Turbulence and Droplet Clustering in Shallow Cumulus: In-Situ PIV Observations

Shallow cumulus clouds over tropical oceans play a fundamental role in the Earth's energy budget. Their microphysical properties influence variations in 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. To address this, we analyze in-situ measurements from the EUREC⁴A field campaign over the tropical Atlantic near Barbados between January and February 2020, focusing on Particle Image Velocimetry (PIV) and holographic images taken within shallow cumulus clouds. By combining planar PIV and 3D holographic droplet data, we examine both turbulence dissipation rate and cloud droplet spatial distribution across a broad range of scales from sub-millimeter to ~10 cm, overcoming the limitations of dimensional reduction that often diminish such signals. The clustering of the cloud droplets was found to occur under intense turbulence dissipation, particularly in precipitating clouds, even in the low Stokes numbers. These findings suggest that turbulence-enhanced collisions may be one of the key processes driving rapid droplet growth in shallow cumulus clouds.