Thursday, 11 June 2009
Stowe Room (Stoweflake Resort and Confernce Center)
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.
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