Fine structure of cloud droplet concentration as seen from the Fast-FSSP measurements. Part II: Results of in situ observations

A new feature of cloud structure has been discovered while analyzing the measurements obtained in situ in 57 clouds by the Fast Forward-Scattering Spectrometer Probe (FSSP). By means of a novel technique of statistical analysis, it is shown that droplets form distinct "communities" of about 1-cm scale that differ in concentration, thus creating a highly inhomogeneous cloud microstructure (inch clouds). Those droplet clusters can be found all over the cloud volume and appear to be induced by droplet inertia within a turbulent flow. An increase in turbulence intensity and droplet inertia results in an increase of concentration fluctuations. The authors believe that this finding is the first direct evidence of turbulence-inertia impact on droplet motion in clouds that leads to formation of microstructure conductive to precipitation formation.