Fine-Scale Droplet Clustering in Stratocumulus Clouds from Airborne Digital Holography: 3D Radial Distribution Functions

Data obtained from the Holographic Detector for Clouds (HOLODEC) during the Cloud System Evolution in the Trades (CSET) experiment was utilized to explore sub-3-cm scale clustering among cloud droplets. The detected spatial positions of individual cloud droplets were used to compute the three-dimensional radial distribution function in individual holograms. Individual hologram radial distribution functions are subject to too much sampling variability to reveal reliable estimates of droplet clustering, but successive holographic images from a single flight can be averaged to give statistically meaningful estimates of the radial distribution function. In each flight interval explored, small but unambiguous evidence of positive correlations among cloud particle positions were observed on mm to cm scales. Several properties of the flight-averaged radial distribution functions qualitatively match expectations from previous theoretical and numerical studies in this weakly turbulent environment.