The Importance of Cloud Top Lifetime in the Description of Natural Cloud Characteristics

Abstract

The microphysical and dynamical characteristics of 156 natural summer cumulus clouds have been documented for three locations in North America: Yellowknife, Northwest Territories; Thunder Bay, Ontario; and Miles City, Montana. The measurements (469 aircraft penetrations) were made in six consecutive years from 1975 to 1980 using state-of-the-art cloud physics instrumentation. All measurements discussed were obtained near ?7°C. Yellowknife clouds had low liquid water contents (0.3 g m?3) and high large (>70 ?m) particle concentrations (0.9 L?1). Thunder Bay clouds had higher liquid water contents (1 g m?3) and low large particle concentrations (0.04 L?1). Miles City clouds, which were similar in dimensions to those near Yellowknife, had low liquid water contents (0.3 g m?3) and low large particle concentrations (0.1 L?1). Yellowknife and Thunder Bay clouds produced precipitation through the warm and cold rain processes but the observed Miles City clouds did not precipitate naturally. Measurements of cloud top lifetime appear to be useful in explaining the differences between locations. Cloud top lifetime is defined in this paper in terms of the persistence of cloud liquid water at the penetration altitude near ?7°C. Lifetime was found to increase with cloud width in each location but did not appear closely related to initial LWC, cloud depth, cloud base temperature, inside-outside cloud temperature difference, environmental humidity, turbulent energy dissipation rate, energy flux, heat flux nor wind shear.