Abstract. The effects of aerosol on warm convective cloud cores are evaluated using
single cloud and cloud field simulations. Three core definitions are
examined: positive vertical velocity (Wcore), supersaturation
(RHcore), and positive buoyancy (Bcore). As presented in Part 1 (Heiblum et al., 2019),
the property Bcore⊆RHcore⊆Wcore is
seen during growth of warm convective clouds. We show that this property is
kept irrespective of aerosol concentration. During dissipation core
fractions generally decrease with less overlap between cores. However, for
clouds that develop in low aerosol concentrations capable of producing
precipitation, Bcore and subsequently Wcore volume fractions may
increase during dissipation (i.e., loss of cloud mass). The RHcore
volume fraction decreases during cloud lifetime and shows minor sensitivity
to aerosol concentration. It is shown that a Bcore forms due to two processes: (i) convective
updrafts – condensation within supersaturated updrafts and release of
latent heat – and (ii) dissipative downdrafts – subsaturated cloudy downdrafts
that warm during descent and “undershoot” the level of neutral buoyancy. The
former process occurs during cloud growth for all aerosol concentrations.
The latter process only occurs for low aerosol concentrations during
dissipation and precipitation stages where large mean drop sizes permit slow
evaporation rates and subsaturation during descent. The aerosol effect on the diffusion efficiencies plays a crucial role in the
development of the cloud and its partition to core and margin. Using the
RHcore definition, it is shown that the total cloud mass is mostly
dictated by core processes, while the total cloud volume is mostly dictated
by margin processes. Increase in aerosol concentration increases the core
(mass and volume) due to enhanced condensation but also decreases the margin
due to evaporation. In clean clouds larger droplets evaporate much slower,
enabling preservation of cloud size, and even increase by detrainment and
dilution (volume increases while losing mass). This explains how despite
having smaller cores and less mass, cleaner clouds may live longer and grow
to larger sizes.