Global Characteristics of On-the-Clock and Off-the-Clock Tropical Cyclone Diurnal Pulses

The radially-outward propagating, cloud-top cooling, diurnal pulse (DP) is a prominent feature in tropical cyclones (TCs) that has significant impacts on structure and intensity. Much of the research conducted on DPs, however, comes from observational studies of Atlantic basin TCs. Our recent study characterized DPs over ocean basins globally and examined their environmental conditions. DPs were found to occur on 52% of TC days globally, most frequently over the Northwest Pacific. The median duration and propagation distance of DPs were 12–15 h and 500–600 km, respectively. Interestingly, the DP signal significantly weakened and became slower while propagating through the 200–400-km annulus during 09–12 local time (LT). Although the mean propagation speed was 11–13 m s^-1, persistent DPs (lasting >15 h) propagated at speeds similar to internal inertial gravity waves (5–10 m s^-1). DPs were more likely to occur over warm sea surface temperatures (SSTs), in low shear, and with a moist mid- to upper- troposphere.

Most DPs initiated in the TC inner core overnight, in phase with deep convection; however, non-negligible numbers (~40%) of DPs initiated during the afternoon to evening hours and/or outside the inner core. This timing and initiation location differs from the diurnal pulse clock proposed by Dunion et al. (2014) and numerical modeling studies of the TC diurnal cycle. This presentation will explore the characteristics of “on-the-clock” and “off-the-clock” DPs including their basin variability, latitudinal distribution, and environmental characteristics. Preliminary results indicate the frequency of off-the-clock DPs increases with latitude, vertical wind shear, and contrastingly, TC intensity, with large interbasin variability.