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

The nature and consequences of mesoscale vortex interactions during tropical cyclone formation are not fully understood. The dynamics is inherently complicated by baroclinic effects and moist convection, and its elucidation generally requires a computational approach. This study offers new insights into the basic mechanics and possible outcomes of midlevel mesoscale vortex interactions over a warm ocean, obtained by way of idealized numerical experiments with a cloud-resolving model. As expected, symmetric binary interactions are sensitive to the initial vortex separation distance D and other parameters that influence the time-scale for an individual vortex to intensify in isolation. 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 is found to exist a special interval of D where binary midlevel vortex interactions can prevent tropical cyclone formation. While tropical cyclones generally form at high RH, the process can exhibit substantial delay at middle-range values of D when the vortices are initially weak. Prevention or inhibition of tropical cyclone development appears to be connected to 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 analogue 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. The main conclusions of this study are based on simulations that employ warm-rain microphysics and are initialized with symmetric vortices. Preliminary numerical experiments that include ice microphysics have supported the key finding that conditions exist where symmetric binary vortex interactions-- culminating in midlevel merger --inhibit tropical cyclone formation. The consequences of more general asymmetric vortex interactions are currently under investigation and will be discussed briefly if time permits. This work is supported by NSF under grant AGS-1250533.