Production of Ice Crystals in Slightly Supercooled Cumulus

Abstract

PERHAPS the most puzzling observation in cloud physics reveals that some slightly supercooled cumulus clouds, with summit temperatures only a few degrees below 0° C, contain high concentrations of ice crystals—two to four orders of magnitude greater than the concentrations of ice nuclei measured in clear air at the same level. According to a series of careful and detailed aircraft observations and measurements by Mossop and his colleagues, such high concentrations of crystals occur only in aged cumulus that has been in existence for some time and acquired widths of several kilometres but are absent from newly rising cloud towers. They are also absent from slightly supercooled stratocumulus clouds. Mossop et al.^1,2 report small ice crystals in concentrations of 10 to 100 l⁻¹ in clouds with summit temperatures as high as − 8° C but always in association with rimed ice pellets of diameter of order 1 mm in concentrations of order 1 l⁻¹ and with large droplets of r>250 µm in concentrations of order 100 m⁻³. The concentrations of ice nuclei active at − 8° C are below those that can be reliably measured by conventional techniques but probably do not exceed 1 m⁻³.