An airborne study of vertical structure and microphysical variability within a small cumulus

On 27 July 1981, as part of the CCOPE experiment, the University of Wyoming King Air research aeroplane made penetrations at six levels ranging from 590mb (‐4.9°C) to 770mb (7.2°C) through a nonprecipitating, unglaciated cumulus cloud whose top was at about 560 mb (‐7°C) and base about 780mb (8°C). the entire flight sequence—from the top downwards—took about six minutes. Principal measurements were of droplet size distribution N(d), number concentration N and liquid water content L, measured at 10 Hz (≃10m spatial resolution); drop‐count N′ (50Hz, ≃2m); temperature T. Also we calculated the fraction F of cloud base air in the mixture sampled (1Hz, ≃100m). the cloud was substantially subadiabatic throughout the volume studied. Paluch analyses, the observed air motions near cloud top, buoyancy profiles and the observed distribution of L and F with altitude demonstrated that the cloud was diluted principally by entrainment from cloud top. Considerable scatter was found in all microphysical and thermodynamic variables on all spatial scales. Large variations in L were primarily attributable to fluctuations in N and N′. Bimodal spectra were sometimes found at the interface between statistically smooth and highly variable regions. the breadth of the droplet spectra and the size of the largest droplets in the condensate spectrum were not systematically related to the degree of dilution. Calculations showed that a small fraction of the droplets (≃100 litres−1) exhibited superadiabatic growth of up to 4 m̈m diameter at the intermediate levels, but not near cloud base or cloud top. Support for the idea that droplets in lower concentrations (≃10 litre−1) grew even faster was provided by 1‐D probe measurements of droplets of up to 75 m̈m diameter in the central and upper regions of the cloud.