Vortex Height Evolution during Tropical Cyclone Rapid Intensification

The vertical structure of the tropical cyclone (TC) tangential wind field is related to intensity change as the minimum surface pressure is hydrostatically linked to geopotential height falls aloft. The dynamic height of the vortex (DHOV), defined as the height at which the maximum winds in the azimuthally-averaged tangential wind field decay to 40% of the maximum at 2-km altitude, is used to quantify vortex height. Airborne Doppler radar observations indicate observed TCs always have large DHOV values prior to rapid intensification (RI). Smaller, or “short”, DHOV values are more common in environments characterized by vertical wind shear (VWS). Observational evidence suggests low DHOV values are not “short” TCs, but rather tilted ones. The VWS tilts the vortex downshear and misaligns the low- and mid-level circulations inhibiting intensification. A tilted vortex reduces the projection of tangential winds in the azimuthal average used to determine DHOV. Large DHOV values likely indicate aligned TCs, which are more likely to undergo RI. DHOV values continue to increase in intensifying storms after alignment, suggesting that growth of the vortex in the vertical is a key structural process linked to warm core development in the upper levels during post-alignment intensification. Our analysis suggests that DHOV quantifies how much of the atmospheric column near the surface pressure minimum participates in intensification in two different ways: first as a proxy for the coupling of the lower- and mid-troposphere prior to alignment, and secondly as a proxy for the upper troposphere and lower stratosphere warm core strength after alignment. Idealized TC simulations are utilized to demonstrate the ability of DHOV to serve as a single metric that can bridge the gap between the pre- and post-alignment TC intensification in shear. Further experiments show how VWS in the upper-levels of the atmosphere can restrict DHOV growth and associated maximum intensity by preventing formation of the upper-level warm core. Characterizing vertical TC structure in moderate VWS and its relationship to intensity change through the DHOV metric can improve our understanding of inner-core processes in low predictability environments.