Moisture Pre‐Concentration in the Boundary Layer: A Key Mechanism for the Rapid Intensification of Simulated Super Typhoon Yagi (2024)

Accurate prediction of the rapid intensification (RI) of tropical cyclones (TCs) remains a major operational and scientific challenge. A primary source of forecast uncertainty lies in the representation of turbulent mixing processes within the planetary boundary layer (PBL), which critically regulates the vertical transport of moisture and heat into the storm core. This study investigates this fundamental issue through high-resolution numerical simulations of Typhoon Yagi (2024). We demonstrate that the vertical profile of turbulent mixing, rather than its magnitude alone, is a decisive control on RI efficiency. Our analysis reveals that, in contrast to PBL schemes which produce excessive or insufficient mixing, the Bougeault–Lacarrère (BouLac) scheme uniquely promotes RI by generating a distinct peak in eddy diffusivity for moisture (approximately 200 m² s⁻¹) within the mid-to-upper boundary layer. This peak effectively confines and concentrates moisture, creating a localized reservoir. This process, which we term "moisture pre-concentration," is pivotal. The pre-concentrated moisture is then efficiently tapped and transported upward by the storm's already well-organized secondary circulation. This coordinated transport fuels robust latent heat release in the core, directly supporting the development of a strong warm core and a rapid pressure fall. Sensitivity experiments systematically modifying the moisture diffusivity (Kq) value within the BouLac framework reveal a non-monotonic response of TC intensity to mixing strength. A key finding is that, provided a sufficiently high moisture concentration is achieved via the "pre-concentration" mechanism, even experiments with substantially reduced bulk moisture transport can generate RI and high lifetime maximum intensity. This underscores that the spatial organization and vertical distribution of boundary layer moisture are more critical for RI onset than the total moisture abundance. Conversely, excessive diffusion (very high Kq) leads to vertical dilution of moisture, stifling intensification, while insufficient mixing fails to organize the moisture reservoir. Moisture-budget diagnostics confirm that RI efficiency is maximized when turbulent mixing primarily acts to organize low-level moisture for coordinated convective uplift, rather than merely strengthening the secondary circulation itself. Ultimately, these findings advance the mechanistic understanding of TC intensification. They emphasize that accurately resolving the vertical structureof PBL turbulent mixing, and its role in preconditioning the moisture field, is paramount for improving the physical basis and skill of future RI forecasts.