Strateole 2 observation of stratospheric equatorial waves and their interaction with QBO winds

Abstract:

Tropical stratospheric gravity waves and Kelvin waves are known to play an important role in driving the quasi-biennial oscillation (QBO) of the zonal winds of the tropical lower stratosphere. The phase of the QBO provides a significant source of forecast skill at seasonal-to-subseasonal timescales, but global forecast models struggle to realize this potential, likely because the simulation of tropical gravity waves still poses a challenge, and accurate observations to inform gravity wave parametrizations are an urgent need.

We report on unique observations of tropical waves from long-duration, super-pressure balloons in the lower stratosphere during the recent Strateole-2 campaign measurements Nov. 2019 through Feb. 2020. These balloons floated on constant density levels at altitudes of 18 - 20 km thus continuously sampling tropical stratospheric air from a quasi-Lagrangian frame of reference over several months. That way the Strateole-2 measurements provide high-resolution observations suitable to analyse characteristics as e.g. the momentum flux and intrinsic frequency, of these waves.

In our study we focus on the flight of the tropical tropopause layer balloon 3 (TTL3) which achieved more than one full circumnavigation of the Earth. This balloon also carried the Reel-down Aerosol Cloud Humidity and Temperature Sensor RACHuTS which provided high-resolution vertical profiles of temperature, humidity, and aerosols in the TTL. The high resolution temperature measurements allow new insight into the fine vertical structure of tropical waves over 2 km depth below the balloon gondola. Combining the in-situ balloon measurements with the RACHuTS observations we identify multiple different wave types and analyse their respective interaction with the QBO zonal winds. One wave event in particular shows evidence for wave-modulated mixing layers and the interaction of a large-scale wave near a critical level that provides new insight into wave driving of QBO winds in the lowermost stratosphere.