A Realistic and Multi-scale Dynamical Study to Understand Thin Tropical Cirrus Cloud Life Cycle

Thin cirrus clouds near the tropical tropopause, at altitudes of about 16–18 kilometers, play an important role in determining how much water vapor enters the stratosphere, which matters for several reasons including the role of humidity in determining the amount of ozone is available to protect us from harmful ultraviolet radiation. Thin cirrus clouds also matter for the heat balance of the tropics as they block outgoing infrared radiation, thereby contributing to the ambient conditions for tropical weather systems. The evolution of these clouds is strongly shaped by atmospheric waves, ranging from small, rapidly varying gravity waves (also called buoyancy waves) to much larger planetary-scale waves that produce cooling and warming over periods from minutes to several days. This project will use unique balloon observations and detailed cloud simulations to determine how these waves control the formation and persistence tropopause-level cirrus clouds. The findings will help improve the representation of these clouds and stratospheric water vapor in atmospheric models, supporting more reliable assessments of their effects on weather and environmental conditions. By linking atmospheric dynamics and cloud physics, the project will also strengthen connections between the communities of researchers studying cirrus clouds and researchers studying wave dynamics who often work in isolation from each other. This project will use balloon observations of both waves and thin cirrus clouds to determine how realistic atmospheric dynamical variability controls cloud lifecycle. The project will combine concurrent observations of clouds, temperature, humidity, aerosols, and atmospheric waves from the Stratéole-2 long-duration balloon campaigns with detailed cloud-microphysics simulations (using the Community Aerosol and Radiation Model for Atmospheres (CARMA)). The investigators will reproduce a well-observed cirrus event that persisted for several days in the cold phase of a large-scale tropical wave and test how the addition of smaller, higher-frequency gravity waves changes ice formation, crystal growth, sedimentation, and cloud lifetime. The project will also examine how gravity-wave spectra vary across the tropics and how that variability affects simulated cirrus properties. Finally, the simulations will be used to estimate cloud optical properties, dehydration efficiency, and radiative effects. 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.