Simultaneous measurements of droplet clustering and turbulence in cumulus clouds

Droplet clustering in clouds due to turbulence may act to accelerate the warm rain process. Specifically, it has been hypothesized that cloud droplets will cluster at scales on the order of the Kolmogorov microscale and below due to finite droplet inertia and strong fluid vorticity. Theory and computations suggest that clustering strength depends on the droplet Stokes number, which is proportional to d² ε^1/2, where d is droplet diameter and ε is turbulence energy dissipation rate. To evaluate this hypothesis we have made simultaneous (in time and space) measurements of droplet spatial distribution, droplet size distribution, and turbulent velocity. The latter allows calculation of local energy dissipation rate. Measurements were made using the Airborne Cloud-Turbulence Observation System (ACTOS) deployed via helicopter. To increase confidence in the clustering measurements two instruments, collocated in space, and based on different operating principles and measurements geometries, were used for the clustering measurements: a Modified-Fast-FSSP and a Phase-Doppler Interferometer. Preliminary analysis shows unambiguous clustering signatures, especially near cloud edges where strong velocity shear is present. The correlation with droplet Stokes number, however, is not as strong as expected, implying that the Stokes number may not be the only dominant dimensionless parameter governing the presence of small-scale droplet clustering.