Characteristics of monochromatic gravity waves in the mesosphere observed by Rayleigh lidar above Logan, Utah

Atmospheric gravity wave (AGW) characteristics were examined using Rayleigh lidar data collected over a period spanning 11 years above Logan, UT (41.7°N, 111.8°W), over an altitude range of 45–90 km. Variations of the relative density perturbations obtained with 3-km vertical resolution and 10-min temporal resolution are used to identify the presence of monochromatic gravity wave features throughout the mesosphere. The measured vertical wavelengths ${\lambda }_z$ ranged over 6–19 km with 12–14 km the most prevalent and the measured wave period $\tau$ ranged over 2–8 h with 5–6 h the most prevalent. The values of ${\lambda }_z$, $\tau$ and mean wind velocity u were used to infer vertical phase velocities $c_z$, horizontal wavelengths ${\lambda }_x$, horizontal phase velocities $c_x$ and horizontal distances to the source region x. There appears to be a clear seasonal dependence in $c_z$, $\tau$, $c_x$, ${\lambda }_x$, and x but not in ${\lambda }_z$. The $c_z$ values maximize in summer, $\tau$ and x maximize in winter whereas $c_x$ and ${\lambda }_x$, maximize in winter and summer but minimize in spring and autumn. The values of x ranged over 1300–5000 km for waves at 60 km and ∼2000–7500 km for waves at 90 km. The source of these AGWs is, thus, far away. Furthermore, for one of these monochromatic waves to exist all night or appear to extend over 45–90 km, it has to originate from a very extended region and persist for a long time.