Mesosphere-stratosphere-troposphere radar observations of characteristics of lower atmospheric high-frequency gravity waves passing through the tropical easterly jet

In this study, we have examined the characteristics of high-frequency gravity waves (5-50 min periodicity) over a tropical region using the mesosphere-stratosphere-troposphere (MST) radar installed at Gadanki (13.5 degrees N, 79.2 degrees E), India. The MST radar (53 MHz) was operated continuously for similar to 15.5 h during 1700-0840 LT on 2-3 June 2005. During this period, a strong unstable wind shear region existed above the tropical easterly jet at the height of tropopause. This has provided an excellent opportunity to study the characteristics of internal atmospheric high-frequency (similar to 15-40 min periodicity) gravity waves, generated in the boundary layer and passing through the shear layer. The study reveals the generation of higher-frequency (5-15 min periodicity) gravity waves from this strong shear region and their vertical propagation both below up to a few kilometers and above the shear layer (similar to 0.5 km thick). These waves showed upward propagation even above 20 km in the lower stratosphere, indicating that unstable shear layers are the important source of momentum and energy fluxes that contribute significantly to the middle atmospheric dynamics in terms of gravity waves. Further, a close association was also observed between the dissipating gravity waves and the distinctly enhanced signal-to-noise ratio and Doppler spectral width of the MST radar echoes. The present observation of radiation of high-frequency gravity waves that propagate vertically upward from a strong wind shear region located immediately below a highly stratified layer is in accordance with the "direct mechanism" explained by nonlinear numerical simulation studies. For the first time, the present study illustrates the existence of layers of polarized refractive index structures in the heights of 10-15 km.