The Development of a Second Circulation Center within Tropical Storm Isabel (1985)

Abstract

Observations in the boundary layer by the NOAA AOC WP-3D aircraft from 8 to 10 October 1985 document the development of a second vortex, which evolves into the circulation center for Tropical Storm Isabel. The new circulation develops just outside the radius of maximum winds and is associated with intensifying convection 90 km from the original center. The original center loses its identity as convection dissipates around it. Low surface pressure, warm, dry air, and low equivalent potential temperature are found in the new center near its formation time. The new center is found beneath the downwind anvil of the intense convection in the rainband and appears to form over a period of 3?6 h, although significant changes in the storm-scale airflow north of the original center are occurring over the proceeding 24 h. The new center moves with a speed and direction similar to that of the original center. The observations of Isabel are compared to beat bursts, subsidence, and midlevel mesovortices that have been observed in tropical and midlatitude mesoscale convective systems. It is hypothesized that subsidence warming beneath the anvil, in the appropriate environment, could lower the pressure by several millibars and serve as an incipient perturbation for a tropical cyclone.