Entrainment and Fine-Scale Mixing in a Continental Convective Cloud

Abstract

Aircraft data from a penetration through several young, nonprecipitating, continental cumulus turrets are examined. Fine-scale measurements, some of a new type, show that mixing is highly nonuniform during the active stages of the turret development. The observations indicate that mixing involves both bulk entrainment, which produces a coarse mixture of cloud and clear air on a scale of meters or tens of meters, and fine-scale (molecular) mixing which changes the local microphysical properties. Both processes take place concurrently, and by the time fine-scale mixing has substantially diluted a cloud volume, bulk entrainment of clear air has broken it up into small patches. Dilution of droplet concentration to less than half of the adiabatic concentration is typically associated with patchy mixing on scales of the order of 10 m or less. These observations, if they can be generalized, have implications for large droplet production through mixing, as modeled in microphysical parcel models: if the air parcel is assumed to be large, then it cannot be diluted. whereas if the air parcel is assumed to be substantially diluted, then it must be small and hence cannot ascend very far without losing its identity through mixing. Estimates indicate that the above constraints severely limit the potential for large droplet production in continental cumuli.