Parameterization of boundary layer height based on helicity and its application in tropical cyclone numerical simulation

Abstract

This study introduces a helicity-based parameterization method for determining the planetary boundary layer (PBL) height to better capture the complex dynamics of the tropical cyclone (TC) boundary layer (TCBL). By integrating this method into the Yonsei University (YSU) PBL scheme of the China Meteorological Administration (CMA) Mesoscale Model (CMA-MESO), the PBL height is dynamically determined using helicity as a proxy for frictional forcing in TCBL regions, while retaining the traditional bulk Richardson number (Rib) method in areas with weak or ambiguous helicity signals. Simulations of 28 Northwest Pacific TCs in 2022 demonstrate that this approach has negligible impact on track forecasts but substantially reduces the systematic underestimation of TC intensity compared to the traditional Rib-based method. The improvements in TC intensity predictions primarily originate from helicity-modulated PBL height adjustments, particularly the distinct elevation of PBL height within the eyewall region. Analysis of PBL tendencies indicates that elevated PBL height enhances low-level stratification instability through deepened heating within the PBL and expanded cooling at the PBL top. Meanwhile, the deepened frictional layer augments low-level convergence through strengthened agradient forcing induced by momentum dissipation. These thermodynamic and dynamic modifications foster convective concentration in the eyewall, where intensified diabatic heating interacts with high inertial stability to elevate heating efficiency, thus driving TC intensification. These findings highlight that the helicity-based parameterization method outperforms the Rib-based method by better determining the eyewall PBL height, whose deeper structure enhances low-level convergence and unstable stratification, providing a practical pathway to improve TC intensity prediction in numerical models.