Large wind shears and stabilities in the mesopause region observed by Na lidar at midlatitude

Unexpected large horizontal winds and wind shears in the lower thermosphere have been observed by rocket soundings and lidars for decades. From 4 years of the Colorado State University sodium wind-temperature lidar data set (2002-2005, total of ~1600 nocturnal hours), we observed an altitude distribution of large wind velocity and wind shears between 80 and 105 km, similar to the results of chemical release experiments. Our lidar data show conclusively that when the observed wind shears are plotted as a function of squared Brunt-Vaisala frequency, N2, they are below the value corresponding to the Richardson number of 1/4, which is a necessary condition of the onset of dynamic instability. This suggests that large wind shears can be sustained in the region of high static stability, e.g., in the lower thermosphere, where large wind shears are often observed by the rocket sounding. The full-diurnal-cycle lidar data enables the extraction of tidal wave components with periods of 24 h, 12 h, 8 h and 6 h, therefore allowing us to reveal the strong correlation of 60%, between large wind shears (>50 m/s/km) and tidal waves. The lidar measured seasonal variation of N2 and tidal amplitudes in the mesosphere and lower thermosphere (MLT) are found to be consistent with the difference in altitude distribution of strong wind shears between winter and summer.