Turbulence effects on warm-rain autoconversion in precipitating shallow convection

A parametrization of the rain formation in warm clouds is developed which includes the effects of turbulence on the collision rate of droplets in a cloud. It is shown that already moderate turbulence with dissipation rates of 400 cm2s−3 can lead to a significant speed-up of the rain formation corresponding to an increase in the autoconversion rate by a factor of 4–6 depending on the size of the droplets. Large-eddy simulations of trade wind cumuli also produce a significant enhancement of rain formation through turbulence effects on cloud microphysics. In small cumulus clouds, the highest turbulent dissipation rates occur near cloud-top, which is also the decisive region of the cloud for the initial rain formation.