Inertial waves in axisymmetric tropical cyclones

TL;DR

This paper investigates how inertial waves influence the outflow and ventilation of axisymmetric tropical cyclones, revealing that inertial stability can excite waves that ventilate the storm core, contrasting with traditional forced descent models.

Contribution

It demonstrates that inertial waves, rather than forced descent, play a key role in high-latitude tropical cyclone outflow dynamics through axisymmetric simulations.

Findings

Inertial waves are excited at the environmental inertial frequency in high-latitude storms.

These waves periodically ventilate the storm core with low entropy air.

Inertial stability offers less resistance to wave formation than buoyant stability.

Abstract

The heat engine model of tropical cyclones describes a thermally direct overturning circulation. Outflowing air slowly subsides as radiative cooling to space balances adiabatic warming, a process that does not consume any work. However, we show here that the lateral spread of the outflow is limited by the environmental deformation radius, which at high latitudes can be rather small. In such cases, the outflowing air is radially constrained, which limits how far downward it can subside via radiative cooling alone. Some literature has invoked the possibility of `mechanical subsidence' or `forced descent' in the storm outflow region in the presence of high inertial stability, which would be a thermally indirect circulation. Mechanical subsidence in the subsiding branch of a tropical cyclone has not before been observed or characterized. A series of axisymmetric tropical cyclone simulations at different latitudes and domain sizes is conducted to study the impact of environmental inertial stability on storm dynamics. In higher latitude storms in large axisymmetric domains, the outflow acts as a wavemaker to excite an inertial wave at the environmental inertial (Coriolis) frequency. This inertial wave periodically ventilates the core of a high-latitude storm with its own low entropy exhaust air. The wave response is in contrast to the presumed forced descent model, and we hypothesize that this is because inertial stability provides less resistance than buoyant stability, even in highly inertially stable environments.