A spatial filter approach to determining the role of convection on the evolution of a mesoscale vortex

Idealized three-dimensional numerical simulations of deep moist convection in the presence of a symmetric midlevel vortex are analyzed to understand the evolution of system-scale vertical vorticity during the very early stages of tropical cyclogenesis. A localized thermal perturbation initiates deep convection and the modified initial vortex is allowed to evolve over the 48-hour simulation period. During this time, surface momentum, heat, and moisture fluxes are excluded to eliminate any possible intensification by CISK or WISHE processes. A spatial filter is applied to the model output to separate system-scale and convective-scale features in an objective manner, whereby the filter scale is determined by a spectral analysis of the vertical vorticity field. It is found that the primary contribution to system-scale vorticity intensification comes from the convergence of convective-scale cyclonic vorticity, supplied by vortical hot towers (VHTs). Vorticity tilting is found to play an insignificant role in this development. The spatial filter approach offers an alternative viewpoint to the traditional symmetry/asymmetry method, and distinctively emphasizes the highly asymmetric evolution of the system-scale vortex.