AbstractObservations from tethered balloon‐borne instrumentation of small‐scale air motion, temperature fluctuations and microphysical structure occurring within nocturnal stratocumulus are presented and discussed. The cloud top was marked by a very sharp and thin inversion layer whose vertical displacement was monitored with an acoustic sounder. The velocity field indicates the presence of strong downwards convection within the cloud, in which radiatively cooled air is transported away from the region near cloud top in discrete plumes. Droplet data show the preferential location of larger cloud droplets (>7 μm radius) within the convection cells. Some aspects of the droplet observations are explained in terms of a simple Lagrangian model. Although entrainment is excluded from the model the predictions are consistent with the observations. Entrainment effects at cloud top result primarily in droplet concentration fluctuations with little change in the shape of the droplet spectrum, which the model suggests is mainly determined by the effects of radiation on droplet growth by condensation and the effects of vertical motion. Finally, some characteristics of turbulence through the cloud layer, and boundary layer generally, are examined and related to observations from the cloud‐free convective boundary layer and data from laboratory exper
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