Mesoscale convective systems (MCSs) contribute a majority of rainfall over tropical oceans. Latent heating associated with tropical oceanic MCS precipitation shapes the evolution of global weather and climate across various spatial and temporal scales. Hence, understanding what controls the precipitation of tropical oceanic MCSs is of central importance when it comes to improving global weather and climate predictions. Despite the importance, satisfactory representation of tropical oceanic MCSs, especially their rainfall characteristics, has been a lasting challenge for global models. This challenge is due, at least in part, to our incomplete understanding of the environmental controls on tropical oceanic MCS precipitation.

Using a 20-year global MCS tracking dataset produced by the Water Cycle and Climate Extremes Modeling (WACCEM) project, we found that MCSs initiating in a mesoscale environment with enhanced lower-free-tropospheric moisture, warmer middle troposphere, stronger low-level ascent, and stronger deep-layer (surface-400 hPa) wind shear tend to produce more precipitation during their lifetimes. While most of these environmental factors are correlated with one another, the deep-layer shear is not. A rapid pickup in MCS lifetime rainfall is found when the lower-free-tropospheric specific humidity exceeds 10 g kg^-1. This nonlinearity is mostly dominated by the nonlinear increase in MCS area. On the other hand, both MCS area and rain rate increase quasi-linearly with the deep-layer shear. The increase in rain rate is related to the enhancement of heavy precipitating convective activity with deep-layer shear. It suggests that the overturning circulation associated with deep-layer shear may widen/enhance convective updrafts and lead to stronger deep convective activity within tropical oceanic MCSs.