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Dependencies of four mechanisms of secondary ice production on cloud top temperature in a continental convective storm


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’ model (AC). 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 (102 to 103) because fragmentation in ice-ice collisions dominates at long time scales, driving the ice concentration towards 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 103 for cloud top temperatures close to the −12°C level, mostly due to the HM process in typically young clouds with their age less than 15 mins. At higher altitudes with temperatures of −20 to −30°C, the predicted IE ratios were smaller, ranging from 10 to 102, and mainly resulted from fragmentation in ice-ice collisions.