Dampening of the precipitation response to aerosol pollution from turbulence in cumulus clouds

The susceptibility of rain formation to aerosol emissions is a key aspect of aerosol-cloud-precipitation interactions. Using aircraft observations and a detailed cloud model with a state-of-the-art particle-based representation of microphysics, it is shown that the enhancement of drop collision-coalescence from turbulent flow in clouds -- which is neglected in current climate models -- leads to earlier onset of rain in warm cumulus clouds and significantly dampens the precipitation susceptibility to aerosol loading. Enhanced drop coalescence from turbulence substantially increases the production of embryonic drizzle drops just above cloud base, which in turn act as seeds that accelerate rain drop growth at mid and upper cloud levels even in highly polluted conditions. In contrast, pollution aerosols strongly inhibit rainfall when the commonly assumed gravitational-only collision kernel is used in the model and turbulent coalescence is neglected. There is also a large impact of turbulent drop coalescence on the mean cloud water amount and optical depth. Overall, turbulence-enhanced drop coalescence strongly influences the response of warm cumulus clouds and precipitation to aerosol loading, suggesting that the effects of turbulent coalescence should be included in climate model representations of aerosol-cloud-precipitation interactions and aerosol indirect radiative forcing.