The role of momentum transfer in tropical cyclogenesis: Insights from a single-column model

A step not fully understood in tropical cyclogenesis is the development of a surface cyclone, which is often preceded by a midlevel cyclone. This paper presents a single-column model to study the role of the transfer of tangential momentum in generating an initial surface cyclone. To isolate momentum transfer factors from thermodynamic factors, diabatic heating is set to be steady. The investigation starts without considering surface friction. The momentum transfer is decomposed into the transport by the vortex-scale circulation and by convection. The convective momentum transport (cumulus friction), when parameterized as a vertical eddy diffusion, leads to a vertical spectral truncation that permits an analytical solution of the single-column model. The analytical solution shows that the production of barotropic vorticity by the vortex-scale circulation is crucial for surface cyclone formation, and cumulus friction plays a dual role. Cumulus friction can enhance the downward momentum transfer, but when the eddy diffusion is too strong, the vortex-scale circulation is too damped to produce a significant barotropic cyclone. Between these two extremes lies an optimal eddy diffusivity that maximizes the growth rate of the surface cyclone. Finally, we add surface friction to the single-column model. Using scale analysis, we identify a critical vortex Rossby number above which surface friction becomes nonnegligible and significantly damps the development of the surface cyclone.