CCN effects on simulated storm electrification and precipitation
A range of CCN concentrations (50 to 15000 cm-3) were tested in a weak CAPE (Convective Available Potential Energy) environment (918 J/kg) that produced weakly multicell convection. Greater CCN concentration has the expected effects of shifting the initial formation of rain drops via collision-coalescence to later times and higher altitudes. Even at the highest CCN concentrations, however, vapor supply in the updraft remains sufficient for droplets eventually to grow large enough for coalescence to become appreciable before the appearance of graupel, so the warm-rain process is not completely shut down in this case. Peak updraft values increased modestly with increasing CCN from 16.8 m/s (50 cm-3) to 19.5 m/s (500 cm-3). Above CCN of 500 cm-3, peak updraft varied little from 19.5 m/s.
Time-integrated mass of graupel increases monotonically with increasing CCN up to about 2000-3 and decreases somewhat at higher CCN concentrations (Fig. 1). Time-integrated updraft volume generally increases with greater CCN concentrations, as well, but reached a plateau for CCN greater than 500 cm-3. Other effects of CCN concentration were variable. The simulated storms had maximum flash rates of 0 to 17 per minute and from 0 to 150 total flashes (Fig. 1). The most intense electrification (total lightning sources) was for CCN concentrations of 1000 to 3000 cm-3, dropping off toward lower and higher CCN values (Fig. 1; no flashes at 50-100 cm-3, and 3-4 total flashes for CCN >= 8000 cm-3.