Simulated diurnal pulses in developing tropical cyclones

The diurnal cycle is one of the most fundamental characteristics of tropical cyclones (TCs), influencing a wide range of processes including cloud coverage, rainfall, and the timing of intensity changes. Recent studies have identified outward-propagating features known as diurnal pulses (DPs). These pulses appear as rings of localized cooling in brightness temperature that propagate radially outward from the storm center over several hours. DPs occur in the majority of TCs and have been observed across all storm intensities, from tropical storms to major hurricanes. Furthermore, DPs have been linked to changes in TC structure and intensity.

Because DPs have mainly been observed via brightness temperature anomalies in the cirrus canopy, their vertical structure remains poorly understood. While prior work indicates that they may extend vertically, the complete depth of these pulses throughout the storm has yet to be determined. Here, we show that DPs are not confined to the upper troposphere but instead represent column-depth features, producing coherent anomalies across multiple atmospheric variables.

Despite the growing body of literature on DPs, their propagation mechanism remains an open question. Several hypotheses have been proposed, with particular emphasis on inertial–gravity waves. Support for this interpretation comes mainly from observed propagation speeds, which are consistent with theoretical inertial–gravity wave speeds in the TC environment, as well as from the strong latitude dependence of the pulses. Here, we investigate this hypothesis using classical dry gravity wave theory, finding propagation angles and inter-variable phase relationships that are consistent with theoretical expectations.

Beyond their propagation, the timing of DP initiation has been a central focus. Originally, DPs were described using a “diurnal clock” framework (Dunion et al., 2014), with initiation typically occurring between 00:00 and 04:00 local solar time (LST) and outward propagation to a radius of approximately 200 km by 04:00–08:00 LST. However, accumulating observational evidence suggests that this timing is not universal. Accordingly, we examine the preferred nighttime initiation of DPs and investigate the physical mechanisms that may underlie this tendency.