On the relative roles of vortex merger, axisymmetrization, diabatic heating, and mid-level moistening in tropical cyclogenesis

Recent studies on tropical cyclone formation and early intensification have proposed that different, though not necessarily exclusive, physical mechanisms control the formation of a self-sustained, warm-core cyclone. The first mechanism emphasizes the role of moist convection in driving a large-scale circulation. Stratiform processes initially dominate the heating profile, leading to mid-level convergence and a stronger mid-level circulation. As the atmosphere approaches saturation, the large scale mean heating profile becomes more convective andthe forced convergence descends to the surface, leading to a stronger surface circulation and thus cyclogenesis. The second mechanism focuses primarily on the evolution and interactions of small-scale vortices generated by deep convection through tilting and stretching of the local vorticity of a pre-existing disturbance. Through events of vortex merger and axisymmetrization, the predominant regions of positive vorticity organize each other while the negative vorticity is expelled, leading to a smaller, stronger, and more coherent circulation. In this study several tools are used to evaluate the roles of these distinct processes in hurricane formation. These include high-resolution, full-physics numerical simulations of tropical cyclogenesis and simulations of inviscid, two-dimensional vortex dynamics with equivalent scales. While the spontaneous merger and axisymmetrization of small-scale vortices does lead to a more coherent vortex, the time scales of this process are too slow unless they are enhanced by local and large-scale convergence associated with deep convection in a nearly saturated column.