Dampening of the Precipitation Response to Aerosol Pollution From Turbulence in Cumulus Clouds

Using aircraft observations and a cloud model with particle-based microphysics, it is shown that the enhancement of drop collision-coalescence from turbulence in clouds not only leads to earlier onset of rain in warm cumulus clouds as past studies have suggested, but also significantly dampens the precipitation susceptibility to aerosol loading. Enhanced drop coalescence from turbulence substantially increases the production of drizzle embryos 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 and turbulent coalescence is neglected. 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 Earth system model representations of aerosol-cloud-precipitation interactions and aerosol indirect radiative forcing.