High-Frequency Gravity Waves and Kelvin-Helmholtz Billows in the Tropical UTLS, as Seen From Radar Observations of Vertical Wind

Abstract

The present study analyzes novel observations of vertical wind $(w)$ in the tropical upper troposphere-lower stratosphere obtained from a radar wind profiler in Cochin, India. Between December 2022 and April 2023, 63 consecutive 4 hr curtains of $w$ were measured with a vertical spacing of 180 m and a sampling time step of 44 s, thus resolving almost the whole spectrum of vertical motions. Spectra of $w$ strongly vary with altitude. They are generally flat up to the local Brunt-Väisälä frequency (BVF), but sometimes exhibit a peak of $w$ variance closer to BVF, a feature which may be attributed to trapped gravity waves. At other times and locations, the $w$ profiles reveal Kelvin-Helmholtz billows. Finally, the variability of $w$ variance over the 4 month campaign period is investigated. Using brightness temperature from geostationary satellites as a convective proxy, it is found that $w$ variance is highly correlated with fluctuations in convective activity.