Strateole-2: Atmospheric Wave Influences on Cirrus, Water Vapor, and Global Circulation Near the Tropical Tropopause

This award supports analysis of observations collected in the first two deployments of the Strateole-2 field campaign, which took place in the winters of 2019/2020 (8 flights) and 2021/2022 (17 flights). Strateole-2 is a long-duration ballooning campaign organized by the Centre National d'Etudes Spatiales (CNES), the French national space agency. The balloons were launched from the Seychelles and drifted around the equator for up to two months following the winds of the lower stratosphere.

The campaign used a variety of instruments to observe the tropical tropopause layer (TTL), the transition zone between the troposphere (the domain of clouds, precipitation, and weather systems) and the stratosphere (the cold and dry region above it which contains the ozone layer).

A primary focus of Strateole-2 is gravity waves, meaning wave motions in the atmosphere for which the primary restoring force is gravity. Such waves are generated by vertical motions occurring in the deep convective clouds over tropical oceans but they can have horizontal wavelengths much larger than convective systems, with sufficient extent in latitude to be affected by the Coriolis force and its variation with latitude.

These waves are of interest because they provide the vertical transport of horizontal momentum which drives the quasi-biennial oscillation (QBO), a reversal of the winds in the equatorial stratosphere that starts at high levels and descends over the next two years. The up-and-down motions in the waves also affect the temperature of the TTL, and the colder temperatures occurring in the upward displacements promote the condensation of ambient water vapor, promoting the formation of thin cirrus clouds which block outgoing infrared radiation and thus have an effect on Earth's energy balance.

The formation of ice particles in cirrus clouds also reduces the amount of water vapor entering the stratosphere, where it has additional effects on the planetary energy balance.

Work performed here focuses specifically on waves which are long, with horizontal wavelengths of perhaps 1,000km, but shallow, with vertical wavelengths less than a kilometer, and periods of a few days. The project takes advantage of the NSF-funded suite of instruments flown in the deployments, including the radio occultation receiver (ROC) supported under AGS-1642650, the fiber optic temperature profiler (FLOATS) developed under AGS-1642277, and the reel-down instrument package (RACHuTS) developed under AGS-1643022.

It also uses data from instruments developed by researchers in France, including a balloon-borne lidar (BeCOOL) and an infrared water vapor sensor (PicoSTRAT), as well as onboard thermometers, barometers, and GPS receivers. These data sources are used in combination with satellite data and reanalysis products, and specialized simulations with enhanced vertical resolution are performed by unfunded collaborators at the European Centre for Medium-Range Weather Forecasts.

One issue to be addressed is the generation of these pancake-like waves, which must somehow be generated by localized convection despite their long horizontal wavelengths. Work on momentum transport considers wave breaking and subsequent generation of turbulence as well as the calculation of wave momentum flux.

The work has broader impacts through its potential for improving the representation of the QBO in models used for long-range weather prediction. Models have difficulty representing the QBO, in part due to the difficulty of representing the effect of gravity wave momentum transport, and the QBO plays important roles in modulating the potentially predictable forms of subseasonal weather variations.

In addition, the radiative effects of cirrus clouds and stratospheric water vapor are among the factors that matter for understanding and anticipating anthropogenic climate change. In addition, the PIs conduct public outreach and education by participating in an effort to correct scientific misunderstanding in social media, working with colleagues to develop easy-to-use tutorials that give users access to scientific data and information.

The project provides support and training to a postdoctoral fellow, thereby promoting the scientific workforce in this research area.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.