Tropical Cyclone Formation

In FY2019, MMM scientists have continued research to further understanding of theories about tropical cyclogenesis, comparing theories about how hurricanes form. Calculations beginning with a weak vortex that forms in radiative-convective equilibrium reveal the importance of a persistent, nearly saturated column of air. This exceptional column water vapor allows the amount of convection to increase (area and intensity), and alters the vertical profile of vertical motion to preferentially increase vorticity immediately above the planetary boundary layer. The radial pressure gradient associated with this vorticity penetrates through the depth of the boundary layer, creating convergence and vorticity spinup at the surface. Enhanced surface drag spins up the cyclone faster, consistent with the proposed mechanism.

Time series of minimum pressure at 50-m above the surface in experiments with a varying drag coefficient showing that spinup occurs for the largest drag, and fails for the smallest drag
Figure: Time series of minimum pressure at 50-m above the surface in experiments with a varying drag coefficient showing that spinup occurs for the largest drag, and fails for the smallest drag. After Wang, Y., C.A. Davis, and Y. Huang, 2019: Dynamics of Lower-Tropospheric Vorticity in Idealized Simulations of Tropical Cyclone Formation. J. Atmos. Sci., 76, 707–727, https://doi.org/10.1175/JAS-D-18-0219.1