CCN effects on simulated storm electrification and precipitation

The effects of concentration of cloud condensation nuclei (CCN) on cloud microphysics have long been recognized, but the resultant effects on storm electrification are relatively unexplored. In the present study, a high-resolution 3D model is employed with 2-moment microphysics (hydrometeor mass and number concentration) and electrification and lightning to simulate a storm observed in Oklahoma during the TELEX-2004 experiment (Mansell et al. 2010, J. Atmos. Sci.). CCN concentration is predicted as a single category monodisperse size spectrum approximating small aerosols. Graupel and hail particle densities are also predicted and are mainly determined by rime density. Rime density in turn is a function of droplet size (affected by CCN concentration) and impact speed. Graupel density is also used as a crude roughness parameter to scale the drag coefficient in the fall speed.

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.