Geographical differences in the tropical precipitation-moisture relationship and rain intensity onset

The precipitation-water vapor (P-r) curve in the tropics is a well-known relationship that highlights the presence of a threshold moisture value— demarcating highly precipitating environments from weakly precipitating ones. A closer inspection of the convective and stratiform components of this relationship reveals that the threshold moisture value could well be an environmental throttle on the intensity and frequency of tropical mesoscale convective systems (MCS). In this study, we present a survey of the precipitation-water vapor curve across tropical land as well as ocean. We find that the P-r curve is not universal in shape, with stark differences between land and ocean and amongst continents and ocean basins. For example, precipitation amounts pick up at lower moisture values and display a less rapid pickup over land than ocean. Amongst oceans, certain regions possess a particularly sharp pickup: the East Pacific, East Atlantic and North China Sea—regions that are marked by more stable environments. These results suggest that entrainment might not be the sole factor that decides the peculiar shape of the P-r curve and that the large-scale environmental forcing could have a role to play. We also discover a separate, earlier pickup for the intensity of convective rain and its subsequent linear relationship with environmental moisture. Ultimately, the study and scrutiny of the tropical P-r curve with emphasis on the components of the quintessential mesoscale convective system—shallow convective, deep convective and stratiform systems—offers a method of statistically quantifying the environmental modulation of MCSs. These results hold potential utility in the parameterization of tropical organized convection, as well as an addition to the suite of climate model diagnostics.