Aerosols are postulated to alter moist convection by increasing cloud
droplet number concentration. Cloud-resolving model simulations of
radiative-convective equilibrium show that increased cloud droplet
number concentration leads to higher convective mass flux, seemingly in
line with a popular hypothesis which links the convective invigoration
to delayed rain formation allowing more cloud liquid water to be frozen.
Yet, the same phenomenon is also present in an alternative model
configuration with only warm-rain microphysics, suggesting that one does
not have to invoke ice microphysics. The key mechanism lies in the
different vertical distributions of the increases in cloud liquid
re-evaporation and in water vapor condensation, causing a dipole pattern
that favors convection. This is further supported by a mechanism-denial
experiment in which weakened cloud re-evaporation tends to mute
invigoration. This work represents a major advancement of the
process-level understanding of aerosol effects on convection.