Speaker: Dr. Morgan O'Neill
Atmospheric convection acts in the net as a thermally direct heat engine across many scales - from a single cumulus cloud, to a tropical cyclone (TC), to the general circulation. It has been shown that TCs operate as a heat engine particularly effectively, producing “about 70% as much kinetic energy as a comparable Carnot cycle (Pauluis and Zhang 2017, JAS)” in the eyewall of a simulated Hurricane Edouard (2014). Outflowing air then slowly subsides as radiative cooling to space balances diabatic warming, a process that does not consume any work. Nevertheless, some literature has invoked the possibility of occasional ‘mechanical subsidence’ or ‘forced descent’ in the TC outflow region in the presence of high inertial stability, which would be a thermally indirect circulation - much like the well-known TC eye. Such mechanical subsidence has not before been observed, measured or characterized.
I present results from idealized axisymmetric TC simulations that show a previously undescribed inertial oscillation at large radii and altitudes. It measurably occurs only in storms of latitude ~22 degrees and poleward, and has the curious property of bifurcating a portion of the overturning circulation into two distinct cells. This behavior is similar to that seen in a diurnal oscillation of recent three-dimensional TC simulations, but occurs as a strong function of inertial frequency instead. This radial oscillation is in contrast to the presumed forced descent model, and I hypothesize that this is because inertial stability provides less resistance than buoyant stability, even in highly inertially stable environments. I discuss implications for the heat engine model of a TC and compare it to a better understood overturning flow: the general circulation of the atmosphere.