Vertical winds and turbulence over Thumba

Abstract

Results of the analysis of barium cloud released over Thumba at an altitude 93 km are presented. In the initial stage, the cloud was in the form of two rings coupled together by a knot and later on the rings were distorted to become elongated loop-like structures. A strong north-south wind shear of 30 m/s/km and a vertical wind gradient are observed which explain the distortion of the initial rings into the loops. The lower loop showed development of turbulence, 280s after the release. However, the upper loop did not show any evidence of turbulence throughout the observed period (10 min), but expanded by wind. The delay of 280 s in the onset of turbulence for the lower loop suggests co-existence of turbulent and non-turbulent regions in the 93–95 km altitude range. The top portion of the upper loop moved upward upto an altitude of about 100km where it recorded maximum upward velocity of 19 ±7 m/s. The values of vertical velocities in the 95–100 km height region coupled with the vertical velocities recorded from other barium clouds released by the same rocket at the lower thermospheric altitudes reveal a wavy profile that is suggestive of the presence of an internal atmospheric gravity wave. The altitude of the turbopause observed in the present experiment is 95 ±2 km, about 10 km lower than that observed earlier. It is suggested that gradients in the vertical wind could cause lowering of turbopause level.