Sensitivity of Tropical Cyclone Intensification to Axisymmetric Heat Sources Part I: The Role of Inertial Stability

The processes responsible for changes in the intensity of a tropical cyclone (TC) remain not well understood. It has been suggested that the location of latent heat release by deep convection, relative to the distribution of the inertial stability of the mean azimuthal flow, affects significantly the acceleration of the tangential wind in TCs. Specifically, when heating occurs in regions of high inertial stability in a hurricane vortex, the acceleration of the tangential wind becomes more pronounced.

However, there are two pathways for the acceleration to take place. The thermodynamic ``efficiency'' pathway stipulates that large inertial stability stiffens the radial inflow to decrease the strength of the diabatically forced secondary circulation. As a result, the adiabatic cooling associated with the upward branch of the secondary circulation decreases and more of the imposed heating is available to locally warm the air column. Through the thermal wind relation, the tangential wind accelerates. On the other hand, the ``dynamic'' pathway stipulates that higher inertial stability implies higher relative vorticity and larger radial transport of angular momentum to spin up the tangential wind. Whether one pathway prevails over the other is not always clear.

The main purpose of this study is to elaborate on how latent heat release in higher inertial stability regions results in larger tangential wind acceleration in TCs. Specifically, the relative contribution of the``efficiency'' versus the``dynamic'' route will be addressed using a three-dimensional, nonlinear, dry, and fully compressible primitive equations model. The model is initialized with a balanced axisymmetric vortex with a prescribed heat source to mimic convection in the eyewall region. A series of sensitivity experiments on varying the strength of the initial vortex and the magnitude of the prescribed heat source are conducted. Azimuthal momentum budget calculations will be presented with the analysis aided by the application of the Sawyer-Eliassen equation. Our results indicate that the contribution of the ``efficiency'' pathway to intensification is non-negligible but remains sub-dominant. The spin up of the low-level tangential wind is always dominated by the azimuthal mean radial advection of absolute vorticity.