Cloud conditions favoring secondary ice particle production in tropical maritime convection

Progress in understanding the formation of ice in lower-tropospheric clouds is slowed by the difficulties in characterizing the many complex interactions that lead to ice initiation and to the dynamic, non-steady-state nature of the clouds. The present study characterizes the conditions where secondary ice particles, specifically identified as needle or thin columnar types, are observed in tropical maritime convection with modest liquid water contents during the Ice in Clouds Experiment-Tropical (ICE-T), based out of St. Croix, U.S. Virgin Islands, and the NASA African Monsoon Multidisciplinary Analyses (NAMMA) in 2006 sampling from Cape Verde, Africa. The properties of the cloud droplet populations relevant to the secondary ice production process and the ice particle populations are characterized as a function of temperature and vertical velocity. These secondary ice particles are observed primarily in regions of low liquid water content and weak vertical velocities. Two situations are examined in detail. First, ice formation is examined by following the tops of a group of ICE-T chimney clouds as they ascend and cool from a temperature of +7° to −8°C, examining the production of the first ice. Then, using the data from a cloud system sampled during NAMMA, the authors elucidate a process that promotes ice multiplication. The intention is that this study will lead both to a better understanding of how secondary ice production proceeds in natural clouds and to more realistic laboratory studies of the processes involved.