Various mechanisms of secondary ice production (SIP) cause multiplication of numbers of ice particle, after the onset of primary ice. A measure of SIP is the ice enhancement ratio ("IE ratio") defined here as the ratio between number concentrations of total ice (excluding homogeneously nucleated ice) and active ice-nucleating particles (INPs). A convective line observed on 11 May 2011 over the Southern Great Plains in the Mesoscale Continental Convective Cloud Experiment (MC3E) campaign was simulated with the "Aerosol-Cloud" (AC) model. AC is validated against coincident MC3E observations by aircraft, ground-based instruments, and satellite. Four SIP mechanisms are represented in AC: the Hallett-Mossop (HM) process of rime splintering, and fragmentation during ice-ice collisions, raindrop freezing, and sublimation. The vertical profile of the IE ratio, averaged over the entire simulation, is almost uniform (10(2) to 10(3)) because fragmentation in ice-ice collisions dominates at long time scales, driving the ice concentration toward a theoretical maximum. The IE ratio increases with both the updraft (HM process, fragmentation during raindrop freezing, and ice-ice collisions) and downdraft speed (fragmentation during ice-ice collisions and sublimation). As reported historically in aircraft sampling, IE ratios were predicted to peak near 10(3) for cloud-top temperatures close to the -12 degrees C level, mostly due to the HM process in typically young clouds with their age less than 15 min. At higher altitudes with temperatures of -20 degrees to -30 degrees C, the predicted IE ratios were smaller, ranging from 10 to 10(2), and mainly resulted from fragmentation in ice-ice collisions.
