Gravity Waves Generated From Convection and Wind Shear as Observed by MST Radar over the Indian Tropical Station of Gadanki
Abstract:Using the 53 MHz very high frequency radar at the Indian tropical station of Gadanki (13.5N, 79.2E), the characteristics of high frequency (period of oscillation of few minutes to few tens of minutes) internal atmospheric gravity waves generated from convection as well as strong wind shears are studied in detail in the heights of 1.5 km to 20 km. Along with that detailed studies are also carried out about the vertical propagation characteristics of low frequency gravity waves (period of oscillation of few hours) passing through strong wind shears and the tropopause. Normally three causative mechanisms are suggested for the generation of gravity waves from convection: (1) latent heating of the atmosphere, (2) oscillation updrafts and downdrafts and (3) air flows crossing deep convective clouds. In the present work, we have taken one convection event (occurred for a few hours during night time on 06 June 2011) as a case study and found from the analyses of the horizontal and vertical winds measured by the MST radar that gravity waves were generated by both the mechanisms of latent heating and mechanical oscillator effects. The dominant periods of oscillation observed are ~13 min., ~17 min. and ~26 min. It is found that these frequencies satisfied well the dispersion relations associated with non-hydrostatic high frequency gravity waves. It is concluded that all the three mentioned mechanisms play significant role in determining the vertical propagation of gravity waves.
In an another event during 23-25 January 2012, it is found that gravity waves with periodicity of ~7 hours, generated in the lower troposphere, propagates to higher heights of lower stratosphere without hindered by strong wind shears and tropopause conditions existed in between. However, high frequency gravity waves (periodicity of few tens of minutes) generated from strong wind shears near the tropopause propagate only a few kms in height both below and above the shear region.