Buoyancy-Precipitation Coupling in the Life Cycle of Tropical Mesoscale Convective Systems

Mesoscale convective systems (MCSs), organized thunderstorms on a scale larger than individual convection, account for nearly half of the tropical precipitation and play an essential role in the tropical hydroclimate. However, the initiation and the thermodynamic states associated with the MCS life cycle remain unclear, which sets the challenging representation of MCSs and related moist dynamics in atmospheric models. To characterize the onset of convective processes and precipitation, a measure of lower-tropospheric buoyancy that accounts for a combination of instability and entrainment has provided baseline empirical buoyancy-precipitation relationships for deep convection. The present work explores such relationships at different MCS life stages, aiming to pin down observational benchmarks for MCS-related thermodynamics. A 20-year MCS dataset based on a satellite-based tracking algorithm (FLEXTRKR; Feng et al., 2023) is applied to identify tropical MCS tracks. The MCS life cycle is then separated into the initiation, growing, mature, decaying, and end stages. GPM-IMERG precipitation and ERA-5 thermodynamic variables are implemented to characterize the evolving rainfall and buoyancy components (instability and subsaturation) in the MCS life cycle. Analysis of buoyancy-precipitation relationships suggests that a higher probability of high-end buoyancy is revealed in MCSs at the growing and mature stage, leading to a considerable contribution to conditional precipitation. At lower buoyancy, the relative precipitation contribution from the decaying stage of MCSs becomes greater. A clear transition of buoyancy components emerges MCSs evolve. The MCS-related buoyancy components initiate at high instability and lower saturation, move towards low instability and high saturation at the mature stage, and end with much lower instability and saturation than the initial stage. Such characteristics of transition in the environmental instability and saturation are consistent between MCSs over oceans and land. The conditional precipitation rate as a function of the thermodynamic buoyancy measure exhibits a dependence on the MCS life stage, reflecting the overall time dependence of the precipitation process associated with microphysics and dynamics. An idealized cloud-permitting simulation of evolving MCSs is performed for comparison.