Development and Nondevelopment of Binary Mesoscale Vortices into Tropical Cyclones in Idealized Numerical Experiments

Abstract

he evolution of two symmetric midlevel mesoscale vortices situated above a warm ocean is examined with a basic cloud-resolving model. Idealized numerical experiments provide insight into how the evolution may vary with the initial vortex separation distance D and other parameters that influence the time scale for an isolated vortex to begin rapid intensification. The latter parameters include the ambient middle-tropospheric relative humidity (RH) and the initial midlevel wind speed of each vortex. At relatively low RH, there exists an interval of D where binary midlevel vortex interaction prevents tropical cyclone formation. While tropical cyclones generally develop at high RH, similar values of D can delay the process if the vortices are initially weak. Prevention or inhibition of tropical cyclone formation occurs in association with the outward expulsion of lower-tropospheric potential vorticity anomalies as the two vortices merge in the middle troposphere. It is proposed that the primary mechanism for midlevel merger and low-level potential vorticity expulsion involves the excitation of rotating misalignments in each vortex. An analog model based on this premise provides a good approximation for the range of D in which the merger?expulsion scenario occurs. Relatively strong vortices in high-RH environments promptly develop vigorous convection and begin rapid intensification. Differences between the interaction of such diabatic vortices and their adiabatic counterparts are briefly illustrated. In systems that generate tropical cyclones, the mature vortex properties (size and strength) are found to vary significantly with D.