Zones of Increased and Decreased Droplet Concentration in Stratiform Clouds

Abstract

The finescale, spatial inhomogeneity of cloud-droplet concentration in stratiform clouds is studied using FSSP-100 data with 10-m or better resolution from 1700 km of cloud penetrations between +5° and ?20°C. The data are analyzed in terms of the sizes and spacings of zones where the droplet concentration is significantly below average (droplet-free being one category) or significantly above average. Such zones typically occur throughout stratiform cloud layers, most frequently near their boundaries. The average length of flight path through zones of significant departure from average droplet concentration is up to several tens of meters. These zones probably arise from turbulent entrainment of clear air at cloud top; inhomogeneities of temperature, humidity, and vertical velocity at cloud base; and turbulent velocity fluctuations within cloud that either activate interstitial condensation nuclei or evaporate significant numbers of droplets. Calculations confirm that vertical velocity fluctuations can produce inhomogeneities like those observed.