Observations of Liquid Persistence and the Development of Ice in Oklahoma Convective Clouds

Abstract

During late spring of 1986 and September of 1987 the University of North Dakota operated an instrumented aircraft in convective clouds over Oklahoma to help assess the potential for rain increase cloud modification over the state. The Oklahoma Rainfall Enhancement Program seeding plan focuses on seeding growing clouds which contain a persistent region of supercooled liquid water but are lacking in ice crystals. The objectives of this study were to determine the persistence of supercooled liquid water in the sampled clouds and to determine where, when and how the ice phase developed in these clouds. The persistence of supercooled liquid water was examined in terms of cloud top lifetime, defined as the projected time from first penetration to zero liquid water content at the sampllng level. Ice particle data were studied in primarily a qualitative sense. In general, the clouds had high liquid water contents initially, but cloud top lifetimes were relatively short. Liquid water decay rates had a large standard deviation. The development of ice occurred very rapidly in many of the clouds, although there were several missions where ice development was not significant. The ice phase was apparently being enhanced in lower portions of the clouds, perhaps through ingestion of ice from neighboring cells or by ice multiplication.