What controls precipitation in shallow cumulus clouds?

We explore macro and micro-physical controls on shallow cumulus convection using large eddy simulation of clouds observed during the Rain in Cumulus over the Ocean (RICO) experiment. We track hundreds of individual clouds, and express rain rates in terms of power-law functions of cloud liquid water path, drop concentration and cloud lifetime. We show that, as expected, liquid water path has a stronger influence (by a factor of about 2) on the ability of a cloud to precipitate than does drop concentration. The inclusion of cloud lifetime into the expressions greatly improves the integrated rainfall estimates and results in robust estimates that should be suitable for use in large scale models. We generalize our results by exploring relationships between the recently proposed concept of precipitation susceptibility (the sensitivity of rain rate to a micro-physical perturbation) and precipitation efficiency (the ratio rain volume to water condensed), and place our results within the context of other studies.