Structure of tropical cyclones at the onset of rapid intensification in the HWRF model

In this study, structure of tropical cyclones (TC) at the onset of the rapid intensification (RI) is examined using the cloud-permitting version of the Hurricane Weather Research and Forecast (HWRF) model. Experiments with different vortex initial vertical structures in different ambient environment show that the RI onset in the HWRF model is determined by a specific constraint in the dynamical and thermodynamic structure including i) an warm anomaly of 1-3K, ii) a moist column with relative humidity > 90 % within the storm central region, and iii) low-level tangential flow ≥12 m s-1. Regardless of the vortex structures or the environment conditions, the model vortex does not intensify if the above constraint is not established. Such requirement in the model moisture and dynamical constraint for the RI onset explains why the RI onset is much delayed in dry experiments as compared to the onset in moist experiments.