Relationships of the Diurnal Pulse to Structure and Intensity of Tropical Cyclones

The radially-outward propagating, cloud-top cooling, diurnal pulse (DP) is a prominent feature in tropical cyclones (TCs) that has important implications for changes in TC structure and intensity. By using an objective diurnal-pulse identification algorithm, this study characterizes DPs both globally and regionally over various ocean basins and examines their relationships to TC structure and intensity. Active DPs (ACTDPs) occur on 52% of TC days globally. They are the most frequent over the Northwest Pacific (NWP, 60.4%). The median duration and propagation distance of ACTDPs are 12–15 h and 500–600 km, respectively. Some ACTDPs (20–25%) last longer than 18 h and propagate as far as 700–800 km. Although the mean propagation speed of ACTDPs is 11–13 m s^-1, persistent ACTDPs (lasting >15 h) mostly propagate at speeds similar to internal inertial gravity waves (5–10 m s^-1). Most ACTDPs initiate in the inner core overnight, in phase with inner-core deep convection. Nearly half of the ACTDPs are coupled with the outward propagation of precipitation within TCs. The TC inner-core deep convection is significantly enhanced on ACTDP days. Specifically, the 20 dBZ echo top in the upshear quadrant of TCs rises the most evidently with the occurrence of the ACTDP, leading to a more symmetric structure of the inner-core convection. The occurrence ACTDPs may promote the rapid intensification (RI) of TCs. The frequency and duration of ACTDPs are strongly correlated with the TC intensification rate. RI TCs have a markedly higher frequency of the very long-duration ACTDPs (≥15h) and longer mean pulse duration than steady-state and gradually intensifying TCs. Overall, the DP is a potentially useful signal for the RI of TCs.