Significant Amplification of Instantaneous Extreme Precipitation With Convective Self‐Aggregation

Abstract

Abstract This work explores the effect of convective self‐aggregation on extreme rainfall intensities through an analysis at several stages of the cloud lifecycle. In addition to increases in 3‐hourly extremes consistent with previous studies, we find that instantaneous rainrates increase significantly (+30%). We mainly focus on instantaneous extremes and, using a recent framework, relate their increase to increased precipitation efficiency: the local increase in relative humidity drives larger accretion efficiency and lower re‐evaporation. An in‐depth analysis based on an adapted scaling for precipitation extremes reveals that the dynamic contribution decreases (−25%) while the thermodynamic is slightly enhanced (+5%) with convective self‐aggregation, leading to lower condensation rates. When the atmosphere is more organized into a moist convecting region and a dry convection‐free region, deep convective updrafts are surrounded by a warmer environment which reduces convective instability and thus the dynamic contribution. The moister boundary‐layer explains the positive thermodynamic contribution. The microphysic contribution is increased by +50% with aggregation. The latter is partly due to reduced evaporation of rain falling through a moister near‐cloud environment, but also to the associated larger accretion efficiency. Thus, a potential change in convective organization regimes in a warming climate could lead to an evolution of tropical precipitation extremes significantly different than that expected from thermodynamical considerations. The relevance of self‐aggregation to the real tropics is still debated. Improved fundamental understanding of self‐aggregation, its sensitivity to warming and connection to precipitation extremes, is hence crucial to achieve accurate rainfall projections in a warming climate.

Keywords

• self‐aggregation
• convection
• precipitation extremes
• microphysics