Droplet clustering via droplet spacing measurements from RICO

During the RICO field project, the NCAR C-130 carried three instruments capable of measuring droplet spacing in one dimension, the direction of flight. Two of these (2D-S and PDI) are relatively new and designed to be free of sample tube artifacts.

The possibility of observable small scale (~1 cm) droplet clustering was first presented by Baker 1992* using a standard FSSP equipped to record droplet spacing. A statistical test was used to determine the existence of centimeter scale in-homogeneities in the recorded data. However, the FSSP sample tube, with a diameter of a couple centimeters, may have been responsible for creating the in-homogeneities. The third instrument of the C-130 during RICO capable of measuring droplet spacing is the FFSSP. The FFSSP has both improved electronics and optics compared to the standard FSSP. The FFSSP uses the same sample tube however.

Data from all three instruments are analyzed and compared to determine whether detectable small-scale in-homogeneities exist in RICO small cumulus clouds. We cautiously report that the cm structure may be real.

* Baker, B.A., 1992: Turbulent entrainment and mixing in clouds: A new observational approach, J. Atmos. Sci., 49, 387-404.