Vertical structure of tropical cyclones at onset of the rapid intensification in the HWRF model

In this study, the tropical cyclone structure at the onset of rapid intensification (RI) is examined using the cloud-permitting version of the Hurricane Weather Research and Forecast (HWRF) model. Idealized experiments with different vortex initial vertical structures in different environments show that the HWRF model vortex possesses a specific constraint in the dynamical and thermodynamic structure at the RI onset including (i) a warm anomaly of 1-3 degrees K, (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 vortex structures or environment conditions applied in this study, model vortex does not intensify if the above constraint is not established. Such a requirement in the model moisture and dynamical structure at the RI onset can explain why the RI onset is much delayed in dry experiments as compared to the onset in moist experiments.