Environmental Controls on MCS Lifetime Rainfall over Tropical Oceans

Mesoscale convective systems (MCSs) contribute the majority of rainfall over tropical oceans. However, a comprehensive understanding of the environmental controls on tropical oceanic MCS precipitation is still lacking. Using 20 years of satellite observations, reanalysis data, and MCS tracking, we found that that MCSs developed in a meso-β-scale environment with higher lower-free-tropospheric specific humidity, warmer middle and cooler lower tropospheric temperature, stronger upward motion, and stronger deep-layer (from surface to 400 hPa) wind shear tend to precipitate more during their lifetime. While most environmental factors are closely correlated with each other through the enhanced convective activity prior to the initiation of MCSs, deep-layer shear is not. Further analyses indicate that the lifetime rainfall of tropical oceanic MCSs is jointly controlled by the environmental lower-free-tropospheric moisture and deep-layer shear. MCS lifetime rainfall shows a non-linear increase with the lower-free-tropospheric moisture, and a rapid pickup can be found as the specific humidity increases above 10 g kg^-1. This can be attributed to the rapid increase of MCS precipitation area with increasing lower-free-tropospheric moisture. However, the MCS lifetime and mean rain rate exhibit a quasi-linear increase with lower-free-tropospheric specific humidity. On the other hand, the increase of MCS lifetime rainfall with the environmental deep-layer shear is mainly contributed by the quasi-linear increases of MCS rainfall area and mean rain rate. In addition, we found that idealized cloud-resolving simulations driven by the composite pre-MCS environments can realistically reproduce the impacts of the environmental lower-free-tropospheric moisture and deep-layer shear on the upscale growth and rainfall characteristics of tropical oceanic MCSs. By leveraging these simulations, we will discuss the underlying physical mechanisms that give rise to the observed quasi-linear and non-linear relationships.