The Role of Depositional Heating in the Transition to Strong Precipitation in Tropical Deep Convection

Over tropical oceans, the transition between weak and strong precipitation is a highly non-linear function of saturation fraction, the column precipitable water scaled by the saturation value. This critical transition accompanies a switch from shallow to deep convection. The transition to deep convection is associated with both a strengthening of dynamic flows within the cloud core and a change of physical size and shape of the cloudy envelope. This study seeks to better understand the microphysical controls on both of these changes through the lens of the precipitation dependence on saturation fraction. The functional dependence of precipitation on saturation fraction is shown to be composed of an exponential pick up and linear growth regime. These different dependencies are found to result from somewhat different physical processes active between these two regimes. Through impacts to both the exponential and linear growth regimes, depositional heating results in a negative moisture-rainfall feedback above the transition point despite its enhancement to convective updrafts and dry surface convergence. This feedback forces wider convective and anvil regions of deep convective clouds.