Inertial Frequency as a Measure of Tropical Cyclone Inertial Resistance

Several studies have recently recognized the role of outflow resistance in modulating the growth rate of a tropical cyclone (TC) as it approaches its maximum potential intensity. Overall, it has been shown that weakened resistance is often associated with jet streams poleward of the TC and even in the context of a TC interacting with the QG circulation driven by a jet stream. Viewing the interaction from an inertial stability perspective, using the Alaka (1961) definition of inertial stability has been implemented to describe inertial resistance, despite the significant flaw that the formulation should only apply to axisymmetric outflow, which at best, is unusual.

It is inertial adjustment to pressure forcing that inhibits the growth of TC divergence at the outflow level, and so defines the “outflow resistance”. The relative strength of inertial adjustment is defined as the inertial frequency, compared to other adjustment frequencies such as the Brunt Vaisalla frequency or the divergence tendency itself. We have derived and tested a formulation for the absolute inertial frequency for the outflow environment, which precisely defines the relative strength of inertial adjustment to pressure forcing and the resistance it causes.

We have have applied this formulation to hurricane outflow, as diagnosed with assimilated TCI observations to demonstrate its effectiveness in determining outflow resistance and intensification potential. Results will be forthcoming at the oral presentation.