A Trajectory-Based Framework for Diagnosing Convective Lifecycle and Organization in Monsoon Rainfall: Coupled Evolution of Moisture, Energy, and Dynamics

Operational forecasts of monsoon heavy rainfall suffer from the inadequate representation of convective lifecycle and three-dimensional organization of moisture. This study introduces a novel trajectory-based lifecycle framework that synchronizes atmospheric profiles with precipitation stages for dynamic tracking of convective. Using 16 years (2005–2020) of ERA5 reanalysis and rain-gauge data over the subtropical monsoon region, Hong Kong, we diagnose the coevolution of Integrated Relative Humidity (IRH), Moist Static Energy (MSE), and dynamical fields across rainfall intensities.  Results show that rainfall intensity correlates with the moist layer depth: organized deep convection (e.g., heavy rain) is coupled with full-tropospheric saturation (IRH ≥0.88 and ≥0.85 at the lower- and mid-upper tropospheric layers) and robust ascent, whereas disorganized shallow convection (e.g., light rain) is confined to the lower troposphere. Critically, the framework reveals an IRH trajectory of an upward-expanding moistening pattern synchronized with peaking MSE gradients (>10 kJ kg⁻¹) and strengthening low-level convergence, underscoring a coupled energy-dynamic sequence fundamental to the lifecycle of organized convective systems. Moreover, IRH anomalies from seasonal baselines are more consistent predictors of intense events than absolute thresholds, highlighting the importance of environmental preconditioning. This trajectory-based approach provides physical insights into convective organization in subtropical monsoons and a process-oriented tool for evaluating and improving the representation of convection in models.