In-Situ Measurements of Turbulence Fluctuations and Droplet Clustering in Shallow Cumulus

Shallow cumulus clouds over tropical oceans play a fundamental role in the Earth’s energy budget. Their microphysical properties strongly influence cloud albedo and climate feedbacks in the tropics. However, the mechanism behind rapid raindrop formation and the role of turbulence in this process remain uncertain, particularly the role of small-scale turbulence in rapid droplet growth. To address this challenge, we analyze in-situ measurements collected during the EUREC⁴A field campaign over the tropical Atlantic near Barbados between January and February 2020. The campaign deployed Max Planck CloudKite, a tethered balloon system, from research vessels, yielding approximately 200 hours of airborne observations within shallow cumulus clouds. A subset of this dataset includes simultaneous planar Particle Image Velocimetry (PIV) and holographic measurements, providing the ability to resolve both turbulent flow properties and cloud microphysics.

The localized measurement of turbulence and droplet size distribution allows, for the first time, the simultaneous investigation of cloud microphysics and turbulence at small scales. Spatial organization of droplets shows a strong correlation with turbulence in the examined clouds. Increased turbulence strengthens voids and clustering regions, particularly in precipitating clouds. We further examine the relationship between droplet size and spatial distribution to comparatively assess the influence of turbulence and entrainment on droplet clustering. This study provides a hint of the crucial role of turbulence in precipitation within the examined shallow cumulus clouds.