PNNL

Abstract

Mesoscale convective systems (MCSs) contribute a majority of rainfall over tropical oceans. However, our understanding of the environmental controls on tropical oceanic MCS precipitation remains incomplete. Using 20-year of satellite observations, reanalysis data, and MCS tracking, 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.