Diurnal Variations on the Initiation Time and Intensification Rate of Rapidly Intensifying Tropical Cyclones

This study investigates whether the initiation time and intensification rate (INTRATE) of intensifying tropical cyclones (TCs) vary diurnally and how they are related to deep convection. TC intensifying events are identified and classified into slowly intensifying (SI) and rapidly intensifying (RI) events. RI events last ∼42 hr on average, much longer than SI events. More importantly, the onset of the TC intensification, especially RI, markedly peaks at 00–06 local time. However, the INTRATEs of both RI and SI events show very weak diurnal variability. The INTRATE continues to increase after RI initiation and slightly peaks in the late afternoon (at 90% significance level). Inner‐core convection of all intensifying events maximizes in the early morning, in phase with the peak initiation time. In short, our results suggest that the nocturnally enhanced inner‐core convection may play a role in triggering TC intensification (e.g., RI), but not maximizing the INTRATE. Plain Language Summary The outbreak of intense convection in the inner core of tropical cyclones (TCs) may promote the rapid intensification of TCs. It is well known that TC inner‐core convection varies diurnally. Recent studies found that the TC intensification rate exhibits a diurnal cycle that is in phase with the inner‐core deep convection. These studies further proposed that the nocturnally enhanced inner‐core convection may simultaneously promote the TC intensification rate. However, it is unknown whether and how the diurnal cycle of the intensification rate varies among different TC intensifying periods. Also, whether the initial time of the TC intensification exhibits an evident diurnal signal? Based on the analysis of 30‐year TC track data, this study found that TC intensification including the RI prefers to initiate in the early morning, well in phase with the inner‐core deep convection. In contrast, the intensification rate of RI TCs continues to increase after RI initiation therefore does not peak immediately with the deep convection. These results suggest that the inner‐core convective outbreak may be important in triggering the TC intensification especially RI, while the intensification rate is impacted or dominated by multiple factors that are not on the diurnal scale. Key Points The rapid intensification (RI) of tropical cyclone (TC) is more likely to initiate in the early morning and lasts ∼42 hr on average TC intensification rates of RI events slightly peak in the late afternoon