Examination of Tropical Cyclone Structure and Intensification with the Extended Flight Level Dataset (FLIGHT+) from 1999 to 2012

The physical processes governing the feedback between tropical cyclone (TC) intensification and structural changes are not fully understood. Recent studies have proposed that the structure and convective activity of intensifying TCs differs from those that are steady state. The efficiency of intensification has been attributed to the radial location of convective bursts with respect to the radius of maximum tangential wind (RMW), where intensifying TCs were characterized by convective bursts radially inward of the RMW as opposed to outward for steady state TCs. Understanding the link between the physical processes responsible for intensity change and radial structure differences could lead to more accurate TC intensity forecasting.

A comprehensive examination of TC kinematic and thermodynamic structure in the Atlantic basin is created from the Extended Flight Level Dataset (FLIGHT+). In situ data collected at the 700 hPa flight level by NOAA WP-3D and USAF WC-130 aircraft from 2000 to 2012 were analyzed. 144 flights consisting of 889 total radial legs were stratified by TC intensity and 12 hour intensity change. A matrix of composite structures was then calculated for hurricanes (Categories 1 and 2 on the Saffir-Simpson scale) and major hurricanes (Categories 3 and above) that were intensifying [intensity increase ≥10 kt (12 h)^-1], steady-state [intensity change between ±5 kt (12 h)^-1], and weakening [intensity decrease ≤-10 kt (12 h)^-1].

Axisymmetric radial composites reveal that intensifying TCs had statistically significant structural differences from TCs that were steady-state or weakening. Intensifying TCs were characterized by steep tangential wind gradients inside the RMW that contributed to a ring-like structure of vorticity and inertial stability. Outside the RMW, intensifying TCs were characterized by a steeper decay of tangential winds and lower absolute angular momentum. The composites indicated more tangential wind structural differences inside the RMW for hurricanes compared to major hurricanes. Thermodynamic structures showed fewer differences than the kinematic structures in general, but weakening major hurricanes were found to have higher equivalent potential temperature overall. Weakening major hurricanes also showed a pronounced ring-like structure of vorticity, similar to intensifying major hurricanes, while steady state major hurricanes had a flatter vorticity profile. These results suggest that structural differences in vorticity play an important role in the intensity change of hurricanes and major hurricanes.