Planetary Vorticity Effects on Cyclogenesis

Absolute vorticity is essential for the development of a tropical storm as additional planetary vorticity from an increase in latitude promotes frictionally-induced convergence in the boundary layer through Ekman transport. Additional convergence should thereby advect more water vapor inward that can be used to support a positive feedback mechanism through latent heat release aloft. This mechanism seemingly establishes a threshold for cyclogenesis as there must exist a latitude for which there simply is not enough inward advection of water vapor to overcome the dissipating effects of friction, diffusion, etc. Present research has established this threshold value to be approximately 7 degrees latitude based on simulations using a pre-existing weak vortex similar to disturbances in the eastern Atlantic.

Interestingly, previous statistical research and research done here shows additional planetary vorticity does not further increase the development time of a storm, nor does it increase the strength of the storm once developed. This seems to belie the previous discussion since a disturbance with additional convergence should overcome dissipating effects more rapidly. By analyzing model simulations at various latitudes, budgets of water vapor and mass reveal that additional planetary vorticity, beyond the threshold value, actually brings in less water vapor. To compensate, budgets of heat and momentum reveal that the loss in water vapor is offset by a higher efficiency of converting heat energy to kinetic energy.