An Explanation for the Existence of Supercooled Water at the Top of Cold Clouds

Abstract

Aircraft measurements in many cold cloud systems have found a narrow layer of supercooled water to exist at the cloud top, even at temperatures colder than ?30°C. We show in this paper that the imbalance between the condensate supply rate and the bulk ice crystal mass growth rate at a wide range of temperatures and updraft speeds is sufficient to produce this liquid layer near cloud top because of the unique property that the ice crystals located there are small. Calculations are also presented to determine the minimum magnitude and maximum depth of a sustained updraft required to produce supercooled water near cloud top from an initially ice saturated cloud containing a population of ice crystals. Potential sources of the updraft circulations required to produce the liquid layer near cloud top are discussed. Finally, we consider the impact of the liquid layer on both cloud microstructure and precipitation processes.