On the Tropical Cyclone Formation from Tropical Waves

It is well-known that tropical waves play a key role in the formation of tropical cyclones (TCs). These disturbances continuously provide a favorable large-scale environment for TC embryos to develop, and it is believed that nearly 80% of all TC genesis events equatorward of 20 degrees latitude are a result of tropical wave dynamics. The simultaneous existence of different types of tropical waves (e.g. equatorial Rossby waves, easterly waves of the Atlantic, equatorial Kelvin waves, and mixed Rossby-gravity waves) within the same region, coupled with the non-linear interaction between the various waves and the background environment makes predicting and understanding TC genesis a difficult problem. The crux of the tropical wave-TC genesis issue is that while tropical waves result in numerous disturbances that satisfy the necessary conditions for TC genesis, only a small fraction of these disturbances actually grow into TCs.

Recent studies by Gall et al. (2009a,b) provide a thorough examination of TC genesis associated with one specific class of tropical waves - the equatorial Rossby wave. In their idealized experiments in an initially-quiescent background environment, the initial amplitude of the equatorial Rossby wave is identified as the main criterion for TC genesis. When the equatorial Rossby waves are placed within a region of background horizontal shear, it is demonstrated that the smaller-amplitude, non-developing waves can interact with the background flow, and ultimately develop into TCs, within a reasonable time scale.

In this study, we extend the work by Gall et al. by first showing that deep convection is an important criteria that could actually spin up the non-developing disturbances into TCs even more effectively than the interaction of the equatorial Rossby waves with the background flows. By increasing the resolution of the simulations up to the cloud-resolving scale, all non-developing cases in Gall et al. grow into TCs, thus revealing the importance of deep convection in TC genesis. Furthermore, it is found that the pure equatorial Rossby waves do not simply evolve directly into incipient TC embryos but project first onto the Kevin equatorial wave modes. The subsequent interactions of these two types of waves give rise to a string of TC genesis events. A further investigation of the Kelvin waves demonstrate that Kelvin waves alone are not as effective as equatorial Rossby waves in triggering TC genesis given the same initial wave amplitude. Such a result implies that westerly wind bursts associated with the Kelvin waves are not an effective mechanism for TC genesis when compared to other types of tropical waves.