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

Abstract:

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 through idealized numerical experiments. The experiments use a traditional cloud-resolving model with warm-rain microphysics. 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 critical interval of D where binary midlevel vortex interaction can prevent tropical cyclone formation. While tropical cyclones generally form at high RH, the process can exhibit substantial delay at intermediate 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. This work is supported by NSF under grant AGS-1250533.