Characteristics of the Quasi-Periodic Outbreaks of Deep Convection During Tropical Cyclone Genesis

Nineteen years of high resolution infrared brightness temperature data are adopted to investigate the characteristics of deep convective activities during the genesis and early evolution of tropical cyclones (TC) in the Northwest Pacific. It is found that quasi-periodic outbreaks of deep convection are common during TC genesis regardless of the large-scale flow pattern or the magnitude of wind shear. Spatially, deep convection concentrates within 3 degrees of the disturbance center at peak time (when convection is active), and will shrink in coverage and become scattered at trough time (when convection is inactive). At the decay stage deep convection in the central area tends to weaken first. Among the quasi-periodic cycles of deep convection, quasi-diurnal cycles are the most distinct feature, followed by cycles with periods of 10-18 hr. The quasi-diurnal cycles with an early morning peak only account for less than 50%. Statistically the amplitude of deep convection cycles will decrease a bit under shear greater than 9 m.s(-1), and period length in the range of 12-28 hr decreases slowly with the increase in wind shear and will rise slightly when shear is greater than 9 m.s(-1). Besides, genesis does not necessarily occur at the peak time of coverage in deep convection, and the oscillatory feature is not an effective indicator of genesis acceleration.