A Theoretical Study of the Evolution of Mixed-Phase Cumulus Clouds

Abstract

A detailed warm and cold cloud microphysical description has been incorporated into a one-dimensional time-dependent, Eulerian cumulus cloud model. The relationship of the ice phase to the development of precipitation, and interactions between cloud microphysical processes and cloud dynamics, have been examined. Ice crystals in the form of plates, columns and dendrites have been allowed to interact among themselves and with water droplets to produce graupel, unrimed and rimed ice crystals and snowflakes. The effects of possible ice multiplication have been investigated by allowing ice splinters to be produced during riming and during the freezing of isolated drops. Comparative studies with shallow, nonprecipitating maritime and continental cumulus clouds showed the importance of the drop size distribution in determining the dominant mechanism for forming graupel embryos. In the maritime cloud, graupel embryos originated on frozen drops produced after collisions with ice crystals. In the continental cloud, graupel embryos tended to originate on ice crystals. The riming-splintering mechanism never produced significant increases in ice crystal concentrations in the continental cloud, but in the maritime cloud it produced orders of magnitude increases after the updraft velocity had decreased sufficiently to allow the formation and sedimentation of larger drops and graupel. The ejection of four ice splinters during the freezing of each isolated drop (>50 µm) in the maritime cloud, increased graupel concentrations at cloud top by 100 times. When the number of ejected splinters was reduced to one per drop in the maritime cloud, the concentrations of graupel were only increased by about a factor of two. Even the ejection of four splinters per isolated freezing drop in the continental cloud did not significantly increase the concentrations of ice particles. Model case studies of artificial seeding indicate that it should be possible to produce large increases in the concentrations of graupel in the maritime cloud we studied by seeding it at cloud top with artificial ice nuclei in concentrations of 250 l?1. The model showed significant increases in the concentrations and sizes of graupel in the continental cloud when it was seeded with 250 crystals l?1 at cloud base.