Impact of varying CCN concentration on the precipitation process in a simulated convective storm

The effects of the concentration of cloud condensation nuclei (CCN) on cloud microphysics have long been recognized, though the impact of CCN on the precipitation process in convective storms has been relatively unexplored. In the present study, the impact of varying CCN concentration on the microphysical structure and evolution of a small multicell storm is simulated with NSSL's 3-dimensional cloud model (COMMAS). The 2-moment microphysics scheme used for this study predicts the mass mixing ratio and number concentration of cloud droplets, rain, ice crystals, snow, graupel, and hail. CCN concentration is predicted as a single-category, monodisperse size spectrum approximating small aerosols. Bulk graupel and hail particle densities are also predicted as functions of rime layer density. Rime density in turn is a function of droplet size (affected by CCN concentration) and impact speed. Particle density (graupel and hail) is also used as a roughness parameter to scale the drag coefficient in the expression for particle fallspeed. The prediction of hydrometeor number concentration is particularly critical to the resolution of secondary ice nucleation at higher temperatures (-5 < T < -20 C) in the mixed phase updraft region, where ice crystals may be produced both by rime fracturing (Hallett–Mossop process) and by splintering of freezing drops in addition to a range of primary nucleation mechanisms. The prediction of cloud droplet and rain drop concentration and mass and their evolution proceeds through condensation growth, quasi-stochastic coalescence, and vertical transport to force the production of graupel embryos via drop freezing (Bigg freezing and crystal contact nucleation).

Model sensitivity tests with a range of ambient CCN concentrations (50 to 2000 per cubic cm) control the mean droplet size at cloud base, thereby modulating drop growth via condensation-coalescence in environments effectively ranging from maritime to continental. Higher CCN concentrations reduce the collision-coalescence formation of rain/drizzle, gradually increasing the proportion of precipitation mass produced by a graupel-based, cold-cloud riming process relative to the warm rain process. Even at the highest CCN concentrations, the primary process of simulated graupel initiation is via drop freezing. Even in the event of high CCN, the vapor supply in the updraft remains sufficient for droplets to eventually grow large enough via condensation to accelerate drop coalescence growth. The time-integrated volume containing graupel at heights above the freezing level increases monotonically with increasing CCN according to a power law relationship.

Precipitation in the simulated storm initiates as raindrops via stochastic collision-coalescence in regions of high cloud water content just below the freezing level. However as expected, formation of significant rain mixing ratios and simulated radar echo are delayed to later times and higher altitudes as CCN concentration is increased. Raindrops lifted in updraft begin freezing at temperatures around -10 deg. C to form graupel. The simulated time-height reflectivity, graupel mass, rain mass, and updraft volume all show systematic variations in their evolutions as base CCN concentration increases. Updraft volume tends to show three maxima at increasing altitudes of 2-3 km, 6-7 km, and 8-11 km at times of about 25-30 min, 40-55 min and 55-65 min in the simulation. Peak integrated rain mass above the freezing level is maximized at CCN concentration of about 500 cm-3, whereas maximum integrated graupel mass tends to increase monotonically with increasing CCN concentration. Average graupel density tends to decrease with increasing CCN concentration above 500 cm-3, as smaller droplets and lower graupel fall speeds lead to lower-density rime formation. As a byproduct of the variation of simulated cloud and precipitation content with CCN, additional cloud simulations in which optional electrification mechanisms are activated manifest a sensitivity of microphysically-based charge separation and lightning production to CCN changes.