Indian MST radar observations of gravity wave activities associated with tropical convection

Abstract

MST radar observations at Gadanki (13.5° N, 79.2° E) of high frequency (few tens of minutes) gravity waves generated most likely by convection are presented. The experiments were conducted during the months of May–June (19 May 1995, 05 June 1995 and 06 June 1996) of summer season, which is likely to be highly convective after the onset of south–west monsoon over southern part of India. The excitation and vertical propagation of gravity waves are found to display specific characteristics pointing convection as a main source. The intriguing characteristics of rising reflectivity pattern, coupled with rise in vertical wind component and turbulence from lower troposphere to upper troposphere within hour of time (on 19 May 1995 and 06 June 1996) are noticed. On the other day of experiment, horizontal large spread in reflectivity pattern confined mostly below 10 km of altitude has been observed. During this period wind disturbances are found to posses comparatively large magnitude of momentum flux at mid-tropospheric levels. Usually enhanced reflectivity regions are found to be accompanied by strong updraft and downdraft in vertical wind component ( w). An interesting feature in the case of 05 June 1995 is the appearance of vertical wind disturbances well up to lower stratosphere. The effect of these enhanced vertical wind is in conformity with the observed increase in the momentum flux values even up to lower stratospheric altitudes. Typical wave amplitude in vertical wind disturbances during convection vary from 1– 3 m/ s with some sudden enhancement in amplitude of the order of 8– 10 m/ s for some short interval of time. An effort has been made to discuss these results in the light of existing theoretical concepts viz. mechanical oscillator effect, obstacle effect and direct thermal forcing for generating the convection waves (gravity waves generated due to convection).