Observed Processes Underlying the Favorable Vortex Repositioning Early in the Development of Hurricane Dorian (2019)

Abstract

Dorian’s evolution from a weak, disorganized tropical storm to a rapidly intensifying hurricane is documented through a unique multiplatform synthesis of NOAA’s P-3 tail-Doppler radar, airborne in situ data, and Météo-France’s Martinique and Guadeloupe ground radar network. Dorian initially struggled to intensify with a misaligned vortex in moderate midtropospheric vertical wind shear that also allowed detrimental impacts from dry air near the inner core. Despite vertical wind shear eventually decreasing to less than 5 m s−1 and an increasingly symmetric distribution of stratiform precipitation, the vortex maintained its misalignment with asymmetric convection for 12 h. Then, as the low-level circulation (LLC) approached St. Lucia, deep convection near the LLC center dissipated, the LLC broadened, and precipitation expanded radially outward from the center temporally coinciding with the diurnal cycle. Convection then developed farther downtilt within a more favorable, humid environment and deepened appreciably at least partially due to interaction with Martinique. A distinct repositioning of the LLC toward Martinique was induced by a spinup of a mesovortex into a small, compact LLC. It is hypothesized that this somewhat atypical reformation event and the repositioning of the vortex into a more favorable environment, farther from detrimental dry midtropospheric air, increased its favorability for the rapid intensification that subsequently ensued. Although the repositioning resulted in tilt reducing to less than the scale of the vortex itself, the preexisting broad mid- to upper-level cyclonic envelope remained intact with continued misalignment observed between the midlevel center and repositioned LLC even during the early stages of rapid intensification.