Date and Time
Research has documented a clear diurnal cycle of cloudiness and rainfall in tropical cyclones (TCs): enhanced convection occurs overnight, precipitation peaks near sunrise, and the cirrus canopy expands radially outward throughout the day, reaching its maximum areal coverage in the early evening hours. Despite these consistent signatures, open questions remain as to how the diurnal cycle is linked to inner-core convective processes and whether it is a column-deep phenomenon or mainly tied to outflow-layer dynamics. Investigating these questions is relevant to TC forecasting as the diurnal cycle of clouds and rainfall, and therefore latent heating, has implications for forecasting storm structure and intensity, as evidenced by the diurnal cycle in objective measures of TC intensity and the extent of the 50-kt wind radius.
To investigate whether diurnal pulses in TCs are column-deep features and to further explore their relationship with convection, our recent research has examined the diurnal cycle using WWLLN lightning data. Outwardly propagating, diurnal lightning signals occurred 61% of the time, with persistently electrically-active pulses (lasting at least 9 h) occurring on 38% of pulse days. Electrically-active pulses had more lightning located right-of-shear, the preferred quadrant for outer-rainband lightning activity, and were associated with more favorable environmental conditions than electrically-inactive pulse days. We conducted case studies of electrically-active pulses in hurricanes Harvey (2017) and Dorian (2019) using HRRR-model output. We found the pulses possessed the characteristics of tropical squall lines, leading to the hypothesis that inner rainbands that propagated outward into a more favorable environment for deep convection reinvigorated into pulses that had tropical squall-line characteristics.