Moist Potential Vorticity Diagnosis of the Tropical Cyclone Boundary Layer

Potential vorticity (PV) has been widely used as a synoptic-scale diagnostic for tropical cyclone (TC) predictions, particularly for genesis and motion. However, its application to studies relating to mesoscale processes has been limited due to strong nonlinearities in the equations that make PV inversion difficult. Nevertheless, PV still retains its utility in tracking the diabatic and frictional forces as they directly impact the mass and wind field respectively, and adjust them locally to a new PV. In this study, we extend the scope of PV analysis to the tropical cyclone boundary layer (TCBL) and demonstrate that a suitable moist potential vorticity (MPV) framework can be used to diagnose moist TCBL processes that are crucial to TC development. We present results from a suite of idealized 3D TC simulations with varying degrees of boundary layer parameterization, along with real TC observations. The simulated TCBL is uniquely characterized as a region of negative MPV with a robust and coherent layer of high-magnitude negative MPV, referred to as the Potential Vorticity Minimum Layer (PVML). The differences in the PVML characteristics across the simulations are in part due to differences in turbulence parameterization schemes. The height of the PVML is primarily set by increased turbulent mixing of heat and moisture and therefore, it decreases in the same order as the vertical diffusivities of the simulations. Changes in the PVML reflect structural changes in the TCBL brought on by parameterized processes, namely diabatic heating and friction. While diabatic heating results in radially inward shifts in the high magnitude MPV inside the PVML, friction causes it to shift radially outward. We also demonstrate that the PVML height can be used as a proxy for TCBL height due to its close association with turbulent fluxes.