A Computational Study of the Evolution of Binary Mesoscale Vortices Above the Ocean

The evolution of two symmetric midlevel mesoscale vortices (MVs) over a tropical ocean is examined with a basic cloud resolving numerical model. The ambient conditions correspond to either the Jordan mean sounding (JMS) for hurricane season, or a variant of the JMS that is humidified in the middle troposphere to provide a setting conducive to rapid vortex intensification. In the humidified atmosphere, the MVs generally develop into one or two hurricanes with details of the process depending on the initial separation distance D. The time scale for the onset of rapid intensification under humid conditions decreases sharply when D is made sufficiently small for immediate coalescence of the interacting MVs. In the drier atmosphere, intermediate values of D can surprisingly prevent hurricane formation for the duration of a relatively long simulation. Nondevelopment appears to be attributable to the detachment (outward radial ejection) of lower tropospheric vorticity from the two MVs as they merge in the middle troposphere where their intensities are maximized. It is proposed that the primary mechanism for the observed midlevel merger and low-level vorticity ejection involves the excitation of rotating misalignments in each MV. The simulated misalignment dynamics compares favorably to an appropriately parameterized analogue model. For cases in which hurricanes emerge, the azimuthal velocity and radius of maximum wind at the time of peak intensity exhibit substantial non-monotonic variation with D. The azimuthal wind speed variation seems consistent with that expected from the attendant modifications of air-sea disequilibrium and unbalanced flow in the vicinity of the eyewall. This work is supported by NSF grant AGS-1250533, and computing allocations from XSEDE/SDSC and NCAR/CISL.