Environmental Factors and Internal Processes Contributing to Interrupted Rapid Decay of Hurricane Joaquin

The objective in this study is to demonstrate how two unique datasets from the Tropical Cyclone Intensity (TCI-15) field experiment can be used to diagnose the environmental and internal factors contributing to the interruption of the rapid decay of Hurricane Joaquin (2015) followed by a 30 h period of constant intensity. After having struck the Bahama Islands, Hurricane Joaquin sharply recurved to the northeast and re-intensified, but then had an extreme decay rate from 135 kt to 75 kt in only 36 h. Even though Joaquin was moving over colder water, this extreme decay was interrupted around 1800 UTC 4 October, and then Joaquin maintained an intensity of 75 kt for 30 hours. Six hourly vertical wind shear (VWS) analyses from the real–time Statistical Hurricane Intensity Prediction System (SHIPS) and also from the Cooperative Institute for Meteorological Satellite Studies (CIMSS) provide the most likely environmental explanation for both the interrupted rapid decay and the subsequent constant intensity period.

A special CIMSS VWS dataset reprocessed at 15-minute intervals provides a more precise documentation of the large (~ 15 m s^-1) VWS throughout most of the rapid decay period, and then a rapid decrease in VWS to moderate (~ 8 m s^-1) values at and following the rapid decay period. During this period, the VWS was moderate because Joaquin was between large VWSs to the north and near-zero VWSs to the south. The moderate vertical wind shear was favorable for sustaining Joaquin at hurricane intensity. A unique dataset of High Definition Sounding System (HDSS) dropwindsondes deployed from the NASA WB-57 flying at 60,000 feet during the TCI-15 field experiment is utilized to calculate zero-wind centers (ZWCs) during Joaquin center overpasses. Deeper lower-tropospheric layers of near-zero vortex tilt are correlated with stronger storm intensities, and deeper upper-troposphere layers with large vortex tilts due to large VWSs are correlated with weaker storm intensities.