency square (N2), and the wave activity in the mesosphere and lower thermosphere (MLT) that cannot be resolved by the models. Since 2020, the Na Doppler lidar at Utah State University has attained several hundred hours of wind and temperature profiles in the MLT up to ∼110 km altitude. This paper focuses on the variations of these large winds and shears (>40 m/s/km), along with their potential relation to the activities of the various scales of gravity waves in the static state upper mesosphere, measured coordinately by the Advanced Mesospheric Temperature Mapper operating alongside the lidar. The investigation reveals that larger shears tend to occur in the meridional direction than the zonal direction and are observed more frequently in winter. The shears are comparable in both directions in summer and more large shears were observed in zonal direction. In addition, small shears occur when the medium‐scale waves are completely or partially blocked by the mean wind, though no convincing evidence relates small‐scale wave activities with these large winds and shears. Furthermore, small and decreasing N2 in the upper mesopause region are associated with insignificant magnitude of the shears. Plain Language Summary: Throughout the year, extremely large horizontal wind shears in the atmosphere occur in the mesopause region near or above 105 km altitude around the globe. Theoretical and numerical simulations suggest their formation is closely related to the atmospheric gravity waves generated in the troposphere, but experimental observations at these altitudes are difficult to obtain. In this paper, an advanced wind and temperature lidar and a temperature mapper are utilized at Utah State University to investigate this topic comprehensively. The coordinated campaigns by the two instruments are capable of observing these large shears and the gravity wave behaviors in the mesopause region simultaneously, providing critical information to understand this dramatic atmospheric feature. Key Points: The large horizontal winds and the associated shears occur more frequently in the meridional wind than the zonal windThe very large shears occur more in the winter than summerThe large shears are highly correlated with medium scale gravity waves with large horizontal phase speed and long horizontal wavelength [ABSTRACT FROM AUTHOR]