Highly Localised Droplet Clustering in Shallow Cumulus Clouds

TL;DR

This paper reports the discovery of highly localized droplet clustering in shallow cumulus clouds on scales of half a metre or less, challenging existing assumptions about cloud microphysics and impacting weather and climate models.

Contribution

It presents new holographic measurements revealing strong local droplet clustering at unprecedented small scales in shallow cumulus clouds.

Findings

Detected clustering on scales of 12 cm or less

Contrasts with previous weak clustering observations at larger scales

Implications for cloud microphysics and climate modeling

Abstract

The growth, lifetime, number density, and size of water droplets in warm atmospheric clouds determine the evolution, lifetime and light transmission properties of those clouds. These small-scale cloud properties, in addition to precipitation initiation, have strong implications for the Earth's energy budget since warm clouds cover large geographic areas. Spatio-temporal correlations on the millimetre scale and smaller may or may not affect these properties of clouds. To date, the pioneering measurements of such correlations in marine stratocumulus clouds have relied on averaging over holographically reconstructed volumes spanning at least ten kilometres. These have revealed weak but widespread spatial clustering of cloud droplets. Here we present results of strong localised clustering on scales of half a metre or less from holographic measurements collected with the Max Planck CloudKite in shallow cumulus clouds in the mid-Atlantic trade wind region near Barbados, with a spatial separation of only 12~cm between measurement volumes. This observation challenges the foundations of our understanding of cloud microphysics at the droplet scale, with implications for cloud modelling in weather and climate prediction.